Rapid Aluminum Alloy Aoalysis Utilizing Inductively Coupied
Plasma Atomic Emission Spectrometry
Elizabeth A Brown
A Thesis submitted to the Faculty of Graduate Studies and Research in partial
filfiIlment of the requuements of the degree of Master of Science.
August 1999
Department of Chemistry
McGill University
Montreal, Quebec
Canada
O Elizabeth A Brown 1999
1*1
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Abstraet
The goal of this study wu the exploration of several techniques for the rapid
analysis of aiuminum pins by ICP-AES (inductively coupled plasma atomic emission
~pectrometry).
Diiect solid sample analysis of the pin by DSI (direct sample insertion) proved to
be unfkasible due to incomplets v a p o ~ i o of
n the sample Born the DSI probe.
A technique called the Real-time Afloy Analysis Technique (MAT) allowed
analyte signals to be monitored during m p l e digestion (in dilute HCI) and using a ratio
method, quantitative resuIts w n obtained afker a few minutes of initiating sample
digestion. This method exhibited k t and simple sample preparation and high presision
of < 3 % relative standard deviation.
An in-solution spark technique (SAD) was used to produce dispersions of the pins
in water. The SAD and DSI proved prornising as a very rapid sampling technique.
Le but de cette étude consistait en l'exploration d e d i v m u avenues pour
l'analyse rapide d e broches d'duminium par SEAP (Spectmmetrie d9EmissionAtomique
par Plasma.
L'analyse directe des broches par IDE (Insertion Directe d e I'Echtillon) s'est
avérée impossible due 1la vaporisation incomplète de I'échantillon dans la sonde IDE.
Une méthode nom& Technique d'Anaiyse des Alliages en Temps Réel (TAATR)
a permis de s u i m I'évolution des signaux d'analytes au cours d e la dissolution de
I'échtillon (dans de 1' HCl dilu&), & par le biais d'une méthode de rapports, des
r6sultats quantitatifs ont ét6 obtenus quelques minutes après le début de la dissolution de
l'échantillon.
Cette méthode possède l'avantage d'une préparation de l'échantillon
simple et rapide, couplée d'une haute précision < 3 % relatifs.
Une technique d'ablation en solution (SAD) a été employée pour disperser les
broches en solution. Cette technique (SAD) et l'IDE se sont avértées prometteuses en
tant que méthodes d'échantillonage rapide.
iii
Acknowledgements
I would fini like to thank rny research supavisor Eric Salin who has always
cncouraged and supported me with my studies and research, as well as my athletic
endeavon. 1 am deeply gratefiil for y w r guidance and fnendship throughout my
undagraduate and graduate studies.
To my parents, 1thank you for your paîience, love and support and for giving me
the opportunity t o study at McGill.
To Martin, m c i beaucoup pour tes nombreuses années dYamitiC et
d'encouragement B l'école, dans le sport, et dans la vie.
To the McGill Martlet Hockey t a m , thank you for providing me with many years
of Wendships, cornpetition, adversity and success.
1 would like to thank Cameron Skinner for trying to pass on his skills as a
handyman when things went awry in the lab. For your endless patience in answering
many questions, I thank you.
To my fnends in the lab, thank-you for your g w d humour, sarcastic wit, support
at my hockey games and fnendship.
Table of Contents
ATOMIC EMISSION SPECXROMETRY, - - - - - - - - - - - - - - - - - - - I ~ - m t - ~ - - - ~ ~ U I ~ . u ~ . u . I
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LIST OP TABLES
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LIST OP PIG~owo-uosoooo-ooosoosooooooooooomo-o.o.monowu-o.wmnwwoHoso.w-o~o-o-ooooooooo-oooooooooooooooooo*ooooo-
.................... ,
.
.
.
................................................................ 1
C'emicuI Dissolution...................................................................................................................
... 3
An: and Spark Ablation.....................................................................................................................
4
X-RqyFftorescience Fw.3................................................................................................................ 6
Laser Ablarion ..................................................................................................................................
7
Direct Sampk Insertion (D.9"........................................................................................................... 7
Aluminum Pins ................................................................................................................................. 9
OBsEcmms ........................
.
.
.......................................................................................................
10
REFERENCES ....................................................................................................................................
11
~WRODUCTIONAND LITERATURE
F~EVIEW
AN EVALUATION OP DIRECT SAMPLE INSERTlON INDUCTMZLY COUPLED PLASMA
ATOMICEMISSXON SPECTROMETRY FOR THE ANALYSIS OF ALUMINUM ALLOY PINS 14
ABsTRAcr ..........................................................................................................................................
...................................................................................................................................
Objective ........................................................................................................................................
EXPERMENTAL
...................................................................................................................................
lnsîmmentation..............................................................................................................................
Reagents md Simples.....................................................................................................................
Osr Cup Design..............................................................................................................................
Di& Sample insertion ..................................................................................................................
Power .............................................................................................................................................
. . Sampie Anabsis...................................................................................................................
Ltqurd
..................................................................................................................
RESULTS AND DISCUSS~ON
S i i d SlanpeAnaiysis.....................................................................................................................
~ODUCI~ON
14
15
16
17
17
17
18
29
19
20
21
21
DSI Cup Dcsign ......................................................................................................................................... -24
Powa .......................................................................................................................................................
24
........................................................................................................25
U p i d SumpleA l l ~ I ' k
CONCLUSIONSAND FWRE WORK......................................................................................................
29
........,
ACKNOWLEDOEMENTS........................................................................................................................
REFERENCES.......................................................................................................................................
DETERMINATION OF AL, CR,CU. FE. MG. MN. SSI AND ZN IN ALUMINUM PINS USING
TEE REALTIME ALLOY ANALYSIS
29
30
31
TECHNIQUEoooœooooo~uo~o~ooeooHoooo~~~oooo~~om~o~~~ooo~o~soo~~oosososoo~oo
o
~
~
~
ABSmAm.......................................................................................................................................
INTRODUCTION ..................................................................................................................................
..................................................................................................................................
EXPERIMENTAL.
Instrumentcllion ..............................................................................................................................
Spack Abletion Device.................................................................................................................................
ICP-AES .....................................................................................................................................................
Reagcnts andSmples............................. ........................................................................................
Spork Ablaifon h p f i n g...............................................................................................................
S e SIrmpIing into L N Pmbe .......................................................................................................
................................................................................................................
R E S U LAND
~ DISCUSSION
Cornpartisonof CidibrufionMefhodologies......................................................................................
Spcwk Ablation ï î m e .......................................................................................................................
Solution Considemtioons..................................................................................................................
Figums of Mefit..............................................................................................................................
Spork Sbnpplg in DSI Probe..........................................................................................................
...............................................................................................................................
CONCLUSIONS
........................................................................................................................
ACKNOWLEDGEMENTS
REFERENCES.......................................................................................................................................
51
52
54
54
54
54
57
57
57
59
59
61
64
67
70
75
75
76
C~~ICWSIONS
AND SUOOE!~~~ONS
FOR FUTURE WORK......................................................................... 77
.......................................................................................................................................
REFERENCES
78
I
List of Tables
1-1: PROPERTIES' OF A U D Y i N 0 ELEMENTS iN THE MANUFACIURE OF ALUMINUM ALIl)YS............... 2
......
............... 17
TABLE
2-1: PERCENT
COMPOSITION FOR ALCAN
ALUIURUUM ALLOY PINS ...................
TABLE 2-2: THE PRECXSION AS ./.RSD OF THE AVERAOE PEAK AREA FROM TWO iNSERTIONS FOR THE
DIFFEXENT AWY s m m.................................... i....................................................................... 24
TABLE 2-3 : ~ M P A C U S O N
OF Yi RSD OF UQüiD ALUMINUM SAMPLES W ï i l i AND WITHOVT THE USE OF
F R E O N - .....................................................................................................................................
~~28
I
M
.......................................................
................................................................-..................................
TABLE
3-1 : PLASMA AND SPECiROUETEROPERATWO PARAMETERS
35
E TABLE
U 3-2: C
~T'ANDARDS
37
TABLE
3-3: SOLUB~EI?I PROPERTES Of ALUMINUM AND VARIOUS ALU)YINO ELEMENTS
.
,
39
TABLE
3-41 ALUWNUM CONCENTRATK)NDETERMINE0 FOR 6 MINUTE DIGESTION AT 22OC
42
45
TABLE
3-5: ELEMENT
/ ALUMINUM RAAND PREC1810N FOR DIOESnONS AT 33*C AND 52OC
TABLE 3-6: L W OF DETECIION(LOD)FOR ICP-AES ANALYSIS AND CONCENTRAnONS FOUND IN
SOLUTIONS FOR ALU)YS DIOESTED AT 6
m
46
TABLE 3-7: DETERMINATION
OF AL, CR,CU, FE,MG,MN, SI.ZN IN VARIOUS ALUMINWM AUI)Y PINS
D I G ~ AT
D A TEMPERATUREOP 600C
47
TABLE
3-8: AVERA~E
PERCENTCOM#)SITK)N DETERMLNEDFOR W
Y 5 182
48
TABLE
4-1: PLASMAANDsPECTROMETER OPERATCNOPARAMEi€RS
56
TABLE 4-2: CONCENTRATIONS FOUND iN SPARK ABLAïION SOLURONS OF ALUMINUM A L U l Y S USiNO
EXIZRNAL SïANûARDS AND STANDARD ADDiïïONS CAUBRATION
60
TABLE 4-3: CONCENTRATION
OF AL FOUND IN ALU)Y SOLUTIONS FROM A 5 MiN SPARK ABLATION
6
4
C0MPARZ:DWITH THE EXPECTEDCONCENTRATION
TABLE 4-4: CONCENTRATION OF AI. EXPFXTED AND FOüND IN D I S ~ ~ U E ~ D E I O M Z EWATER
D
AND ACIDlFIED
SPARK ABLATION SOLUïiONS
66
TABLE
4-5: CONCENTRATIONSDETERMINED FOR D I S ~ ~ L E ~ D E ~ O N I ZWATER
ED
AND ACIDtFIED SPARK
ABLATION SOLWIONS
66
TAEUS4-6: D E ~ ~ L(MITS
O N FOR ICP-AES ANALYSIS
6 7
68
TABLE
4-7: DETERMINA= OF AL, CR,CU, FE. MO,MN ZN IN ALUMINUM ALLOY PINS
TABLE
4-8: AVERAGE
PERCENT COMPOSIT~ONDETERMINED FOR AUDY 3 104
69
................ ........
........................
...............
.....................,.. ...........................................................
............................................................................................
.........................................
........................................................
...............................................
......................................................................
...........................................................................................................
......................................................................................................................
........................................................................
.................
..........................................
List of Figures
............................................................................................. 9
18
2-2: DIRECT SAMPLE iNSERn0N DEVICE IICHEMA1K: DIAGRAM................................................. 19
FWRE 2-3: LIQUI)SAMPLE ANALY SIS: .................................
... .............................-..............................20
FiavRE 24: FIW ALUMINüh4 TRACEâ FOR 2.0 MO OF ALtrDY 5 182 USmO A THIN-WALLED CUP AT 1.0 KW
FOWER ....................
...,...
.... ..,............................................................................................... 22
ZiNC TRACES FOR 2.0 MO OF U
Y 5182 USiNO A THIN-WALLED CUP AT 1.0 KW POWER
FIGURE 2-31
...................................................................................................................................................
22
5 182 U S M A T H I N - W U D CUP AT 1.0 KW
FIC3URE 2-6: TW0INSERnoNS OF 2.0 M o SlXiMENTs OF
POWER ...........................................................................................................................................
23
FKfURE 2-7: ~ ~ M ~ A I U ETRACES
SE
AT W W AND HK)H #IWER ............................ .
.
..................................25
RHIRE2-8: TRANSIENT SIONAU FROM UQUtDSAMPLE ANALYSIS OF ALU3Y 7010.................................. 26
FICK~RE
2-9: EFFECT OF FREON
ON ALUMP(UM TRANSIENT SIONAIS.......................
.......
............... 27
FIGURE 3-1: h AND ZNSI<INALS MONlTORED THROUOHOVTTHEDIOESTION OF AN ALUMINUM PiN AT ROOM
TEMPERATlRLE.....................
.,..
................................................................................................... 40
FIGURE
3-2: RATIO OF SIGNAL iNï€NSITIES (ZN/&) MOMTORED THROUCIHOiJï THE DIOESllON OF AN
ALUMIIUUM PiN AT ROOM TEMPERATURE........................................................................................... 40
FIGURE
3-3: RATIO O F SIONAL INTENSïllES (MN/&.) MONlTOREDTHROUOHOW THE DIOESllON OF AN
ALUMINUM PiN AT ROOMTEMPERATURE.......................................................................................
41
FIGURE
34: TRANSIENT SIONALS ( R A W E D TO AL) FOR CU.FE,MO, hnN AND ZN(N A L m Y 70 10 DIOESTED
AT A TEMPERA~UREOF 33OC............................................................................................................44
FlCXJRE 3-5: ~ S L E N SI<INA~S
T
(RATIOED n> AL) FOR CU,FE, MO.
AND ZNIN ALU)Y 7010 DIOESTED
AT A TEMPERATUREOF 52 C...........................................................................-.............-..................
44
RQURE
4-1 : PH-RAPH
OFTHE SPARK AB~ATION DEVICE.................................................................... 55
FI~WRE
4-2: DSI PROBE DESION FOR PERFORMING N S T U SAMPU ABLATION USMG T)iE SPAiUC ABLAnON
DEVICE............................................................................................................................................ 58
FKiuRe 1-1: &SAN ALUWNUM ALWY PiNS
FICRIRI3 2-1:
DSI CUP DESIONS: ...............................................................................................................
O
a
.
FIC~URE4-3: b h S s OF ALUlY 3104 DISPERSED N i l J SOLUnON AS A FUNCIlON O F SPARK ABLAnON R M E 62
LINEARRELATIONSH~P
BETWEEN MASS DISPEilSED AND SPARK ABLATION TME THE MASS
62
DISPERSED FOR A 3 MIN SPARK WAS ESTIMATED AS THE AVERAGE OF THE 2 AND 4 MIN VALUES
~ ~ U 4-5:
R ECONCENTRATIONS OF AL, CU.FE. MOAND hfN FOUND AS A FUNCTION OF SPARK ABLATION
FIWM 44:
.
.........
mat............................................................................................................................................... 63
FIOURE
f i : RATIO OF SIGNAL CNTENSmES MEASURED (RATIoED TO AL) FOR CU. FE, M G AND MN AS A
FUNCTTlON OF SPARK ABLATION TIME................................................................................................ 63
FIGURE
4-7: CROSSSWTiONAi, VIEW OF SPARK ABLATION OF AN AtUMINUM WlRE PERFORMED INSiDE A DS1
PROBE: (A) BEFORE INITIA'1WO ABLATION. (8) ûiJRiN0 SPARK ABLATION.......................................... 71
FIOURE
4-8: SPARK
DlSCHAROE OCCURRLNO BETWEEN THE ALIJMINUM WüW AND ï I i E DSI PROBE WALLS.72
FIGURE
4-9: OVERHEAD
WEW OF SPARK ABLATION BETWEEN ALWlRE AND DSI PROBE............... 72
FIGURE
4-10: TRANSIENT SIONALS OBTAINED FROM AN MSERRON OF ADSI PROBE CONTAININO A SAMPLE
OF
6 11 1 PRODUCED FROM A 30 S ABUTiON PERFORMED iN SlTll............................................. 74
Chapter 1
Introduction and Literature Review
Ancient civilizations including the Egyptians, Greeks and Romans discovered and
put to use the first m d s known u the maals of antiquity. These metals included gold,
copper, silver, lead, t h , uon and merairy and were al1 discovered between 6000-750 BC.
Metals during this time were used in the maiüng of jewelry, tools, implements, weapons,
containers and even 9 e paint. These rnetals did not always occur in their elementai fonn
and in the case of gold it was quite dinicult to obtain the pure metal. One of the first
metal alloys was called eiectrum, containing both gold and silver. An alloy comprised of
copper and tin becaime known as bronze. These metals and alloys were so crucial to the
development and survival of ancient civilizations that the Bronze Age (about 2000-1000
BC) and the Iron Age (about
1200 BC-1200AD) were aptly named.
It was not until the early 19th century, however, that one of today's most
important metals was discovered. Aluminum was discovered in 1809 by Sir Humphrey
~ a v and
~ ' first pmduced commercidly in 1854 by Sainte Claire ~ e v i l d . Deville
exhibited a bar of this alurninum at the 1855 Paris Exhibition. Even with Deville's
discovery7 aluminum production did not really becorne feasible until 1886. Charles
Martin Hall fkom the USA and. Paul Louis Touissant Héroult fkom France independently
developed an electrolytic process for producing aluminum'. This process is still used
today in the aluminum industries.
The demand for aluminum grew as more metallurgical techniques pmvided
methods to improve the strength of the metal.
Pure aluminum is not very strong*
therefore the production of aluminum alloys becarne desirable. During World War II, the
demands for alloys of rigid specification2 increased, leading to the development of
aluminum or 'light' Jloys. These alloys require only small amounts of other elements in
o r d a to change their physi& properties. The main elements used in aluminum alloys
are magnesium, silicon, zinc, wpper and manganese4. ûther elements such as nickel,
chrornium, iron, titanium, vanadium and zirconium are also used. Table 1-1 sumarizes
the main properties of some of these alloying elements.
Table 1-1: ~ r o ~ e r t wof
i ' iUoying dementr in the maaufacture of dumioum dloys.
Magnesium
decreases melting point to 451°C, increases corrosion resistance to salt
Silicon
Zinc
increases strength and ductility, decreases me1ting point
dramaticall y incrases strength
COPF
increeses strength, but decreases wirosion resistance, weldability and
ductiiity
Nickel
Titanium
increases strength under high-temperature conditions
provides decreased grain size
Zirconium
stabilizer eIement influencing temperability
Chrornium
increases resistance to stress corrosion
Iron
usually an impurity which can increase strength of pure aluminum
Aluminum alloys can be divided into two categories: wrought and cast alloys.
Each category can be firther differentiated based on the mechanism of property
development (heat-treated and non-heat-treated alloys)'. Wrought and cast alloys have a
nomenclature known as the Numinum Association systemS developed in the United
States.
A description of this nomenclature system for the identification of different
aluminum alloys is presented in Appendix 1.
In order to produce and reproduce these alloys, a method of analysis was
necessary since the chemical composition of an alloy determines its propetties. This led
to the development of various techniques for aluminum alloy analysis. There exist many
instrumental, volumetnc and gravimetric techniques for determining the constituents of
an alurninum alloy. These methods can be time consuming, involve considerable sample
prepmtion and may only be applicable to one element.
Copper in aluminum olloys cm be analyzed by the electmlytic (gravimetric) test
method6. This method involves acid dissolution followed by filtration to remove Si. An
etectrolysis is perfomed and the cathode deposit is weighed. The iodate titrimetrîc test
method7 is used to determine tin in duminum dloys. The alloy sample is dissolved in
=id, filtered and tin is reduceâ. The stannous tin is then titratecl with an iodate solution.
Similar techniques a i s t for many' other dloying elements, however, most of these
'
techniques are element specific.
Chemical Dissolution
Chemical dissolution typically involves laborious sample preparation, introduces
the possibility of contamination dtie to handling and reagents and increases the cost of the
analysis. The advantages of chernical dissolution include the production of easy t o
d y z e liquid samples, calibration techniques utilizing liquid standards and relatively
inexpensive commercially available instruments depending on the method of anaiysis.
Aluminum is soluble in hydrochloric (HCI) and hydrofluoric (HF) acid.
An
duminum alloy sample can be dissolved using HCI and heating, however silicon is
dinicult to digest and is usually lost using this type of dissolution. Difficult to dissolve
silicates can be digested using W and nitnc acids. Special containers must be used
beceuse the HF will leach out the silicbn from typical glassware. The use of HF also
poses a problem if inductively coupled plasma OCP) anal ysis is to be done because of the
glass spray chambers and torches that are used.
Once an alloy sample has been digested, the liquid can be analyzed using the
technique of ICP atomic emission spectrometry (ICP-AES). The ICP is a partially
ionized g
(usually Argon) produced in a quartz tube. The tube or torch is surrounded
e d fiequency (RF) generator. The generator
by an induction coi1 that 4s ~ ~ e ~tota radio
typically operates at 27 M H z (ranges 6om 4-50 M h ) and at output powers of 1-5 kW
(usually 1-2.5 k~)!
An Argon
ICP ranges in temperature &om
this source much hotter than flames or graphite Lmaces.
5000-9000 K, msking
The ICP can be used for
sample atomization and excitation followed by atomic emission detection with a
spectrometer. Analysis of liquid samples using ICP-AES exhibits good detection limits
on the order of parts per trillion (ppt) to parts p a billion @pb) and high precisions of 1 %
RSD~.Another important advantage of ICP-AESis that it provides simultaneous multielement analyses.
Wud and ~ a r c i e l l o " d y z e d alwninum alloy samples by inductively coupled
plasma optical emission specfrometry (ICP-OES). The samples were acid-dissolved
using hydrochloric acid and heating. Bames et ai1' d y z e d primary, refined and alloy
aluminum using ICP-AES.
The aluminum sampla were dissolved using various
combinations of hydrochloric, nitric and hydrofluoric acids. Broekaert and ~ e i s "used an
alkali diasolution in order to dissolve aluminum alloys with a high silicon content. The
major problem with c h e r n i d dissolution methods is the time requued for sampling,
digestion and analysis.
As more instrumental techniques are developed, the use of wet chernical methods
is decreasing.
The direct analysis of solid sarnples provides many advantages over
conversion to liquid.
Time consumhg and costly sample preparation steps can be
minimized by directly analyzing the sample in the solid fom. Another advantage of
solids analysis is the reduced risk of contamination through reagents and handling
involved in the sample preparation. Dilution erron are eliminated and detectability may
be enhanced by analyzing the sunple in its natural state. For the alloy industries, it
would be best to have r d - t i m e monitoring in the melt in order to minimize energy and
time resources, however present systems do not allow for tbis. Present state of the art
systems for solids analysis typically requin a sample in the form of a disk. The first step
in the production of this disk is to withdraw a sample fkom the alloy melt. This sample is
then poured into a bar and rapidly moleci before becoming available for testing.
Inhomogeneity may occur due to segregation on ~olidification'~.A sample disk is
machined from this bar and can then be analyzed. These sample disks are typically
inhomogeneous on a small s a l e and this presents a problem with traditional direct solid
metal analysis techniques like spark. Methods of direct solid rnetal analysis will be
disaissecl, including arc and spark ablation, x-ray fluorescence, laser ablation and direct
sample insertion.
Arc and Spark Ablation
The use of arc and spark discharges as excitation sources for both qualitative and
quantitative analysis began as early as 1920". Since this tirne, these two techniques have
developed into methods that are still used today. Arc is mostly used for qualitative and
semiquantitative anaiysis, whaas spark his found widespread use for quantitative
analysis.
Arc and spuk ablation have been used for solid sampling and combind with ICP
excitation. This process involves aôlation or emsion of the sunple through interactions
with the arc or spark discharge, creating a m p l e plume of vaporiad and partiailate
matter which can then be transportexi tb the ICP via a gaa stream". A requimnent for use
of either technique is that the sample be condudive. Samples are either naturaily
condudve or are wrnbined with a condudive rnatezial such as graphite usually in a
p o w d d fortn.
There are différent types of arc discharges, including dc arcs, ac arcs, controlledatmosphere arcs and gas-stabilized arcs"!
The most cornmonly used arc is the dc arc.
The dc arc employs a continuous discharge between two electmdes made of graphite or
metal. The sample is placed into a cup shaped electrode in the fom of powden, filings
or chipsI6. The discharge is then fonned behveen a counter electrode and the electrode
containing the sample. Dc arc exhibits good detectability and poor precision. The poor
presision can be attributcd to sahple .inhomogeneity, source stability, selective
volatilization and arc wander. Precisions of better than 10 % Relative Standard Deviation
(RSD) are rare1' and therefore limit the use of arc discharge for quantitative analysis.
The high-voltage spark discharge is more commonly used than arc discharge.
The problems of themarc's poor precision and selective volatilization are improved upon
Spark sampling employs an intermittent discharge between the two
electrodes. Multiple discharges at various spots on the sample surface enable numerous,
with spark.
random samplings which aren't evident in arc discharge. These multiple samplings allow
an average measurement which helps improve precision.
The problem of sample
heterogeneity is lessened by sarnpling at various spots on the sample surface. A prebum
or prespark is typically used for about 1-2 minutes in order to achieve a reproducible
sampling. High-voltage spark sampling combined with ICP excitation can achieve
precisions of 1-10 % RSD with r d sarnplesl'.
Thenno Jarrell-Ash manufactures a
commercial spark ablation system known as the separated sampling and excitation
amlysis (SSEA) sYsteml8.
S j w k systems have bcen extensively used in the metals industry, including steel
and aluminum industries. These systems am offen located on-site at the founâry, where
~ m p l e scan be rapdly analyzeâ, most often using a major constituent as an intemal
stuidrrrd. Calibration is usually done using a matrix matched standard of similar
composition to the sarnple. The total analysis time for this method is imeased due to the
requinment for the rnanufpchin of the sample bar, rnachining of this bar and subsequent
malysis of the sample disk.
Advantages of this technique include relatively good
precision and aâequate detection lirnits for many elements.
Aziz et al. anaiyzed variws aluminum dloys using spark ablation ICP-OES.
They concluded that spark ablation ICP-OES is a viable alternative to ICP-OES
following sample dissolution or other methods for direct analysis of solidslg. Prell and
~ o i r t ~ o h a n nanalyzed
~'
aluminum alloys and found pglg deteetion limits, precisions of
5-10 % RSD and linear calibration curves over four orders of magnitude. Precisions
improved to 1-3 % RSD when the major element was used as an interna1 standard.
A variation of the spark ablation technique has been investigated. This technique
involves dispersion of a metal sample using a spark oblation device (SAD) operated in a
liquid medium21322U4. The SAD is used to prepare mlloidal solutions22 of the metallic
samples that cm be analyzed using atomic absorption methods. Ghiglione et or"
detennined concentrations of minor elements of an aluminum alloy with a relative
accuracy of about 5 %. L'vov and ~ovichikhin" determined trace elements in various
alloys by graphite fumace atomic absorption spectrometry (GFAAS) after spark ablation
of the metal samples in water.
X-Ray Fluorescence (XRF)
The sample is irradiated with a beam of x-rays 60m an x-ray tube, whereby the
elements in the sample are excited by absorption of this beam and emit characteristic
fluorescence x-rays. The XRF technique provides relatively simple spectra, therefore
spectral line interference is not usually probletnatic. The technique is non-destructive
and therefore has important applications for the analysis of valuable objects such as
jewelry, paintings and archaeological artifacts. The sample size is not important and
allows analysis of micro samples as well as very large samples and multi-element
analyses can be achieved within a few minutes. Some of the disadvantages of XRF
include poor ddection limits and poor detection of light elements especidly below
atomic nimber 23?
The sample surface must be prepared by machining, grinding d
polishing before pafonning an analysis therefore requiring sarnple preparation and time.
Dick and
rase?
rndyzed aluminum alloys using XRF and found that the
analysis was satisfactory for Cu,Fe, Mn, N i Ti, Zn, Bi, Cr, Pb and Sn and unsatisfmory
for Mg and Si.
Laser Ablation
Lasa ablation is used as a solid sampling technique and can be combined with
ICP-AES or ICP-MS excitation and detection. A Ruby or Nd-YAG laser beam is
focuseci ont0 the sample causing some material to be vaporized from the surface. Similar
to arc and spark techniques, a phme of sample vapor and particdates is forrned and then
carriecl in an argon stream to the ICP. Unlike arc and spark ablation, the sample does not
have to be c d u c t i v e and very small surface areas are sampled. Laser ablation requires
the use of rnatrix-matched reference materials for calibration, however, even with this
calibration methodology, precision is still poor and varies âom 5-10 % RSD for
metallurgical and geological samples2'.
Liu and ~ o r l i c kanalyzed
~~
aluminum and steel samples using an in situ laser
ablation ICP-AESsystem and found that precision was of the order of 10 % RSD. The
quality of the calibration curves was not great due to the susceptibility of LA to problems
sssociated with sample inhomogeneity.
Direct Sample Insertion @SI)
The first DSI device for ICP spectrometry was reported by Salin and ~ o r l i c k 'in~
1979. Direct sample insertion is implemented by inserting the sample on a probe (usually
made of graphite) directly into the load coi1 of an ICP. DSI allows one to analyze
mnples in both liquid and solid fonn therefore providing mon flexibility over techniques
which are only available for liquids. The DSI technique exhibits precisions of 3-10 %
R S ~ ' , which is poonr than liquid nebulization, however, DSI offers improved detection
limits over liquid nebulization of about one order of magnitude.
The type of sample probe used for DSI plays an important role in the efficacy of
the analysis. Both the probe material and geometry are important design characteristics.
Graphite is typically used as the sample probe material due to t s case of machining,
ability to withst~dthe high temperatures in the plasma, resistance to chemical attack by
acids and relatively low cost. Wire loops have also been used by Sing and salin" for
liquid sunple d y s i s . Graphite probes or cups are p o n e to carbide formation with
elements such as Ca, Cr, Si, Ti, V,-W and Al. The anaiysis of carbide fonning elements
may suffer 60m low sensitivity ruid memory effeds.
Chernical modification can be employed in order to enhance analysis of rehctory
and carbide forming elements. Karanassios et ai. examined the use of KCI, KF, NaCl
and NaF as modifiers for the determination of Al, Ca, S r and Zr from solution residud2.
Studies have demonstrated that carbide formation can be overwme and sample transport
greatly enhanced using gaseous halocarbons usually Freon, to form volatile halides of
most elements33.34 .
Kirkbright aiid Li-Xing utilized Freon in order t o enhance
volatilization of r e h c t o r y carbide fonning elements such as U, Zr, Ti, Mo, B and
c??
The use of Freon has been s t ~ d i e d 'and
~ has it has been demonstrated that decomposed
Freon-12 (CCI2F2) is en extremely corrosive reagent that can break down virtually any
material but cubon.
Anotha way of decomposing difti~cult solid samples can be
accomplished by using small amounts of oxygen andor nitrogen in the argon ICP. Liu
and Horlick found that the use of a mixed-gas plasma facilitated the release of nonvolatile constituents from the DSI probe37.
The probe geometry has been studied38.39 and found to be critical to the analytical
performance of the method. Karanassios et al. found that a long undercut graphite
electrode was the best design because of its favorable heating chara~teristics~~.
The analysis of aluminum alloys by DSI-ICP-AES has not been extensively
researched. Liu and Horlick analyzed 0.2 mg of aluminum alloy filings using an Ar=&
(20.h) plasma at high power (1.5 to 2.0 k ~ ) ~ ' Aluminurn
.
was used as an intemal
standard and linear log-log calibration curves were obtained for Si, Cu,Fe, Ti and Mn.
This work was encouraging because the aluminum filings are close to the real aluminum
fonn, however, the focus of this investigation was on the use of a mixed gas plasma It
seems that more information and experimentation is necessary in order to evaluate the
potentid for the analysis of solid aluminum alloy samples using DSI-ICP-AES.
Clearly none of the ifonmeationed direct sdids techniques offer solutions for
anaiysis 'in the mlt' because a prepared sdid sample is required, dthough laser
sompling followed by excitation may o f f a an .evenaial solution.
Aluminum Pins
Alcon has developed a
aist method of sampling £kom the alloy melt which
provides homogeneous alloy samples.
In o r d a to minimize the problem of
inhomogeneity, a small sample is taken during the alloy melt and chilled so that the metal
solidifies very rapidly. These s~mplesare in the fonn of a cylindrial pin weighing
approximately 10-150 mg and ranging in sire from 10-25 mm in length with a diametex
of 0.7-2.2 mm. F i p r e 1-1 shows two different s h e d pins provided by Aican for this
study.
Figure 1-1: A l u n duminum dloy pins.
Objectives
The goai ofthis shidy was to analyze the alloy pins using a mahod that combined
both speed and ptecision. Due to the pins imgular size and shape, these spmples are not
convenient for traditional spark anaiysis. It seemed thit DSI wwld be a g d method to
investigate for the analysis of pins due to its capabilities for fast, direct solids analysis.
Refennces
Handbodr of Aluminum, Second Edition, Aiuminum Company of Canada, LTD.,
Montmû, Canada, 1%1, p. d.
Anaiysis of Aluminurn Alloys A Compilation of Modem Methods, Chemical Publishing
Co.,Inc., New York, USA, O h m , G.H. and Stross, W. Editors, 1953, p. 1.
Ahiminum Alloy Stmctures, Sezond Edition, M m l a n i , F. M., E & FN Spon, London,
WK, 1995, p. 3.
W.
p. 7.
'Aluminum and Aluminum Alloys, ASM International. Ohio, USA, Davis, J.R. Editor,
1993, p. 3.
Annual Book of ASTM Standards: Metals Test Methods and Analpical Procedures,
ci
Section 3, Volume 3.05, ASTM, Philadelphia, USA, 1994, p. 116.
'ibid. p. 96.
'Spectrochemical Analysis, Ingle Jr., J. D. and Crouch, S.R, Prentice-Hall, Inc., New
Jersey, USA, 1988, p. 227.
ibid. p. 25 1.
'O
Ward, A.F.and Marciello, L.F.,Am&!Ckm., 51, 1979, p. 2264.
l1
Barnes, RM., Fernando, L., Jing, L. S. and Mahanti, H.S., Appl. S p c . . 37, 1983, p.
389.
l2
Broekaert, J.A.C. and Leis, F., Analysl, 108, 1983, p. 717.
l3
Analysis of Aluminum Alloys A Compilation of Modern Methods, Chemical
Publishing Co., Inc., New York, U S 4 Osbom,G.H.and Stross, W. Editors, 1953, p. 7.
" Spectrochemical Analysis,
Ingle Jr., J. D. and Crouch, S.R, Prentice-Hall, Inc., New
Jersey, USA, 1988, p. 257.
'Indurtively Coupled Plasmas in Analytical Atomic Spectrometry, Second Edition,
McLeod, C.W.,Routh, M.W.and Tikkanen, M.W.,
VCH Publishers, Inc.,New York,
USA, Montaser, A. and Golightly, D.W. Editon, 1992, p. 74i.
16
Spectmcheemical Analysis, Ingle Jr., J. D. and Crouch, S.R, P.rentice-Hall, Inc.,New
Jersey, USA, 1988, p. 258.
l7
ibid. p. 269.
*
l8 Inductively Coupled Plasmas in
Anaiytical Atomic Spectmmetry, Second Edition,
McLsod, C.W.,Routh, M.W. and Tüdunen, M.W.,VCH Publishers, hc., New York
USA, Montam, A and Golightly, D.W. Editors, 1992, p. 743.
Mz,A., Broekaerî, LAC.,Laqua, K. and Leis, F., &wctrochim. Ac&., 39B,1984, p.
l9
1091.
" Prell, L.J. and Koktyohann, S.R.,Applied WC.,
42, 1988, p. 1221.
'' Ghiglione, M., Eljuri, E. and Cuevas, C., Applied >ec.., 30, 1976, p. 320.
az Karyakin, V. Y.,Kharlamov, I.P. and Pchelkin, A.I., Zm>od h b . , 54, 1988, p. 36.
23
Pchelkin, A.I., Kharlamov, I.P., Gusinskii, M.N.and Shipova, E.V., B.Anal Khim.,
42, 1987, p. 2138.
L'vov, B.V.and Novichikhin, A.V., Atonric Spctrmcopy, 11, 1990, p. 1.
" Principles of Instrumental Analysis, Fourth Edition, Skwg, D. A. and teary,J.J.,
"
Saunders College Publishing, Orlando, USA 1992, p. 378.
"Dick, J.O. and Fraser, AR., C.J. Spectroscopy., 17, 1972, p. 135.
Inductively Coupled Plasmas in Analytical Atomic Spectrometry, Second Edition,
2'
McLROd, C.W., Routh, M.W. and Tikkanen, M.W.,
VCH hiblishers, Inc., New York,
U S 4 Montaser, A. and Golightly, D.W. Editors, 1992, p. 764.
Liu, X.R. and Horlick, G., Spctruchim. Acta, SOB, 1995, p. 537.
Salin, E.D.and Horlick, G., AmL Chem., 51, 1979, p. 2284.
29
Inductively Coupled Plasmas in Analytica Atomic Spectrometry, Second Edition,
McLeod, C.W.,Routh, M.W.and Tikkanen, M.W.,
VCH Publishers, Inc., New York,
USA, Montaser, A and Golightly, D.W.Editors, 1992, p. 732.
" Sin& R.L.A.,and Salin, E.D.,Anal.Chern., 61, 1989, p. 163.
" Karanassios,
V., Abdullah, M. and Horlick, G.,Specb.ochin. Acta, 458, 1990, p 119.
''Ren, J.M.and Salin, E.D.,Spctrucin. Acta., 1994, p. 555.
34
Re4 J.M. and Salin, E.D.,Spectruchm. Acta., 1994, p. 567.
Kirkbright, G.F.and Li-Xing, Z., Anu&t, 107, 1982, p 6 17.
''
36
Mary, 3-F., Hemandez, G., and Salin, E.D., Appl. Spcrra~c.,
49, 1995, p. 1796.
37
Liu, X.R.and Horlick, G., J. And. At. Spctrom., 9, 1994, p. 833.
38
Karanassios, V. and Horlick, O., Spctrochim. Acta, 45B. 1990, p 85.
'' Skinner, C. D.and Salin, E.D., J. Ami. A t Spechanr., 12,1997, p. 725.
An Evaluation of Direct Sample Insertion Induetively Coupled
Plasma AtomicEmission Spectrometry for the Analysis of
Aluminum Alloy Pins
Abstract
Direct sample insertion @SI)
Inductively Coupled Plasma (ICP) Atomic
Emission Spectrometry (AES) was used to analyze aluminum alloy pins. A rapid and
precise (1 % RSD) method of analysis was the objective for these experiments. It was
demonstrated that power levels of 1-1.5 kW were not suficient to vaporize a 2 mg pin
segment within 10 minutes. The solid sample analysis did not provide the necessary
speed and precision, therefore a liquid digestion of the aluminum pin was examined.
Analysis of the digested aluminum sarnples provided results within 30 s and the precision
rang4 ftom 2-14 ./.RSD for the elements studied (Al, Cu,Fe, Mg, Mn and Zn). The use
of Freon-12 as a halogenating agent enhanced the removal of the Al from the cup and
increased the reproducibility to 1-5 % RSD.
Introduction
The aluminum alloy industry is interestecl in tesiing for alloy composition during
the alloy melt. The molten aluminum that is produced fiom the electrdytic proass must
be maintaineci at a temperature of approximately 730°C in a holding fimace, until the
alloying ingrrdients have been added. A sample must be taken fiom the melt and
analyzed as quickly as possible in order to minimize the use of energy and monetary
resources. If changes are necessary in order to meet specifications, these changes to the
composition must be made before the casting and fabricating stages while the aluminurn
is in molten fonn. A f ' analysis time is not the only important cnteria for the aluminum
industry, high precision on the order of 1 % relative standard deviation (RSD) is also
desired.
A spark system is most comrnonly used in the metals industry for the analysis of
alloys. As mentioned in Chapter 1, the production of a sample disk for spark analysis is
time consuming and laborious. Cwling of the sample taken fiom the melt causes
inhomogeneity in aluminum alloys. Obtaining a homogeneous sample is a major step
forward in improving the technique used for analysis. The aluminum pins produced by
Alcan are more representative of the alloy melt than the large machined disks used in
spark analysis. A technique that combines both speed and precision and is capable of
handling solid samples would be desirable to m l y z e the aluminum pins.
The DSI-ICP-AES technique can be used to rapidly analyze both liquid and solid
samples. Analysis times of less than one minute are typical and automation is feasible.
Direct sample insertion offers the advantage of 100 % m p l e transport eficiency
because the whole sample is introduced directly into the ICP, therefore detection limits
are about one order of magnitude lower than liquid nebulization ICP-AES. Low
detection lirnits allow one to analyze for trace constituents that may not be accurately
determined othenrise.
As previously mentioned, DSI can be u s 4 for solid samples
thereby elirninating time consuming sample preparation steps. Furthermore automated
DSI systems have exhibitcd precisions of better than I % when used with liquid samples,
suggesting that the DSI device can be quite reproducible when given a homogeneous
sample. It would seem that a pin or pin segment could be placed inside a DSI cup and be
d y z e d using DSI-ICP-AES.
paformed inside the
Simple and expeditious sample preparation could be
DSI cup if required. For these nasons DSI would seem to be
advantageous for analyting the aiuminum dloy pins.
Objective
A technique would be valuable if it could provide both speed and precision for the
uulysis of the aluminum pins. We want to develop a solid ample technique that will
enable this type o f andysis to be done. In this study DSI wes used with the ICP-AES
system to analyze severai types of duminum dloy pins in order to determine whether the
DSI-ICP-AES arrangement might provide rapid automated analyses on-site to improve
the analytical process used in support of the alloy formation procedure.
Experimental
Instrumentation
Al) DSI experiments w a s dom using an HFP-2500 Plasma Thenn (Plasma
nKnn Inc., Kresson, NJ, USA) inductively coupled plasma with a modified Jarre11 Ash
(division of Fisher Scientific) Atomcornp Model 750 diest r d i n g spectrometer.
Modifications to the spectmmeter included high speed electronics and a galvanically
dnven quartz rehctor plate for -id
background ~orrection'~
'.
Reagenb and Samples
The acid solution was prepered fiom Fisher ScientSc brand trace metal grade
HCI (Fisher Scientific, Nepean, ON, Canada) and MilliQ distilled deionized water
(MiIlipore Corp., Bedford, MA, USA).
The halogenating agent Freon-12 (CCkF2)
(Matheson, Ville St. Laurent, PQ, Cenada) was used to facilitate the removal of the
aluminum fiom the graphite cup. The gas waslOOO ppm Freon-12 in argon and was
introduced into the cup via a charnel drilled through the stem.
The aluminum alloy pins were Akan alloys 3 104, 6 111, 5 182 and 7010 (Alcon,
Jonquiere, PQ, Canada) that had been sampled during the alloy melt.
compositions for these four alloys are s h o w in Table 2- 1.
Table 2-1: Percent composition for A l u n .luminum aIIoy pins.
The alloy
DSI Cup Design
DSI cups used w e n machin4 on a lathe sterting fiom high density graphite
ekctrodes (Bay Carbon Inc., Bay City, Mt,USA). The various foms are presented in
Figure 2-1. The first cup design was used for prehinary experiments 6 t h diffaent
sample sizes and varied ICP power settin-gs. This aip was also used for liquid samples.
The second a i p design was used in order to introduce the haiogenating agent through the
stem of the cup. The graphite foam insert enabled the Freon gas to flow through the aip
and provided a surface for liquid sarnple deposition prior to sample insertion. The third
aip design wntained a cavity in order to accommodate the aluminum pin. The fourth
design had a similar cavity as the former cup drilled into a standard cup. This cup was
then fitted with a boiler cap in order to control the melt down and vaporization of the
duminum pin.
Ci)
( iii )
( ii )
Figure 2-1: DSI cup dmigns:
(i)Narrow tbin-wiiled cup; (ii) Thin-walled bollow cup with graphite foam iasert;
(iii) Thin-walled cup with sample uvity;
Standard cup with sample u v i t y and
boiler cap.
ci)
Direct Sample Insertion
The DSI device @Sm) used was a modified design by Skinner and ~ d i n ? The
origind design was done by Sing and salin4. Figure 2-2 shows a shematic diagram of
the DSID used. An insertion involves placing the DSI cup ont0 the DSI shaft and raising
the shatt into the load coi1 of the ICP (not ignited). The sample is then deposited inside
the aip and the shaft is retracted to its lowest position. The plasma is then ignited and the
cup is inserted into the plasma using the cornputer wntrolled stepper motor.
The gas
fittings at the bonom of the DSI shaR allow gas to be introduced through the hollow sh&
and into the stem o f the hollow DSI cups.
H O ~ ~ O DSI
W shaft
bn
Mounting plate
Gas fittings on shaft
Guide shaft
Stepper motor
Figure 2-2':
D i m t sampk insertion device schematic diigram.
Power
The effect of plasma power was examined by using a standard setting of 1.0 k W
and a high power setting of 1.5 kW. In each case a full pin was p l d inside a thinwalled narpw cup and then a sample insertion into the ICP was performed.
Liqvid Sampie Analysis
An Al pin (7010) weighing 30 mg was digested in 15 ml of concentrateci HCI and
then diluted to about 60 ml with Milli-Q distilled deionized water. A 20 pL aliquot was
pipetted into a thin-walled cup and dned inductively in the Joad coi! of the ICP for 30 s at
50 W prior to insertion into the ICP (Figure 2-3).
Five consecutive insertions were
pefiormed using the &ove procedure.
A 10 pL aliquot of the Al (7010) solution was deposited into a hoilow cup and
dried inductively in the load coi1 of the ICP for 3 0 s at 50 W. The plasma was ignited
and Fnon-12 gas was introduced, via the gas fittings at the bottom of the DSI shafk,
through the hollow stem into the cup during the insertion in the ICP. Four consecutive
runs were perfonned in order to examine reproducibility.
(iii)
Figure 2-3: Liqrid umple analyris:
(i)Sampk deposition; (ii) Sample dying; (iii)Sample vaporiution.
Results and Discussion
Soüd Sample Analysis
Expeciments with a full pin weighing approximately 10 mg were done using a
thin-walled aip. Aiulysis of an entire pin required the use of an opticai density filter
(O.D. 2: to reduce the signal intensities by two orders of magnitude) in order to prevent
signal saturation. The aluminum did not burn off using a power of 1.0 kW within ten
minutes. Therefon smalter sample sizes were used. Five consecutive insertions of
smalla alurninum pin pieces (about 2.5 mg) were pedormed ig a thin-walled cup in order
to examine reproducibility. This cup had previously been used for a k l l aluminum pin of
the same sample type (5 l82), therefore residual alumi num remained. After an insertion
time of 40 s at 1.0 kW, aluminum still remained in the cup and therefore the precision for
the five runs was low due t o memory effects.
The precision was determined by
integrating the area under the sample trace and then evaluating the ./.RSD of the average
area value fiom the five mns.
The precision was quite poor for Al, Fe, Cu and Mn
ranghg from 32-38 % RSD. The elements Mg and Zn displayed better precision ranging
nom 12-15 % RSD. Figures 2-4 and 2-5 show the five traces for Al and Zn respectively.
In the next set of experiments two consecutive insertions o f approximately 2-2.5
mg were done for each sample type using a fiesh DSI cup (for each sample type) in order
to examine reproducibility. Traces for each element studied in alloy 5 182 are s h o w in
Figure 26. The figures show fairly reproducible peak shapes with higher intensities for
the second mn. The higher intensity was due to memory effects fkom the first run. The
precision for Al, Cu, Fe and Mn in alloy 5182 has improved, most notable was the very
high precision for M n at 1 .O % RSD. The results for the four alloys are listed in Table 22. These results still did not exhibit the 1 % RSD that was desired.
Figure 24: Five duminum traces Cor 2.0 mg of aUoy 5182 using a thin-w alled
at 1.0 k W power.
F i e2
Five zinc traces Cor 2.0 mg OC dloy 5182 using a thin-walled cup at 1.0
kW power.
Aluminum
lron
Magnesium
Manganese
Zinc
Figure 2 4 : TWOinsertions of 2.0 mg segments o f alloy 5x82 using a thin-walled cup
at 1.0 k W power.
Table 2-2: The precisïoi ir 3C RSD of the average p
for the differcnt d o y s rtiidied.
Element
7010
5 182
6111
3 104
Al
31
28
6
23
Cu
Fe
19
13
17
2
40
28
10.
33
12
1
5
1I
10
Mn
2
14
Zn
3
18
8
12
Mg
k a m h m two insertions
9
hlPICyp D e n
The DSI cup with a cavity ddled into it was used to contain the pin during the
physical process of melting. Preliminary experiments involved placing the pin inside the
cup where it resteù dong the aip walls. Therefore the physical change in the pin during
the melting could not be controlled. By placing the pin in a well of the same diameter,
the pin would melt fiom top to bottom and bum off more uniformly than before.
Although the melting process was now contained, the same problem of slow burning off
of the Al occurred. Long insertion times (greater than 10 min) were required in order to
remove the Al fiom the cup.
In earlier experiments with a fùll pin in a thin cup, there appeared to be small bits
of aluminum coming off, creating more than one peak in the tnices. A boiler cap was
placed onto a standard electrode in order to allow a slower vaporization process. A pin
was placed into a cavity drilled into the bottom of this cup and d e r fitting the cup with a
boiler cap, an insertion was done at standard power (1 .O kW). Due to the large size of the
electrode with very thick walls, the plasma energy was diminished and heat transfer to
the cup was inefficient. There was still Al in the cup after 5 minutes, therefore this
method was not investigated fùrther.
Power
A higher power setting of 1.5 kW increased the signal intensity for al1 of the
elements except for Mg. The traces for the higher power setting also returned closer to
the badine than those at I o w a power (1 .O kW), thereby indicating that a more complete
burn was achieved. Figure 2-7 shows the M n trace h m sample 5182 at low and high
power. The high power sdting was still not sufihient
cup within 10 minutes.
to remove al1 of the Al fiom the
Figure 2-7: Mangantse traces at iow aad high power.
Liquid Sampie Analysis
As a question of curiosity, a chernical digestion was examined.
Aqueous
hydrochloric acid was used to digest a fiill pin (7010). The use of either 10 or 20 pL
aliquots allowed the deposition of small samples that could be bumed out of the cup
within 1-2 minutes. In order to detemine the precision of this process, 5 consecutive
mns were completed. Figure 2-8 shows transient signals for each element studicd. For
most elements (except Cu and Zn) double peaking occurred. This was due to analyte
vaporization oaxcmng first from the outer aip walls followed by vaporkation fiom the
Aiuminum
Zinc
Figure 2-8: Transient signais from liquid sample analysis of alloy 7010.
imer cup walls and base. As the liquid sample wu, deposited into the thin-walled cap,
thc liquid penneated through to the outer wall of the graphite cup. Upon insertion of the
cup into the ICP, this portion of the sample vaporized fint, followed by the remainder on
the inside of the aip. The absence of double peaking for Zn can be attributed to its low
boiling point of 907OC. The results w a e much improved compared to the solid sample
uiaiyses with precision vuying Grom 2-14 % RSD.
In order to enhance the liquid sample analysis, Freon gas was introduced into the
stem of a hollow stem aip during the sample insertion. There was a significant
improvement in the reproducibility of the Al signal with the use of Freon-12 attributable
to the mmplete vaporization of Al fiom the cup. This complete vaporization of Al is
evidenced by the transient signal retuming back down to the b a d i n e within the analysis
time. A cornparison of the AI signal with and without the use of Freon is shown in
Figure 2-9. There was also improvement of the Cu and Zn signais, while the Fe, Mg, and
!
Mn signals did not improve. Table 2-3 contains the 96 RSD for the analytes âom both
sets of experiments using liquid samples. Most importantly in these experiments is the
removai of the aluminum which could not be achieved easily without the use of Freon-12
as well as the very high precision of Zn.
-no
Freon
.----Freon
Figure 2-9: Effkct of F m n on rluminum h n s k a t signala
Tabk 2-3: Coipanion o f 3C RSD of üquid Ilumirum samplcr with and without the
use of Frcon-12.
Etement
% RSD (no Freon)
% RSD (with Freon)
AI
14
5
Cu
Fe
3
2
2
4
Mg
4
Mn
Zn
2
4
4
4
1
Conclusions and Future Work
Power ievels of 1-1.5 kW will not vaporize a full pin (approximately 10 mg), nor
a 2 mg segment o v a a 10 min period. Freon-12 would d a n c e the vaporustion process,
however long d y s i s times would still be required and precision would still not be
adequate. Therefore the anaiysis of a pin or pin segment using DST-ICP-AES did not
prove to be satisfactory.
The analysis of liquid samples using DSI-ICP-AES provided rapid and precise
results within 30 s wing chernical modification with Freon-12 to enhance volatilization
of Al. The reproducibility for the elements studied varied from 1-5 % RSD, however
most elements exhibited either 4 or 5 % RSD. One of the objectives for this study was to
develop a technique providing very high precisions of 1 W S D .
Therefore fihue
aperiments will examine the use of liquid nebulization for sample introduction into the
ICP. This method has proven to be very precise with better than 1 % RSD. Sarnple
digestions using HCl will be exarnined in order to provide a liquid sample for analysis.
Acknowledgements
1would like to thank Alcan and specitically Dr. Tom Belliveau for supplying the
aluminum pin samples. 1would also like to thank Dr. Cameron Skinner for his guidance
in experimental design and DSI cup machining.
' Lég&e, O. and Burgener, P.,ICP It$ N i e t t e r . , 1988,13, p. 521.
'Skinner, C.D. and Salin, E.D.,J. A d Ar Spectrom., 1997,12, p. 725.
Ibid.
'Sing,, R.L.A. and Salin, E.D.,AmLChem., 1989,6l, p. 163.
'Skimer, C.D.and Salin, E.D., J. A&L
AL Spechom., 1997.12, p. 725.
Chapter 3
Determination of Al, Cr, Cu, Fe,Mg, Mn,Si and Zn in
Aluminum Pins Using tbe Real-Time Alloy Analysis Technique
Abstract
A method for the andysis by inductively wupled plasma atomic emission
spectrometry of homogeneous aluminum alloy pins actively digesting in solution is
describeci. This work demonstrates that the sample does not need to be completely
digested before obtaining analytical results, thereby reducing the sample preparation and
analysis time fiom hours to minutes. A digestion temperature of <50°C allowed results to
be obtained after 2 minutes of digestion initiation. Alloy composition was determined
using ratiometnc measurements relative to the principle component, Al, in the sample
matrix. Relative detection limits were of the order of 1-100 ppb.
Agreement with
referencs values for the aluminum alloy pins studied ranged from 0-2 % for Al and Mn,
1-14 % (when above the detection limit) for Cr, Fe, Mg and Zn and 7-76 % for Cu and Si,
when using aqueous extemd standards. The precision ranged fiom 0-3 % RSD for d l
elements studied.
Introduction
The carefùl d y s i s of aluminum alloys in industry is necessuy to ensure proper
dloy composition, because the physical properties of an alloy are highly dependent on
the dloy composition. Physicd properties such u corrosion tesistance, strength and
ductility can be enhanced o r reduced by different alloying elements and composition,
thereby a é c t i n g an alloy's industrial applications and uses. A rapid and precise method
of analysis that can be implemented 'on-site' at the foundry f l w r is desirable in order t o
determine dloy composition during the alloy melt when alterations in composition can be
made in a cost-effective manner.
Techniques aimmonly u d for routine analyses of aluminum and other metal
alloys include spark emission and x-ray fluorescence (XRF). Both of these techniques
can be used to analyze solid samples, which are usually in the form of a solid disk. The
fmt step in the production of this disk is withdrawal of a sample fiom the alloy melt.
a
This sample is then poured into a bar and rapidly cooled, then a sample disk is machined.
Both techniques regularty utilize matrix matched standards in order to achieve accurate
results. This method of calibration requires the availability of appropriate standards,
which can be nontrivial and expensive. The use of spark emission and XRF techniques to
monitor alloy composition during the melt and mixing stage is not ideal at the foundry
floor due to the time wnsurning sample preparation.
Inductive1y coupled plasma atomic emission spectrometry (ICP-AES) is a
technique that is capable of providing rapid, simultaneous, multi-element analysis of
liquid samples, often by simple extemal standards calibration. Solid samples such as
aluminum alloys can be analyzed using this technique by first perfonning a digestion.
Ward and ~ a r c i e l l o ' described a method for the determination of 12 elements in
aluminum alloys using ICP-AESaiter HCI dissolution. A concentration ratio method of
analysis was used in order to elirninate the requirement for accurate sample weighing and
volume measurements. Barnes et
al.' examined the use of different reagents for acid
digestion (including HCl, H
N
a and HF) of aluminum alioys in order to develop a
0
methoci of uulysis utilking ICP-AES. The probla with both of these methods is the
lengthy m p l e preparation time rsquired for wmplete sample dissolution.
The development of the new homogesiawis aluminum alloy sampling technique
would be complemented by a rapid mahod of adysis. We have developed the ml-time
alloy anaiysis technique W T ) , which d o m the pins to be analyzed in a highly
reproducible hshion. The RAAT eliminates the need for sample weighing and complete
sample digestion by using ratiometric measurements.
Experimental
Instrumentation
The RAAT mcasurements w a e made using an HFP-2500 Plasma Therm (Plasma
Tham Inc., Kresson, NJ, USA) inductively cwpled plasma with a modified Jarrell Ash
(division of Fisher Scientific) Atomcomp Model 750 duect reading spectrometer.
Modifications to the spectrometer include high sped electronics and a gafvanically
driven qusrtz r e m o r plate for rapid background correction3.
The steady state measunments were pdonned using a Thermo Jarrell Ash (TJA)
(Thenno Jarrell Ash, Franklin, MA, USA) Mode145 0.75 rneter scanning spectrometer
system with a TJA high salt nebulizer. Table 3-1 üsts the operating conditions for both
instruments and the elemental lines studied.
Reagents and Samples
a
Multi-element liquid standards were prepared fmm 1ûûû ppm AA single element
reference standards (Fisher Scientific, Nepean, ON, Canada). The acid solution was
prepared from trace metal grade HCI (Fisher Scientific, Nepean, ON, Canada) and Mill iQ distilled deionized water (Millipore Corp., Bedford, MA, USA).
The aluminum pins were Aican alloys 3 104, 61 11,
5 182 and 7010 (Alcan,
Jonquiere, PQ, Canada). The smaller sized pins shown in Figure 1-1 were used in this
study. These pins weighed approximately 10 mg and were 10 mm in length with a
diarneter of 0.7 mm.
Parrmeter
TJA Mortcl 25
Plasma power
Observation height
Plasma gas flow
Auxiliuy gas flow
Sample uptake
Flush time
Integration tirne
Màsma Tk&Jmll
Ash
Atomeonp M d 7 5 0
Plasma power
Observation height
Plasma gas flow
Auxiliary gas flow
Sample uptake
Exposure time
Number of on-line and off-line
exposures per trace
Galvanometer settle time
Lànasdudi.d(W
Setting
1150 W
15 m m
(ATOLC)
15 l min-'
1 l min-'
1 mi min"
45 s
2 s* 10 repeats / elernent
1000 W
15 mm (ATOLC)
13 i min"
0.8 1 min"
1 ml min-'
0.1-1.5 s
300
Red-Time AMoy Analysis Technique (RAAT) Procedure
A digestion vesse1 containing a 5% HCI solution was placed inside an ultt8sonic
bath. The uftrasonic bath was used to pmvide mixing and elevated tanpentures during
subsequent sample digestion and analysis. A small volume of the acid solution w u used,
t y p i d l y between 25-50 ml. The aiuminum dloy pin was placed into the acid solution
and sample digestion was initiateû. The sunple solution was aspuateci (while digestion
was underway) into the ICP using a fixed cross flow nebulizer. The elemental lines were
monitored using a direct reading spectrorneter throughout the sample digestion, b e g i ~ i n g
at 2 minutes after initiating the digestion. Transient signal acquisition was done using
SF20 software by G. W
e and transient signal proassing was perfonned using
Grams/32 software (Galactic Industries, Salem, NH, USA).
Method of Ratios
The concentration ratio method describecl elsewhere4 and applied by Ward and
~ a r c i e l l o can
' be represented by Equation 1:
CCi+Cc,
+cm
= 100
where: Ci =concentration of the ith analyzed element,
C, =concentration of thejth unanalyzed element,
Cm= concentration of the matrix (principal wmponent) element, and
ALI concentrations are in percentages.
Equation 1 can be rearranged to give:
The concentration ratios (Ci /Cm) can be detennined fiom the individual calibration
cuwes. Equation 2 can be used to calculate the concentration of the mat& element
(C,), once the concentration ratios and unanalyzed concentrations (Ci)have been
determined. If the sum of C, is less than 0.5%. it can be represented by the probable
total as a constant4. The absolute concentration of the ith analyzed element cm now be
determined fiom Equation 3:
ci = t e c m
w h : k = (C,
/Cm)*
During the analpis, the signai intensities inaease rs the alloy digestion proceeds.
The elenentai signal intensitia arc d o e d ta the aiuminum signal intensity in o r d s to
obtain a steady state value throughout the digestion time. nie ratio obtained can be
applied to the concentration ratio method in o r d a to acquire quantitative results. The
average value of this ratio over time was alculated in order to improve precision. The
precision was detennined by alailating the percent relative standard deviation (% RSD)
of the ratio value. Analytical cdibration curves were constructecl by analyzing multielement liquid standards. In order to utilize the concentration ratio method, the
concentraîion of the major matrix element (Al) was kept constant while the
concentrations of the analytes were varied. The standards used for calibration are listed
in Table 3-2.
Table 3-2: Cdibrrtion standards.
Concentration
a
Elements
ms/i
% w/W.
Cr
O, 0.01,0.05,0.1, 1
0, 0.003,0.017, 0.03,0.33
Fe, Mn,Si
0,0.1, 0.5, 1, 10
O, 0.03,0.17, 0.33, 3.3
Cu,Mg,Z n
Al
0, 0.5, 2.5, 5, 50
0, O. 17, 0.83, 1.67, 16.7
300
100
Where w/w is the weight of analyte / weight of aluminum.
Results and Discussion
The ability to dissolve aluminum in an acid solution depends more on the nature
of the anions than the hydrogen ion concentratiod. Both hydrofluoric and hydrochloric
acids rapidly dissolve duminum. Oxygen-containing acids such as sulfuric and nitric are
not as effixtive in dissolving ahiminum and have very slow dissolution rate?. A
comparative study was done to confinn that HCI was an appropriate solvent for an
aluminum pin as wmjweâ to H
N
a
. Hydrochloric acid digested the pin at m m
temperature within one hour while HNG at room temperatun was not aôk to digest a
pin within 24 hours. Table 3-3 lists the solubility properties for aluminum and some of
the principle alloying elements. Aluminum and the majority of the elements listed are
sduble in HCl, whereas Cu is soluble in m O 3 but only slightly soluble in HCI. Silicon
is soluble in a combination of HF and H N 0 3 acids. Silicon containing compounds such
as g l a s and quartz are wmmon in ICP instruments. Highly corrosive HF could damage
the torch, nebulizer and spray chamber components in our system. For this reason, we
wanted to avoid using HF to maintain a relatively simple and safe analysis procedure. A
senes of timed digestions w e n done using alloy 5 182 with acid solutions containing both
HCI and H m to determine if the addition of nitric acid would aid in the digestion of Cu
and Si. The four acid solutions used had the following compositions: i) 5% HCI, ii) 4%
HCl + 1% H m , iii) 5% HCI
+
1% H N 0 3 and iv) 4% HCI
+ 2%
HN03.
The
concentration of Al extractecl was highest in the 5% HCI solution (333 ppm) and lowest
U
Ia solution (101 ppm). The presence of
in the 4% HCI + 2% H
combined with a
decreased level of HCI, hindered the digestion of the aluminum alloy, thereby decreasing
the concentrations of alloying elements in solution. For this reason the concentrations of
Cu and Si (which are soluble in HNO,) did not increase as would have been expected due
to the addition of H m . Therefore a 5% HCI solution was chosen as the appropriate
solvent for digesting the aluminum alloy pins.
a
Tabk 3-3: Sdublity prapertitr4olduminum and variour iIloying demeots.
Element
Solubility
At
s alkali, HCI, HzS04; i conc HNOj
Cr
Fe
s dilute H2SO4, HCI; iH
N
a
,qua regia
sH
N
a
,hot HzS04; v SI s HCI, =OH
s acid; i alkali, alcohol, ether
Mg
Mn
s mineral acid, conc HF,
s dilute acid
Si
sHF+Hm;iHF
Ti
s dilute acid
Zn
s acid, alkali, acetic acid
Cu
salts; i alkali
Where soluble, i=insoluble, v SI m e r y slightly soluble and wnc-concentrated
Preliminary Digestion Study
Figure 3-1 shows typical traces for Al and Zn signals recorded as the aluminum
alloy pin was digesting at rwm temperature. As can be seen this was a dynamic process
whereby the analytes in the sample solution continued to increase in concentration with
time until a plateau was reached. From this figure it can also be seen that the profiles
mimicked each other. As expected, this indicates that as more Al was extracted into the
solution, more Zn was also extracted into the solution and likewise for the other alloying
elements. By examining the ratio of these two profiles (Zn/Al), Figure 3-2,it is evident
that this ratio was fairly constant over the analysis time. High precision (2% RSD) was
obtained by taking the average value of this ratio (Figure 3-2). The value was calculated
using 350 on line and off line measurements with an integration time of approximately 3
S.
It becornes apparent that the aluminum alloy pin does not need to be hlly digested,
because the ratio rernains constant throughout the digestion. The sample does need to be
digested to a point where the analytes in the solution become quantifiable, i.e., well above
the detection limit.
2
7
12
17
22
Time (min)
Figure 81: Al and Zn aignais monitored througbout the digestioii of an rluminum
pin at rooa temperature.
2
7
12
17
22
Time (min)
Figure 3-2: Ratio of signal intenaiticr (ZnlAl) n o n i t o r d throughout the digestion of
an aiuminum pin at room temperatun.
For this digestion paformed .at m m temperature, Zn was partiailady well
behaved as compared with the other elements analyzed. Manganese for example,
exhibitcd a poor precision, 24% RSD. If an integration time of 30 s is used, the RSD ia
13%. Figure 3-3 shows that the ratio of MdAl signal intensities fluctuates greatly early
in digestion and then stabilizes after approximately 12 minutes. Thaefore a room
temperature digestion was not sufncient in order to obtain precise results within a few
minutes.
2
7
12
17
22
Time (min)
F
r 3 : Ratio of signai iitensitia ( M d A I ) monitored thmughout the digestion
of an duminum pin at m m temperature.
nie digestion process for the different dloy types wu examineci by digesting a
pin fiom each sample type. The aluminurn pin was pertidly digested for 6 min at m m
tvperature (2z0C) in 5% HCI without stirting. The =pie solution was diluted to 50 ml
d e r ranoval of the remaining undigested duminum pin. The average sample mass on
insertion was 10 mg and varied by 7 % for the different olloys. Table 3-4 lists the
aluminum concentrations for the four alloy solutions determineci using extemal standards
calibration. The digestions of dioys 7010 and 5 182 were observed to be more rapid and
vigorous than the digestions of alloys 3104 and 611 1. It was found that the aluminum
concentration
WPS
much Imger (1-2 orders of magnitude Iarger) for the 7010 and 5182
dloy solutions. The difference in sample mass did not appear to be significant, in fm the
5182 pin weighed the least (9.4 mg) and the 611 1 pin weighed the most (1 1.2 mg). Even
with the largest sample mas, the 6111 solution contained the lowest concentration of
extracteci Al. In further studies, similar observations repeatedly ocairred irrespective of
the sample m a s . Upon examination of the alloy composition for these four alloys, it
appears that the digestion proceeds faster for the mon "impure" aluminum alloys.
Table 3-4: Aluminum concentration determined for 6 minute digestion at 22OC.
AJloy
% Al (Alcan)
6111
97.3 19
0.39
3 104
97.055
0.41
5182
94.129
4.16
7010
89.082
39.86
Al Conc. (ppm)
Real-Time Analysis
The digestions perfiorrned at m m temperature were not fmt enough to obtoin
rapid resutts for al1 elements as shown in Figure 3-3. T h d o r e a higher digestion
temperature was tested Ui order to imease the rate of digestion.
The RAAT was
pafonned while digesting ailoy 7010 at elevated temperatures of 33OC and 52°C using an
ultrasonic bath for both rnixing and heating of the acid solution. The alloys were digested
for 2 min before the elemental lines were monitored. Figure 3-4 shows the signals
(ratioed to Al) for the low temperature digestion. The traces exhibit non-unifonn ratios
before 8-9 min of digestion, indicating a heterogeneous digestion early in time. The
ratios of Cu and Zn were relatively stable as compared with Fe, Mg and Mn, which
fluctuateci significantly early in the digestion. The use of higher temperature for the acid
solution increased the rate of digestion and enabled a homogeneous digestion to be
achieved d e r 2 min of initiating sample digestion (Figure 3-5). The results of the low
and high temperature digestion are compared in Table 3-5. The ratios presented in Table
3-5 are average values obtained over the tirne interval of 5-9 min. With high temperature
digestion, the precision ranged fiom 1 4 % RSD for al1 elements except Mn. Manganese
is the lowest in percent composition (0.083 %w/w) and was monitored using a less
sensitive atomic line (393.3 nm).
These results indicate that precise results can be
obtained within 2 minutes of initiating the digestion when a temperature of >50°C is used
for sample digestion. This translates into a total anatysis tirne of the order of minutes
using this technique compared to hours when complete sample digestion is required.
7
112
Time (min)
Figure 3-4: Transient sigoah (ratiocd to Al) for Cu, Fe, Mg,
7010 digestcd at a temperature of 3J°Ce
7
M n and Zn in alloy
12
Time (min)
Figure CS: Transient signih (mtioed to Al) Tor Cu, Fe, Mg, M
7010 digtsted at 8 tempemture o f 52'Ce
n and Zn in aUoy
Table 3-5: Eltamat / dumiaum ratios and pmision for digestions at U0cand
sz'c.
Average Ratio
% RSD
Element
33OC
52°C
33OC
52°C
Cu
0.64
0.65
5.2
1.1
Fe
1.16
0.9 1
20
2.2
Mg
0.59
0.22
57
1.9
Mn
0.46
0.47
47
10
Si
11.3
3 .O1
70
4.3
Zn
3.34
3.38
4.5
1.1
Figures of Merit
The relative iimits of detection ranged fiom 1-100 ppb (Table 36).
The
concentrations detennined in steaôy state solutions (after removal of undigested pin) for
an 8 min digest at 60°C are also summarized in Table 3-6. It can be seen that for Cr in
alloy 3 104 and 7010, the concentration is below the detection limit in this instrument.
Regardless of how long the alloy is allowed to digest, the Cr concentration remains below
the detection limit, therefore accurate results will not be obtained. The concentration of
Zn in alloy 3104 is right at the detection limit; therefore quantitation will also be
problematic.
Table 3-6: Limit of Dtttction (LOD)for ICP-AES andysis and concentmtions
found in mlutioas for dloyr digestcd at WC.
Concentration @pm)
Etement
LODa
3 104
5182
6111
7010
w s i n g solutions firom Table 3-2.
The concentration ratio method was used to determine the composition of the four
alloys using aqueous external standards. The percent composition detemined and the
percent difference fiom the tme value for the alloys studied are listed in Table 3-7. The
determinations of Al and Mn were quite accurate (within 0-2 % difference fiom the true
value) for al1 alloys studied. High accuracy was also obtained for Mg (within 0.5-3 %) in
three out of four dloys. The detennination o f Zn was within 2-5 % accuracy for the
alloys with the highest Zn composition.
For alloys 3104 and 6111, the absolute
concentration of Zn in solution is either at or near the detection limit as shown in Table 36.
Relatively poor accuracy for Si was not surprising due to the difficulty in
solvating/digesting the element in HCI as well as the diff~cultyin determining Si using
ICP-AES.The determinations of Cr in alloy 3 104 and 7010 were quite F r . As s h o w
in Table 3-6 the absolute concentration of Cr in solution is below the detection limit when
digesting these alloys. The acairacy for Cu seemed to improve with decnasing percent
composition of Cu in the alloy. As disaisseci previously, Cu is soluble in HN03 therefore
the HCI solution does not seem sumcient to quantitatively digest the Cu.
a
Tibk 3-7: Dcterminitioa of Al. Cr,Cu, Fe,Mg, Mn, Si Z n ia various duminum
dloy piiu digestnt i t a temperatan of 60°C.
Alloy3104
Elclllcllt
% whiv
%wfw
M o y 5 182
%end
(Al-)
%w/w
O
/.
cmr
(Al-)
Al
97.055
96-94
4.1
94.129
94.14
0.0
Cr
Cu
Fe
Mg
0.002
0.0004
-81.1
0.0237
0.021
-1 1.7
O. 175
0.113
-35.5
0.084
0.078
-7.2
0.42 1
0.453
7.6
0.284
0.278
-2.1
1.20I
1.34
11.3
4.765
4.82
1.1
Mn
0.93 1
0.920
-1.1
0.368
0.360
-2.2
Si
0.2084
0.227
9.2
0.0887
0.041
-53.9
Zn
ûîher
0.002
0.007
240.9
O. 137
O. 144
S. 1
0.0061
0.0061
N/A
O. 1205
0.1205
N/A
%wfw
Alloy 61 11
96w/w
Elemait
a
96 whn
AUoy 70 10
% enor
% w/w
Whcre ./r error is the dinerem f h m the tme value expressed as percent.
% w/w
% error
High precision is obtiined by detamining the average value for the intensity
ratios over the time intaval monitored (typically 2-3 minutes). The average value was
calculated fiom 30 mwurements with an integration time of approximately 4.5
S.
For
most elements the prrcision of this average raîio is between 1-5 % RSD unless the
percent composition for that element is low. The.detemiinations for 3 digestions of alloy
5182 thaî were pediormeci on the same day reved that the reproducibility of these
determinations ranged nom 0-3 % RSD for al1 etcments studied (Table 3-8). Examining
the results thae appean to be a relationship between the % composition and the % RSD.
The general trend is an increase in the % RSD with a decrease in % composition. The
major elements, Al and Mg, exhibit very high precision of < O S % RSD. Two minor
elements, M n and Fe, exhibit slightly lower precision than the major elements, although
the RSD i s still l e s thon 1%. The remaining minor and trace elements show precisions
of 1-3 % RSD. This indicates that the digestion process is very reproducibte. Even
though high uxuracy for al! elements was not achieved, highly reproducible results were
obtained.
Table 3-8: Average percent composition determined for alloy 5 182.
Element
% w/wa
ab
% RSD
Al
94.16
0.08
O. 1
Mg
4.83
0.025
0.5
Mn
Fe
Zn
0.363
0.003
0.7
0.268
0.002
0.9
O. 156
0.004
2.5
Cu
0.076
0.002
3 .O
Si
0.060
0.001
2.2
Cr
0.02 1
0.0002
1.1
" Where ./.w/w is the average value determined (n=3).
Where o is the calculated sample standard deviation.
Conclusions and Future Work
The duminum industry would benefit fiom a technique for alloy melt analysis in
ordex to produce alloys of precise specifications. Idedly this technique would be an online anaiysis of the alloy melt itself, not requiring the removd and preparation of a
representative sample. Aithough the RAAT does require a sample to be prepared Rom
the alloy melt,it has k e n shown to be much more rapid than the methods of spark optical
emission and conventional dissolution methods. Use of a ratio technique enables precise
results to be obtained rapidly. Complete sample digestion is not &cesssry because the
digestion is homogeneous afker 2 min. Accurate results were not obtained for al1 of the
elements studied, however, the precision is quite high for replicate analyses (0-3 % RSD).
Calibration with matrix matched standards of similar composition could be done in order
to improve the accuracy.
Acknowledgements
The authors wish
to
thank the Aluminum Company of Canada (Aican),
particularly Dr. Tom Belliveau, for supplying the dloy sarnples and the Natural Sciences
and Engineering Research Council of Canada (NSERC)for hinding this research.
References
' Wuci, A.F. and Marciello, L.F.,And. Chem., 1979, 5 1,2264.
1
Barnes, RM., Fernando, L., Jing, L.S.and Mihanti, H.S., Appl. Spec., 1983,37,389.
1
3
Lég&re,0.and Burgener, P., ICP Inf: Newdetter., 1988, 13,521.
Methods for Emission Spectmchernical Analysis, ASTM, Easton, MD, 7th edn., 1982,
Recomrnended Practice E-158, pp. 102- 106.
5
A Jemy, The Anodic Oxidation of Aluminum and its Alloys, Charles Griffin and
Company Limited, London, 2nd edn., 1950, pp. 50-52.
6
CRC Handbook of Chemistry and Physics, eds. RE. Weast, CRC Press, Inc.,
Cleveland, 55th edn., 1974, pp. B63-B 156.
Chapter 4
Aluminum Alloy Analysis by Spark Ablation ICP-AES
Abstract
The use of a spark ablation device to produce dispersions of aluminum alloys in
an aqueous medium for analysis by inductively coupled plasma atomic emission
spectrometry is describeci. It was found that a sampling time of 5 min produced colloidal
solutions o r sols with an average dispersed mass of 4.7 mg and concentrations of the
o r d a of 90 ppm.
The apparent concentration of Al found in sols produced in
distilled/deionized waier was 36-57 % of the expected concentration based on the total
mass of alloy dispersed. The addition of HCl to adjust the pH to about 2.6 increased the
apparent amount of Al found to aimost 90 ./.of the expected value. Relative detection
limits were of the order of 1-150 ppb. Agreement with reference values for Al, Cu, Fe,
Mg and Mn ranged fkom 0-43 %. Results with l e s than 1 % e m r were achieved for Ai
in three of the four dloys. The determinations of Mn in alloy 3 104 and Fe in alloy 7010
exhibited high accuracies of less than 1 % -or.
The precision ranged from 0-2 % RSD
for Al, Cu,Fe, M g and Mn.
The in situ spark sampling of an aluminum pin using a DSI probe was achieved.
Reduced ablation times of 3
0 s were found adequate to ablate sunicient sample for
analyte detedon. Near Gaussian transient peak shapes were observed for Al, Cu, Mg,
0
Mn, Si and Zn. Transient signais of Cr and Fe suffered fiom significant tailing.
Introduction
Metals analysis by spark optical emission spcctrometry typically requires large
disk-shaped samples. A promising technique has been studied that is capable of sampling
much smdler cylindrid shaped samples. This technique is basai on ablation of an alloy
in a spark discharge peflormed in a liquid medium to pmduce dispersions or colloidal
solutions.
Ghiglione et al' described a spark based method to prepare metallic dispersions
for atornic absorption (AA) analysis. It was found that these dispersions were stable
wlloidal suspensions. Cu, Mg and M n in an aluminum alloy were determineci with a
relative aîairacy o f 5 %. These elements were at concentrations of about 1 96; no trace
components were detennined. PcheUcin et ap also found that the dispersion of metallic
sarnples in a liquid medium produced colloidal solutions or sols. Electron-microscopie
studies showed that the sols contained particles no greater than 1 pm. Karyakin et alf
0
used electric sparking to produce stabk solutions of nickel and steel alloys. The
concentration of analytes in these solutions was high enough to enable trace element
detemination by electr~thermalatomization atomic absorption spectrometry (ETA-
AAS). L'vov and ~ovichikhin' analyzed disperseci samples of alloys and pure metals
using graphite fumace atomic absorption spectrometry (GFAAS). Cu, Fe, Mg, Mn, Pb
and Zn in high-purity aluminum were d e t e n ~ n e dwith an RSD of 1-16 %. ~ e n d i c h o '
used spPrk dispersion in a liquid medium to prepare samples of electrolytic iron. Trace
impunties of Mn, Cr and Cu in the iron were determined using GFAAS. Bendicho found
that calibration with aqueous standards was feasible.
The majonty of this previous work has focussed on methods of AAS. Atomic
absorption methods do not have the capability o f pefiorming simultaneous multi-element
deteminations like methods such as atomic emission spectrometry (AES). The first aim
of this study was to evaluate the use of a spark ablation device for sampling aluminum
alloys in an aqueous medium prior to analysis by the method of inductively coupled
a
plasma (ICP) AES.
Direct sample insertion @SI) c i be used to analyze solid or liquid samples in
small quantities. If the ablation of the alloy wuld be done inside a DSI probe then the
possible sources of e m r due to sample dilution could be eliminated.
Due to the
improved detedon limits rssociated with DSI (about one order of magnitude wmpared
with conventional nebulization), a reduction in spadc sampling tirne would be expecteà in
ords to produce a sarnple with analytes that are quantifiable. The second aim of this
study was to investigate the use of this device to perform in situ sampling of alloys inside
a DSI probe prior to ICP-AES analysis.
Experimeatal
Instrumentation
S p d Abloti011Dcvice
The spark ablation device (SAD) used was similar to that described by L'vov and
~ovichikhin'. Figure 4-1 shows the SAD that was used to disperse alloy simples into a
liquid medium by applying a spark discharge between two samples. The SAD contains
two electrodes equipped with adjustaôle holders that c
m accommodate spherical shaped
sarnples. The gap distance between the electrodes is adjustable and the electrode holders
can be tightened to secure the samples. The platfom located beneath the electrodes is
used to support the sample container during sample ablation. The SAD operates on a 220
V / 15 A supply and outputs a voltage that is adjusted using a wntrol knob on the fiont of
the instrument. The output voltage was measured at 4444- V for this study.
a
ICP-Am
Two different methods of sample introduction into the ICP were used. The fust
method was liquid nebulization and the second method was DSI. The liquid nebulization
experiments were done using a Thermo JarreIl Ash (TJA) Model-61 ICP. The DSI
device @Sm) used was the same as that described by Skinner and salin6. The DSI
experiments were done using an HFP-2500Plasma Therm inductively coupled plasma
with a modified Jarre11 Ash Atomcomp Model 750 direct reading spectrometer.
Modifications to the spectrometer include high speed electronics and a galvanically
driven quartz refiactor plate for rapid background correction7. The operating parameters
and wavelength seledion for both ICP instruments are shown in Table 4-1.
p
p
p
p
p
p
p
p
-
-
-
-
-
-
-
-
-
-
Figun Cl: Photograph OC the spark ablation device.
Plasma power
Observation height
Plasma gas flow
Auxiliary gas flow
Sample uptake
Flush time
Integration time
Io00 w
15 mm (ATOLC)
15 1 min-'
0.5 I min1
10 8.5 repeats / element
Plasma power
Observation height
Plasma gas flow
Auxiliary gas flow
Exposun time
Number of on-line and off-line
exposures per trace
Gaivanometer settle time
Sample insertion tirne
1.6 ms
200
10 ms
15 s
Al (1) 309.2, Cr (II)
267.7, Cu O 324.7,
Fe (LI) 259.9, M g 0383.2, M n (il) 293.3,
Zn (1) 213.8
Rcrigents and Samples
Multi-element liquid standards were prepared B-om 1ûûû ppm AA single element
reference standards (Fisher Scientific, Nepean, ON, Canada), trace metal grade HCI
(Fisher Scientific, Nepean,ON, C d )and Milli-Q distilled deionized water (MiIlipore
Corp., Bedford, Mq USA).
HPU: grade methanol (Cdedon Laboratones Ltd.,
Georgetown, ON, Canada) was used to rinse the samples and graphite sheaths. The DSI
cups and sheaths were machined fkom highdensity graphite electmdes (Bay Carbon Inc.,
Bay City, MI, USA).
The aluminum alloy srtmples were Alcan ailoys 3 104, 6 1 11, 5 182 and 7010
(Alcan, Jonquiere, PQ, Canada).
These alloy samples were in the form of a pin,
approximately 20 mm long and 2.3 mm in diameter.
Spark Ablation Sampling
In order to place the aluminum pin in the electrode holders of the SAD, a graphite
sheath was fashioned by drilling a well in which to place the aluminum pin. The pin was
placal in the graphite sheath with about 1-2 mm of pin lefk exposeci. The sample was
prepared by washing with 5% HCI, rinsing with Milli-Q water followed by a final rinse
with MeOH. The sample was dried in a beaker using a hot air gun and allowed to cool
before weighing.
After two sarnples (consisting of pin and graphite sheath) were
weighed, they were placed into the electrode holders. The holders were immersed in a
Teflon container with Milli-Q water, which was situated on the platform shown in Figure
4-1.
The gap distance between the samples was adjusted until sufticient sparking
occurred and ablation of the sample was done for 5 min with wntinuous sparking. The
sample solution was poured into a Nalgene container and diluted to 50 ml. The pins and
graphite holders were weighed after rinsing with Milli-Q and MeOH in order to
detennine the mass of sample ablateci. The sample solution was then analyzed using both
DSI-ICP-AES and liquid nebulization ICP-AES. The methods of extemal standards and
standard additions were petformeci in order to detemine the appropriate calibration
technique.
Spark Sampiing into DSI Probe
0
An exploratory study was done to detennine whether the SAD might be used to
sample ahiminum alloys directly inside a DSI graphite probe.
The DSI probe was
attacheci to one electrode and an aluminum wire was attached to the other. The DSI
probe design for this study is sbown in Figure 4-2. The graphite plaâonn inside the cup
was designed to dlow the sperk discharge to fom between the graphite and the sample.
Further experiments were done using a pin of dloy 61 1 1 in place of the duminum wire.
Distilled d e i o n i d water was pipetted into the DSI cup so that sample ablation could
ocair in solution. The s p u k time was between 30-00 s in order to ablate sufficient
sample. The DSI probe was inserted into the load mil of the ICP and the sample solution
w u drkd inductively. The probe was removed fiom the load mil and the plasma was
ignited before a sample insertion was perfomed.
Figure 4-2: DSI probe design Tor pedorming in situ sample ablation using the spark
ablation device.
Results aad Discussion
Cornparison of Caübmtion Methodologils
The spark ablation of the rhuninum pins was pafonned in distilled deionized
water. The gap distance between the two pins was adjusted until a continuous discharge
was obtained. It was neceseary to adjust this gap distance f i e r a few minutes of sparking
in order to maintain a continuous ablation throughout the sunpling time. The solutions
produceci fmom a 5 min spark were introduced using both liquid nebulization and DSI as
methods of sample introduction for ICP-AES analysis. The deteminations for sample
solutions using the liquid nebulization method of sample introduction are presented in
Table 4-2. The concentrations found using both standard additions and extemal standards
agree within experimental error for most analytes studied in the different alloy sarnples.
These results suggest that when calibrating with aqueous standards, an extemal standards
calibration is sunicient. This is advantageous due to the laborious nature of the method
O
of standard aâditions. The sample solutions were also analyzed using DSI-ICP-AES.
Because an automated system was not available, this method of analysis proved more
laborious than liquid nebulization. However, similar results were observed for both
methods of calibration using DSI; thereby wnfirrning the ability to calibrate with extemal
standards. Due to the high precision and relatively simple analysis associated with liquid
nebulization, it did not seem advantageous at this time to utilize DSI for fùrther
experiments with these sample solutions.
Tabk 4-2: Concentrations Cound iii spuk i k t i o i soiutions of duminom dtoys
using esterna! st.ndards rad standard additions ulibration.
Concentration found @pm)
Alloy 3 104
Alloy 5 182
Extemal
Standard
Extemal
Standard
Standards
Additions
Standards
Additions
AI
38.70 (1.01)
37.05 (0.97)
37.80 (1.42)
41.73 (1.57)
Cr
0.01 (0.01)
<LOD
0.01 (0.01)
<LOD
Cu
0.05 (0.04)
0.05 (0.04)
0.37 (0.04)
0.41 (0.04)
Fe
0.13 (0.03)
O. 15 (0.04)
0.06 (0.04)
0.07 (0.05)
Mg
0.56 (0.04)
0.55 (0.04)
1.78 (0.06)
Mn
0.39 (0.04)
0.39 (0.04
O. 10 (0.04)
0.09 (0.04)
Zn
<.LOD
<LOD
1.67 (0.04)
1.75 (0.04)
Elernent
Ailoy7010
-
1.86 (0.07)
Alfoy 61 1 1
.
External
Standard
Standards
Standard
Additions
Standards
Additions
Al
42.80 (0.84)
41.43 (0.81)
43.40 (1.85)
43.89 (1.87)
Cr
0.10 (0.07)
0.06 (0.04)
0.03 (0.03)
<LOD
Cu
0.98 (0.04)
1.21 (0.05)
0.32 (0.04)
0.38 (0.04)
Fe
0.32 (0.05)
0.3 1 (0.04)
0.07 (0.04)
0.08 (0.04)
Mg
Mn
Zn
1.38 (0.04)
1.34 (0.04)
0.39 (0.04)
0.39 (0.04)
0.06 (0.07)
0.03 (0.03)
0.06 (0.05)
0.04 (0.04)
3.87 (0.09)
3.77 (0.09)
0.01 (0.01)
<LOD
Element
Extemal
Spark Ablation Time
The eff- of spwk time on sample ablation and concentration
was examined.
Five sample solutions of alloy 3104 were prepared by perfonning the spark ablation for 2,
4, 6, 8 and 10 min. The mass of sample ablatd for each solution wm detennined and
plotted as a fiinction of spuk time in Figure 4-3. From this gnph, it might B r n that the
total mass ablated is not linearly proportional to the spark time. However, if the 6, 8 and
10 min A u e s are examined then t h a e does seem t o be a linear relationship between
mass dispersed and spark time. The average mass dispersed for the 2 and 4 min ablations
wos determined and estimated as the mass dispersed for a 3 min ablation. This estimated
value is plotted with the 6, 8 and 10 min values in Figure 4 4 and a linear relationship is
now evident. The alloy dispersion rate or mass dispersed per unit tirne was calculateci for
the 6, 8 and 10 min solutions. The average rate of dispersion for alloy 3104 was 810
pglrnin.
nie dispersion rates for 5 and 10 min ablations of al1 four alloys were
determined and the average value was also 810 &min.
a
The average dispersion rate may
be used to detennine the ablation time required for producing a solution of a certain
concentration.
The concentrations of Al, Cu, Fe, Mg and Mn in the five solutions were
determined and plotted in Figure 4-5 as a fiinction of time. The signal intensities for Cr
and Zn in these solutions were similar to blank levels and therefore the concentrations
were not quantifiable. Figure 4-5 shows that the concentrations of the analytes in the
sample solutions of alloy 3 104 increased with increasing spark time as was expected.
In order to determine if the ablation of the various elements is proceeding
homogeneously throughout time, the ratio of signal intensities was examined. The blank
subtracted signais for Cu, Fe, Mg and Mn were ratioed to the blank subtracted signal of
Al and plotted as a function of spark time in Figure 4 6 . These ratios rernained relatively
constant for the five solutions, varying by l e s than 3 % RSD. Therefore, it appears that
the ablation of the sample does p r d homogeneously over time.
2
4
6
8
10
Time (min)
Figure 4-3: Mass of' aüoy 3104 disperscd into solution as a ruaction of spark
ablation time.
2
4
6
8
10
Time (min)
Figure 4-4: Linur relationslip bctwcea mas disperscd and spark ablation timc
Tbe m u r dispend for a 3 min spark was estimated as the average of the 2 and 4
min vduts.
6
Time (min)
Figure 4-5: Concentrations of Ai, Cu, Fe, Mg and Mn found as a runction of spark
ablation time.
6
Time (min)
Figure 4-45: Ratio of signal intensitia measureâ (ratioed to Al) for Cu, Fe, Mg and
M n rn 8 function of spark ablation timc
Solution Considerations
The arnount of ailoy ablated waa determineci tiom the difference in weight before
and .tta sample ablation. By biowing this weight and the volume of dilution, the
concentration of dispersed alloy
cui be
determineci. The expected concentration of Al
can then be calculated since the percent composition is known.
The expected
concentntion of Al fkom a 5 min spark for each ailoy type is wmpand with the actual
concentration found in Table 4-3. The amount of Al found in solution was 36-57 % of
the expected amount.
Table 4-3: Concentration o f Al faund in alloy solutions from a S m i n spark ablation
c o m p r r d with the expectd concentration.
Concentration Al @pin)
AIloy
Found
Expected
% Al found
3104
38.70
85.39
45
5182
37.80
69.66
54
7010
42.80
119.36
36
6111
43.40
75.91
57
The aluminum pins were weighed with the graphite sheaths to avoid excessive
handling and possible contamination of the pins. Experiments were done to detennine
whether there was ablation or loss of graphite during the spark procedure. If the mass of
the graphite was changing then the calculated weight of sample ablated would be
erroneously greater than expected. The weight of sample ablated is used in determining
the expected concentration of sols and therefore the expected concentration of Al and
other ailoying elements in solution. The graphite sheaths and aluminum pins were
weighed separately before and afier sample ablation in order to detemine the true
difference in a m p l e mass. It was found that the mass difference in graphite was minimal
(<0.3 mg) and accounted for about 5 % of the total mass difference. Therefore, this small
difference in mass of graphite did not account for the 50 % difference in Al found in
Table 4-3.
Revious workf hm shown that the aâdition of acids (HNthe dispersion medium
(W)
to pH
and HCl) to acidify
2.8-3.3 allowed staôle colloidal solutions to
be
obtained with concentrations of up to 1 msfml (1000 ppm). Stable solutions with these
concentrations dlowed trace a d y s i s to be petformeci. When the pH of the dispersion
medium was not acijusted, the sols were only stable when the concentration was below
20-50 ppm2.
The concentration of sols in solution for a 5 min spark ranged fiom about 75-140
ppm for the four different alloys. These solutions were prepared in distilled/deionized
water ody and therefore sol coagulation and subsequent precipitation may have occurred.
From the results in Table 4-3 it appears that these solutions were not stable for sol
concentrations of this level because the concentration of Al found was much lower than
expectedThe spark ablation was repeated for the four alloys using a spark time of 6-10
min. The solutions that were produceci were analyzed before and aiter the addition of
0
250 )il of concentrateci trace metal grade HCl. The addition of HCl adjusted the pH to
about 2.6. The acid was added afkrperforming sunple ablation because of the design of
the SAD, which requires the electrode holders containing the pins to be immersed in
solution during the sample ablation.
If the dispersion medium was acidified,
contamination due to acid attack on the electrode holders could occur because of their
metallic nature- The concentrations of Al found in the water and acidified solutions are
shown in Table 4-4. For al1 of the alloys studied, the concentration of Al in the acidified
solution was higher than in the water solution. The % Al found increased fkom about 50
% to close to 90 % of the expected value when the solutions were acidified.
The
concentrations of Al and the other elements studied in the water and acidified solutions
are presented in Table 4-5. The concentrations found were higher for the acidified
solutions for al1 of the elements studied. The amount of increase, however, was not
uniform for the different elements in the four alloys.
TaMe 4 4 : Conctntntion o f Ai upected and îound in distüleâ/dtionized water and
a c i d i f d sprrk ablation rolutionr
Concentration Al @pm)
Expected
Water
Acid
Alloy
% Al found
Watet
Acid
3 104
306.55
209.45
266.29
68
87
5182
181.18
85.41
133.32
47
74
7010
191.90
91.29
170.80
48
89
6111
233.44
136.24
21 1.37
58
91
Table 6 5 : Concentrations determincd for dîstilled/deionued watcr and acidified
spark ablation solutions.
Concentration (ppm)
Alloy 3 104
Element
AUoy 5182
AIIoy 7010
Water
Acid
Alloy 61 11
Water
Acid
Waîer
Acid
Water
Acid
Al
Cr
209.45
266.29
83.4 1
133.32
91.29
170.80
136.24
21 1.37
0.003
0.012
0.01 1
0.016
0.011
0.013
0.056
0.113
Cu
0.34
0.35
0.84
1.31
1.75
2.63
0.94
1.41
Fe
0.79
1.O8
O.16
0.27
O. 12
0.26
0.22
0.42
Mg
2.87
3.58
4.57
5.70
3 .50
4.75
1-42
1.77
M.
Zn
1.94
2.57
0.20
0.33
0.08
O.16
0.20
0.30
0.025
0.043
3.59
5.60
7.5 1
13.72
0.01
0.05
'
,
Figures of Merit
The limits of detection (LOD)for the TJA Modd 61 w a e alculated using the
following equation
where q, is the standard deviation of the blank signal and m is the dope of the
dibration curve. The calibntion curves w a e constructed using aquawis extemal
standards. Table 4-6 shows the limits of detection detemineci for the elements studied.
Table 4-6: Detectioi ümib for ICP-AES uulysis.
Concentration
Element
PPm
P P ~
Al
O. 147
147
Cr
0.011
11
Cu
0.007
7
Fe
0.011
11
Mg
0.0005
0.5
Mn
Zn
0.002
2
0.004
4
The concentration ratio method described in Chapter 3 was used to determine the
percent composition for the spark solutions mentioned previously. These results for the
water and aciditled solutions are presented in Table 4-7. In general the accuracy for the
detemination of Al is quite high with a relative error less than 5 % for al1 four alloys.
Manganese also exhibited a high accuracy of 0-4 % for dloys 3 104 and 6111. When
cornparhg the water and acid solutions, thae an several observed trends in the accuracy.
Tibk 4-7: Detaminition of A!, Cr, Ca, Fe, Mg, M n and Zn in duminum iPoy pins.
AIloy 3104
AUoy 5 182
Acid
Water
Acid
Water
Elemeat
%w/w
%errof
96wh
%errer
./.w/w
./.error
96w/w
% error
AI
97.04
0.0
97.03
0.0
89.73
-4.7
90.63
-3.7
Cr
0.005
135
0.005
149
0.0 12
-48.0
0.0 12
31.0
Cu
O. 162
-7.0
O. 133
-24.0
0.868
936
0.873
942
Fe
0.367
-12.9
0.393
4.7
O. 182
-36.0
0.193
-32.1
Mg
1.321
10.0
1.294
7.7
4.937
3.6
3.98 1
-16.4
Mn
0.89 1
4.3
0.925
4.6
0.220
4.3
0.230
-37.6
Zn
N/A
N/A
0.00 1
-23.0
3.839
2700
3.875
2735
'
AUoy 7010
Ailoy 6111
Acid
Water
Acid
Water
EIement
./a w/w
% error
% wiw
./remor
% w/w
% emr
% w/w
% ermr
Al
87.29
-2.0
88.56
4.6
97.3 1
0.0
98.04
0.7
Cr
0.0 11
265
0.007
139
0.04 1
-28.2
0.054
-4.8
CU
1.625
-1 1.6
1.322
-28.1
0.664
-12.4
0.640
-15.6
Fe
Mg
Ma
ZU
O. 123
-12.4
0.139
4.8
O. 168
-22.0
O, 197
-8.3
3.432
43 .O
2.509
4.6
0.99 1
20.5
0.788
-4.2
0.090
8.2
0.088
6.6
O. 147
2.2
0.145
1.4
7.3 10
15.4
7.258
14.6
0.00 1
-71.8
0.01 1
263
The acairacy for the determination of Cu degrades in the acid solutions for al1
four alloys. Copper is soluble in HN03 and only slightly soluble in HC1 and therefore
would not benefit from the addition of HCI. Since the addition of acid will digest or
partially digest the sols, the use of HCl would not increase the concentration of Cu in
solution to the same extent as the other elements. This was observecl in Table 4-5 where
the concentration of d l elements did increase but not uniformly.
Conversely, the accuracy for both Fe and Mn improved in the acidified solutions.
The most notable improvement in accuracy is for Fe in alloy 7010, which improved fiom
12 % to c 1 % in acid solution. The Pccuracy for the determination of Mg improved for
0
all of the ailoys except alloy 5182. The determination of Al improved for alloys 5182
and 7010 nmained the srme for dloy 3 104 and degradeci slightly for alloy 6111.
Overail, the detamination of Al in either water or acid solution wu quite occurate.
The determinations of Cr and Zn were relatively poor for both the water and acid
solutions. The exception is the high accuracy (4.8 %) for the detennination of Cr in the
acid solution of alloy 6 111.
The precision of this method of analysis was examined by performing replicate
analyses of the sarne alloy. The average results for four replicate analyses of alloy 3104
The % RSD for replicate determinations using water
sotutions varied fiom 0.2-2.4 % for al1 elements except Cr which has a % RSD of 38 %.
an presented in Table 4-8.
It was shown previously that the concentration of Cr in solution for alloy 3 104 was neat
the detedion limit. Therefore inamrate and varying results would lead to poor precision
for replicate analyses.
The precision varied fiom 0.2-33 % RSD for replicate
determinations in acidified solutions. There is a slight improvement in precision for Mn,
Fe and Cr in the acid solutions. On the contraiy the precision for Cu is higher in the
water solutions. T h e n is no change in precision for AI when the solutions are acidified.
Table 4-8: Average percent composition determilied for alloy 3104.
Water
Acid
Element
% w/wa
ab
% RSD
% w/wa
ab
% RSD
Al
96.91
0.2 1
0.2
96.91
0.22
0.2
Cr
0.0070
0.0027
38
0.0037
0.0012
33
Cu
O. 165
0.003
1.9
0.144
0.011
7.5
Fe
0.374
O. 009
2.3
0.399
0.007
1.7
Mg
Mn
1.32
0.02
1-6
1.30
0.03
2.2
0.920
0.022
2.4
0.942
0.0 15
1.6
Zn
<LOD
N/A
N/A
<LOD
N/A
N/A
" Where Y. w/w is the average value d e â e d w i ( ~ 4 ) .
Wberr u is the calculatexi samjde rtandvddeviatioa
Spark Sampling in DSI Probe
An investigative study was W o r m e d in order to ddamine whether it was viable
to perform the spuk ablation ôetween the lhuninum samples and graphite. Sarnple
solutions were prepareâ by patorrning sample ablation in solution as done previously.
The first solution was prepared by placing samples of alloy 5182 in both electrode
holders and perfonning sample ablation for 2 min. One of the alloy samples was then
replaced with a graphite r d and sample ablation was repeated? this time between the
alloy sample and graphite. The solutions were then analyzed for the Al content and it
was found that the first ~olutioncontained about twice as much Al as the second (62 ppm
versus 30 ppm). Therefore, it was possible to penonn ablation of the aluminum alloy
when graphite was used to replace the second alloy sample. If a graphite rod could be
used as an electrode, then it seemed possible that a DSI probe could also be used in this
manner.
The SAD was tumed sideways 90' so that the electrode holden were situated
0
horizontally rather than vertically. This arrangement was necessary in order to attach a
DSI probe to the electrode holder in an upright position. The DSI probe was placed in
the lower electrode holder and a piece of aluminum wire was attached to the upper
ekctrode holder. Figure 4-7 (a) shows a cross sedional view of the aluminum wire and
the DSI probe attached to the electrode holders. The gap between the wire and the
bottom of the probe was adjusted and after voltage was applied to the etectrodes, sparking
ocnirred as shown in Figure 4-7 (b). A steady ablation of the sample occurred with a
controlled sparlc concentrated between the wire and the graphite platform at the bottom of
the probe. When the distance between the w k and probe bottom was too great, sparking
occurred between the wire and 'the cup walls (Figure 4-8). This sparking was observed to
be quite erratic and caused degradation of the probe walls. An overhead view of the
spark ablation between the win and probe in Figure 4-9 shows that there is no sparking
between the wire and the probe walls when the gap distance is properly adjusted.
7 : Cross sectiood view o f spark ablation o f an iluminum wire pedorm
a DSI probe: (a) berore initiatingablation, (b) during spark ablation.
Figure 4-8: Spark discharge o c r u M g behvccn the aluminum wirc and the DSI
probe wdls.
Figure 4-9: Ovtrhead view o f spirk ablation between aluminum wirc and DSI
probe.
In orâer to avoid sunple lom via volatiliution during the spark ablation, water
was dded to the DSI probe. This d l o w d the . m p l e ablation to be pedonned in
solution rather than in air. A pin of alloy 61 11 wu attached to the upper electmde holder
and adjustexi so that the tip of the pin was just above the graphite plauorm inside the
probe. Water was pipetted into the probe until the probe was about % full. The sample
ablation was observed t o ocair between the sample and the graphite platfonn. Because
of the higher sensitivity of DSI, a shorter ablation time was used. An ablation time of 90
s ablated sufficient sample to cause saturation for some elements. A shorter ablation time
of between 30-40 s was found appropriate in order to obtain signds well above blank
levels without saturating.
Figure 4-10 shows transient signals obtained fiom an insertion of a probe
containhg a 40 s ablation sample of alloy 61 11. Most of the elements exhibited Gaussian
peak shapes with slight tailing and analyte vaporization over a period of less than 10 S.
The low boiling points for Zn and Mg,
907°C and 1090°C respectively, riccount for the
rapid volatilization and namow peak widths for these elements. The traces for Cr and Fe
exhibited significant tail ing and grrater vaporization times were observed. Incomplete
vapontation of Ai, Cr and Fe bom the probe is evidenced by the transient signals, which
did not retum to the b a d i n e within the 40 s sample insertion time. This problem of
incomplete vaporization of these elements 60m the probe was seen previously in Chapter
2. A halogenating agent could be added to aid in volatilization. However, the addition of
a gas such as Freon during sample insertion could not occur through the base of the probe
used in this study. The DSI probe required for in situ sarnple ablation would not allow
for the introduction of a gas Stream.
This work shows promise for perfonning in situ sarnple ablation using DSI
probes. The sample ablation time was on the order of seconds and no sample dilutions
were requird. However, this study was exploratory and quantitative analysis was not
perforrned .
1
1
Aîuminum
Chromium
Magnesium
O
10
2a
30
40
(ml
Silicon
Zinc
Figure 4-10: Transient signais obtained from an insertion of a DSI probe containing
a srimpie of alloy 6111 producd from a 3 0 r ablation perfijrmed in situ.
Conclusions
The analysis of aluminum alloy dispersions by ICP-AES offers simultaneous
rnulti-element determinations which AAS methods do not offer. A sample ablation tirne
of five minutes w u sunicient to produce quantifiable samples for introduction by liquid
nebulization. This method of sarnple preparation can be considerably more time and cost
efficient compared with conventional digestion methods, which typically require houn
for complete digestion. The method of extemal standards was used for caiibration and
uxniracy was high for some analytes but poor for others. It may be necessary to prepare
sols of certifieci standards rather than using aquanis standards in order to achieve high
accuracy for al1 analytes.
The spark ablation device was successfùlly used to pedonn in situ sampling of
alloys inside a DSI probe pnor to ICP-AES analysis. This method of sampling wuld
prove advantageous due to the short sampling times (on the order of seconds), avoidance
of emmr due to sample dilution and the capability of DSI to perform automated batch
0
analyses. Future studies will be necessary in order to evaluate the performance of this
method for quantitative analysis.
Acknowledgement s
The author gratefully thanks Dr. Douglas Goltz and Dr. Cameron Skimer for their
collaboration and support for this research as well as Dr. Mike Hinds fiom the Royal
Canadian Mint for his generosity in loaning the spadc ablation device.
' Ghiglione, M., Eljwi, E. and Cuevas, C., AppledSpc., 3û, 1976, p. 320.
Pchekn, AI., KharIamov, I.P., Gusinskii, M.N.and Shipova, EN.,2k A m L Khim.,
42, 1987, p. 2138.
Kmyakin, V.Y., Kharlarnov, I.P. and Pchelkin, AI., Zmiad Lob.,54, 1988, p. 36.
'L'vov, B.V.and Novichikhin, AV.,A t d c Spctroscopy, 11, 1990, p. 1.
Bendicho, C., Fres. J. A m / . CChe., 348, 1994,353.
Skimer, C.D.and Salin, E.D., 1Am1 At. Spcfrom., 12,1997,727.
'LLgere, G. and Burgener, P., ICP 1i# Newslefter.,13, 1988, 52 1.
Chapter 5
Conclusions and Suggestions for Future Work
The results in Chapters 2 and Chapter 4 for the d y s i s of aluminum samples
duectly using DSI-ICP-AES show that the vaporintion of Al is problematic. A recent
studyl in our laboratory has shown that the application of a pyrolitic coating to the DSI
probe d u c e s the sample intercalation into the probe. The use of a pyrolitically coated
probe may prove beneficial to prevent Al intercalation and to enhance volatilization.
The RAAT proved to be a rapid, simple and precise method of analyzing Al pin
samples. The alloy digestion proved reproducible and the ratio of signal intensities
remained constant throughout the digestion enabling quantification during sample
digestion. The acairacy was not high for dl of the analytes and could be improved if
0
calibration was performed using matrix matched standards rather than aqueous standards.
The use of the SAD to sample the aluminum pins exhibited great potential. It
would be interesting to use the RAAT to analyze the dispersions that are produced fiom
the spark sampling in solution. The feasibility of using the SAD to sarnple the pins inside
the DSI probe and perform quantitative analyses needs to be examined. It would be
beneficial to use DSI, due to its irnproved detection limits as cornpared with liquid
nebulization, in order to improve the detection of analytes such as Cr (0.00 1 %) whch
are now "at the edge" for traditional methods.
In order to irnprove the detection of trace analytes in aluminurn alloys, ICP-Mass
Spectrometry (MS) could be used rather than ICP-AES. However, if sample digestion or
sample ablation is performed using HCI, problems may arise due to spectral interferences
from chloride species2.
References
' Ryhdq ME.and Salin, E.D., J. A
d Ar Spechm., 13,1998,707.
Inductiveiy Coupkd Plasma Mass Spectmmetry, Montaser, A, Wiley-VCH Publishers,
New York, USA, 1998, p. 524.