Spectrochimica Acta Part B 55 Ž2000. 629᎐635
Elimination of boron memory effect in inductively
coupled plasma-mass spectrometry by ammonia gas
injection into the spray chamber during analysis
Assad S. Al-Ammar, Rajesh K. Gupta, Ramon M. BarnesU
Department of Chemistry, Lederle Graduate Research Towers, Uni¨ ersity of Massachusetts, Box 34510, Amherst,
MA 01003-4510, USA
Received 23 June 1999; accepted 10 March 2000
Abstract
Injection of 10᎐20 mlrmin of ammonia gas into an inductively coupled plasma-mass spectrometry ŽICP-MS. spray
chamber during boron determination eliminates the memory effect of a 1 ␮grml B solution within a 2-min washing
time. Ammonia gas injection also reduces the boron blank by a factor of four and enhances the sensitivity by
33᎐90%. Boron detection limits are improved from 12 and 14 to 3 and 4 ngrml, respectively, for two ICP-MS
instruments. Trace boron concentrations in certified reference materials agree well using ammonia gas injection.
䊚 2000 Elsevier Science B.V. All rights reserved.
Keywords: Memory effect elimination; Washout time; Inductively coupled plasma-mass spectrometry; Boron
determination
1. Introduction
The high sensitivity of inductively coupled
plasma-mass spectrometry ŽICP-MS. makes this
technique suitable for reliable and rapid boron
determinations. However, measuring boron at
ultratrace levels by ICP-MS often is plagued a
significant memory effect. Recently, we character-
U
Corresponding author. Fax: q1-413-545-3757.
ized the boron memory effect mechanism in ICPMS and demonstrated that introduction of aqueous ammonia solution with the sample reduced
memory effect w1x. The memory effect appears to
result from boron’s tendency to volatilize as boric
acid from the sample solution layer covering
the spray chamber inside surfaces. The mass
spectrometer skimmer, sampler, ion lenses,
quadrupole and other components did not contribute to the memory effect at trace boron
concentrations. Aqueous ammonia apparently
0584-8547r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 5 8 4 - 8 5 4 7 Ž 0 0 . 0 0 1 9 7 - X
630
A.S. Al-Ammar et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 629᎐635
converted volatile boric acid to non-volatile
ammonium borate. The ammonia was introduced
by injecting a small solution volume simultaneously with the sample solution through a junction
immediately before the nebulizer. This arrangement was effective in eliminating the boron memory effect completely and reducing boron signals to blank levels instantaneously. Sun et al. w2x
recently applied this approach with mannitol in
the determination of B in serum, plasma and
urine. However, introducing ammonia solution
just before the nebulizer is not always suitable
when the sample solution contains matrix or analyte elements ŽCa, Fe, etc.. that precipitate in
basic solutions. The precipitate can block the
nebulizer orifice. Although boron is not prone to
precipitation in basic solution, it can co-precipitate with the matrix.
The present investigation demonstrates an
arrangement to eliminated boron memory effect
by introducing ammonia gas in the spray chamber
without blocking the nebulizer or co-precipitating
analyte. Direct ammonia gas introduction inside a
spray chamber provides an optimum, universal
approach to eliminate boron memory. Even if
appreciable matrix element precipitation were to
occur, it would follow sample nebulization. Nebulizer blockage or analyte loss would be prevented. The effect of ammonia gas introduction
on the boron blank value and sensitivity is investigated, since a lower blank and thus a better
detection limit could be achieved by converting
the volatile boric acid on the inside surface of the
spray chamber to the non-volatile ammonium
borate.
2. Experimental
2.1. Instrumentation
Two commercial ICP-MS instruments ŽElan
5000a, Perkin-Elmer, Norwalk, CT; Spectromass
2000, Spectro Analytical Instruments, Inc., Fitchburg, MA. were used to measure boron Ž mrz 10.
with ammonia gas since these instruments have
different ion lens design and sample introduction
systems. The experimental operating parameters
Table 1
ICP-MS operating conditions
ICP system
Elan 5000a
Spectromass 2000
r.f. power ŽkW.
Frequency Žfree running.
ICP torch
Torch injector
Outer argon flow rate Žlrmin.
Intermediate argon flow rate Žlrmin.
Central Žaerosol. argon flow rate Žlrmin.
Nebulizer
1.05
40 MHz
Fassel type
Ceramic alumina
15
1.0
1.0
Gem tip cross-flow
Sample pump rate Žmlrmin.
Spray chamber
1.0
Ryton Scott double pass
Drain
Detector voltage ŽV.
Resolution
Scanning mode
Replicate time
Dwell time
Sweepsrreading
Readingsrreplicate
No. of replicates
Ammonia flow rate Žmlrmin.
Pumped
2500
0.8 amu ŽNormal.
Peak hop
1s
200 ms
5
1
3
20
1.35
27 MHz
Fassel type
Quartz
15
1.5
1.0
Concentric ŽGlass Expansion Ltd.,
Hawthorne, Australia.
1.0
Glass Scott double pass coolant jacketed
ŽSpectro Analytical Instruments .
Pumped
2250
0.8 amu ŽNormal.
Peak hop
᎐
1s
᎐
᎐
3
10
A.S. Al-Ammar et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 629᎐635
ŽTable 1. are selected based on manufacturer’s
recommended optimizations as used in routine
analysis.
Ammonia was introduced through a port in the
spray chamber cap near the nebulizer tip, so that
the nebulizer flow would mix with and distribute
ammonia efficiently and homogeneously to all
spray chamber surfaces. The ammonia supply was
connected to the spray chamber with Teflon
Ž6.35-mm o.d.. and Tygon tubing Ž0.76-mm i.d...
The ammonia flow rate was metered with a needle valve ŽNupro. and flow meter ŽBrooks ShoRate.. The optimum ammonia flow was determined by trial and error with a blank 2.5%
nitric acid solution and a 1-␮grml B solution in
2.5% nitric acid. The nitric acid concentration
was selected to match the final acid present in
biological sample solutions after acid digestion.
The 1-␮grml B solution was nebulized to introduce a memory effect in the presence of a certain
ammonia gas flow rate. Then the blank was nebulized at the same ammonia flow rate to
determine the washout time to remove the boron
signal completely. The 10 B sensitivity was also
monitored as a function of ammonia flow rate,
since the boron signal is enhanced at low ammonia flow rates Ž10 mlrmin for the Spectromass
2000 and 20 mlrmin for the Elan 5000a. but
depressed at higher flow rates Ž) 10 and 20
mlrmin, respectively.. The ammonia flow rate
was increased gradually from 3 mlrmin in 2mlrmin steps until an optimum ammonia flow
rate is achieved. The ammonia flow rates that
gave suitable compromise values for both boron
sensitivity and memory washout time were 10
mlrmin for the Spectromass 2000 and 20 mlrmin
for the Elan 5000a systems.
2.2. Reagents
Standard reference material boric acid ŽNational Institute of Standards and Technology,
SRM 951. was used to prepare 1-␮grml B solution in 2.5% sub-boiled nitric acid. Standard reference materials ŽCorn Bran RM 8433, Whole
Egg RM 8415, and Trace Elements in Water
SRM 1643C. were prepared with sub-boiled nitric
acid to test the accuracy of the ammonia intro-
631
duction procedure. Premium grade anhydrous liquid ammonia ŽLaroche Industries, Inc., Atlanta,
GA. was used to generate the ammonia gas.
Distilled de-ionized water was used for cleaning,
diluting and preparing solutions.
Boron-containing samples are usually decomposed with strong oxidizing acids Žtypically nitric.,
and the final boron solution analyzed by ICP-MS
typically contains approximately 2% acid. With
this oxidative sample decomposition, boron should
exist as boric acid in the final acidic solution.
Thus the use of boric acid and dilute nitric acid
for preparing the blank, standard and other solutions simulates practical sample solutions. Following the procedure described by Sah and Brown
w3x for analysis of biological samples for boron,
approximately 1.0-g aliquots of corn bran or 2.0-g
aliquots of whole egg powder were extracted with
20 ml of 1 M nitric acid in a sealed Teflon
container ŽTeflon PFA microwave vessel, CEM
Corp., Matthews, NC. for 1 h at 80⬚C in a conventional oven. Beryllium was added to the final
solution as an internal reference for boron to
eliminate the non-spectroscopic matrix interference resulting from the dissolved organic carbon
ŽDOC. and other matrix constituents such as Na,
K and Ca w4x. This solution was then diluted to 40
ml. The solution contains 100 ␮grl B nitric acid
matrix matched with the sample solution. A
method blank is prepared by applying the same
sample treatment procedure to a 20-ml aliquot of
1 M nitric acid. Water SRM 1643C that contains
2.5% nitric acid was analyzed directly using a
blank and standard with same acid concentration.
No further treatment other than the addition of
the Be internal reference is required with this
SRM. The effect of introducing ammonia on the
B detection limit, sensitivity and accuracy of boron
determination in biological materials was investigated with these Corn Bran RM 8433, Whole Egg
Reference RM 8415, and Trace Elements in Water SRM 1643 C solutions. The experiments were
repeated with and without ammonia gas introduction.
2.3. Memory
The boron memory effect was measured with
632
A.S. Al-Ammar et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 629᎐635
and without ammonia introduction. A solution
containing 1 ␮grml B in 2.5% nitric acid was
nebulized for 1 min at the rate of 1 mlrmin to
create the memory effect. A blank solution Ž2.5%
nitric acid. was nebulized immediately after, and
the 10 B signal was recorded for an extended time.
3. Results and discussion
The washout curves with and without ammonia
introduction are shown for the two ICP-MS systems in Fig. 1. When ammonia is not used, these
curves show a two-part extended washout re-
Fig. 1. Ža. Washout curve for 10 B using an Elan 5000a ICP-MS. Žb. Washout curve for boron using a Spectromass 2000 ICP-MS
with standard glass spray chamber and concentric nebulizer ŽTable 1.. Žc. Washout curve for boron using a Spectromass 2000
ICP-MS with Ryton spray chamber and Gem tip nebulizer. ŽI. Boron washout curve without ammonia; Ž^. boron background
without ammonia; Ž䉫. boron washout with ammonia; Ž`. boron background with ammonia.
A.S. Al-Ammar et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 629᎐635
633
Fig. 1. Ž Continued..
sponse. An initial period lasting approximately 2
min of ‘fast’ washout represents the cleanout of
most of the boron. This segment is followed by a
slow washout period which, although represents
the smaller fraction of the memory effect, is ruinous to the analysis, because of the prolonged
time required for the boron signal to return to
the blank level. Even after 14 min of continuous
washing with the blank solution ŽFig. 1b., the
boron signal is still 17% higher than its blank
value. This memory effect introduces 32% error
on a boron sample containing 25 ngrml B when it
was measured after 17 min of continuous washing
with the blank after the measurement of 1 ␮grml
B solution. The 25 ngrml B concentration is
typical in the final sample solution usually prepared after digesting biological samples for boron
determination. The elevated B blank level in Fig.
1b was significantly reduced in Fig. 1c by substituting a Ryton spray chamber and Gem tip nebulizer for the glass spray chamber and concentric
nebulizer.
The same washout curve experiments were repeated with 10 mlrmin ŽSpectromass 2000. and
20 mlrmin ŽElan 5000a. of ammonia gas introduced continuously to the spray chamber. The
washout curves in Fig. 1 obtained with ammonia
gas introduction indicate complete elimination of
the memory effect within 2 min of washing. The
initial washout rates are 50% faster and B blank
levels are lower with than without ammonia. Substituting the Ryton spray chamber and Gem tip
nebulizer for the glass spray chamber and concentric nebulizer again reduced the final B blank
level. The 2-min washout is similar to the period
by which two consecutively measured samples are
separated in routine measurements.
The concentrations of B in Corn Bran RM
8433, Whole Egg RM 8415, and Water SRM
1643C determined with and without ammonia introduction ŽTable 2. indicate that ammonia addition does not affect the boron measurement accuracy. The boron concentrations obtained with and
without ammonia agree with the certified boron
concentrations. The boron sensitivity is higher
when using ammonia gas. The sensitivity increases by 90% with the Spectromass 2000 and
33% with the Elan 5000a. The boron blank values
with ammonia addition were lower by a factor of
4 and 3 for both instruments ŽFig. 1.. The
increase in sensitivity coupled with the decrease
in blank value improves the B detection limit
from 12 to 3 ngrg for the Elan 5000. With the
Spectromass 2000 the B detection limit decreased
A.S. Al-Ammar et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 629᎐635
634
Table 2
Boron concentrations determined in reference materials with and without the addition of ammonia gas
Reference material
Corn Bran 8433
Žall concentrations
in mgrkg.
Sample
aliquot
1
Using
ammonia
Yes
No
2
Yes
No
Whole Egg RM 8415
Žall concentrations
in mgrkg.
Trace elements in water
SRM 1643C Žall
concentrations in ␮grl.
a
1
Yes
1
No
1
1
Yes
No
Instrument
Boron concentration
Measured
Certified
a
Elan 5000
Spectromass 2000
Elan 5000
Spectromass 2000
Elan 5000
Spectromass 2000
Elan 5000
Spectromass 2000
3.19" 0.05
3.23" 0.07
3.22" 0.05
3.05" 0.07
2.90 " 0.04
2.85" 0.06
3.01" 0.05
3.06" 0.08
2.8" 1.2
Elan 5000
Spectromass 2000
Elan 5000
Spectromass 2000
0.38" 0.01
0.38" 0.01
0.38" 0.01
0.37" 0.02
0.4" 0.26
Spectromass 2000
Spectromass 2000
113.4" 0.4
113.6" 0.6
119 " 1.4
Standard deviation of repeated reading of a single sample.
from 23 to 7 ngrg with the standard glass spray
chamber and nebulizer and from 14 to 3.6 ngrg
with the Ryton spray chamber and Gem tip
cross-flow nebulizer.
The enhancement in boron signal resulting from
ammonia addition may be due either to an increase in the degree of ionization as a result of
the increase in thermal conductivity in the plasma
arising from hydrogen formation from ammonia
decomposition w5x or a charge transfer reaction in
the central channel between the positively charged
nitrogen species and boron atoms w6x.
Ammonia addition has been applied for boron
determinations in practical samples including inorganic reagent chemicals, animal diets, wheat
gluten, soy concentrate, cellulose, sugar, edible
oils, water and seawater, and in boron isotope
ratio determinations in seawater. Details of these
analyses will be reported elsewhere.
4. Conclusion
Introduction of ammonia gas into the spray
chamber during analysis efficiently removes the
boron memory effect. It also improves the detec-
tion limit, because it simultaneously lowers the
blank value and enhances the boron signal sensitivity. Ammonia addition does not introduce a
bias to boron determination. The procedure
employs a simple arrangement, can be used on a
routine basis and permits rapid B determinations.
No nebulizer blockage, matrix precipitation, or
analyte loss were observed in the spray chamber
following sample nebulization with ammonia
introduction.
Acknowledgements
The authors thank Spectro Analytical Instruments ŽFitchburg, MA. for providing the Spectromass 2000 ICP-MS system and technical support
and Dr Eva Reitznerova
´ for the valuable discussions and assistance with the boron determinations. This investigation was funded by US Borax
Inc. ŽValencia, CA. and the ICP Information
Newsletter, Inc. ŽHadley, MA..
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