Fuel Processing Technology, 1 (1977/1978) 209--215
209
Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
©
COMBINED GAS CHROMATOGRAPHIC-MASS SPECTROMETRIC
ANALYSES OF NITROGEN BASES IN LIGHT OIL FROM A COAL
LIQUEFACTION PRODUCT
C.M. WHITE, F.K. SCHWEIGHARDT and J.L. SHULTZ
Energy Research and Development Administration, Pittsburgh Energy Research Center,
4800 Forbes Avenue, Pittsburgh, Pa. 15213 (US,A.)
(Received August 20th, 1977)
ABSTRACT
Nitrogen- base components of light oil produced during the catalytic hydrodesulfurization of coal were isolated by precipitation with hydrogen chloride and analyzed by
combined gas chromatography-mass spectrometry. Anilines and alkyl pyridines, 71 and
16 weight percent, respectively, were the major components. This is the first quantitative
report of anilines and pyridines in materials produced by the hydrogenation of/coal. Analytical techniques described provide a rapid and precise method for the analysis of pyridines and anilines.
INTRODUCTION
The advent of the energy crisis has hastened development of alternate
forms of fuel to supplement our dwindling petroleum reserves. One of the
technologies presently under development is the liquefaction of coal. Part of
the program at the Pittsburgh Energy Research Center of the Energy Research and Development Administration (ERDA) involves an active investigation into the nature of nitrogenous components in synthetic liquid fuels
derived from coal [1,2]. These components are largely undefined, although
the tar base fraction from coal carbonization processes has been extensively
investigated {3--6] , and other fractions of the light oils produced by coal
hydrogenation have been studied [7].
There are many reasons for determining the specific nitrogen-containing
compounds present in products from the liquefaction of coal. Identification
and quantitation of the nitrogen-containing materials is necessary to evaluate
denitrogenation properties of liquefaction catalysts and to determine the
amenability of the liquefaction product to further processing since nitrogen
compounds can adversely affect some refining processes by reducing the
activity of both cracking and hydrocracking catalysts. Nitrogen components
also have deleterious effects on the properties of fuels. They can lead to undesirable combustion products; such as NO x , and are, to some degree, asso-
210
ciated with the instability of liquid fuels during storage [8] . Many nitrogencontaining compounds are environmentally undesirablec Identification of
these components in a synthetic fuel will permit the environs of future synthetic fuel plants to be monitored specifically for these compounds. Furthermore, coal products are a prime source ofpyridines because of their absence
or low concentration in petroleum fractions [9}. Pyricl:ines are particularly
important as raw materials in the manufacture of pharmaceuticals and herbi.
cides, while an~lines are important in the manufacture of dyes.
EXPERIMENTAL
The coal-derived oil investigated was recovered from the one-half ton per
day Synthoil [10J Process Development Unit (PDU) at Bruceton, Pa.,
during the liquefaction of a Homestead Mine, Kentucky, coal (hvBb) at
450 0 C and 4000 p.s.i. total pressure of hydrogen in the presence. of a
CoMo/Si02~Al203 catalyst. The liquefaction product passed from the reactor
to a high-pressure receiver from which it was intermittently discharged into a
low-pressure receiver and held at 1000 C, in four-hour batches. Thus, when
liquid flowed from the high- into the low-pressure receiver, gases in the lowpressure receiver were displaced. The condensable components in the displaced gases were collected using a condenser; the nitrogenous materials investigated were recovered from this con~ensate.
The nitrogen bases were recovered from the light oil by precipitation with
hydrogen chloride [11] . The usual procedure was modified because of the
volatility ofthe sample and is described herein detiill. The light oil (167g)
was weighed into a 2-liter distilling flask and a PTFE-coated stirring bar was
added to enhance agitation. The flask and gas handling equipment, roughly
sealed with PTFE tape,. were placed in a fume hood. Electronic grade hydrogenchloride was bubbled through the solution at a flow-rate of 1-2 SCFM
for 2 hours at room. temperature; dry nitrogen was then used to purge the
system of hydrogen chloride vapor. This process resulted in the formation of
a two-phase system. Salts formed by reaction of the nitrogen bases with
hydrogen chloride, being insoluble in the motherliquor, formed a thick oil
beneath the supernatant liquid. The reaction products were centrifuged at
27,578 g for 10 min at 6 C. The supernatant liquid was decanted, leaving
the salts. These salts were transferred to a flask and Govered with 30.0 ml of
anhydrous diethyl ether. An amount of 150 ml of sodium hydroxide solution (0..5 N) was added in increments of 50 ml with stirring. The ether. became a dark red color within 2-3 min and was removed after 30. min with a
pipette. A second. 300-ml portion of ether was added, allowed to mix with
the base for 30 min, and removed. The final wash of the hydrogen chloride
adduct and sodium hydroxide solution w~ made with 50 ml of pesticide
grade benzene. Ether extracts were combined and the solvent was removed
by a gentle flow of nitrogen in a Rotavap*. The benzene extract was flash
frozen in liquid nitrogen and the benzene removed by sublimation. The
0
*Use of brand names facilitates understanding and does not necessarily imply endorse·
ment by the U.s. Energy Research and Development Administration.
combin
light oil
Thes·
spectro:
interfac
80:20. s
210o.A
masssp
tage of
tograpb
ent W1l.'S
second:~
Gas .(
tD,gh,c
lOG-I::;
wasop"
progrm::
and de:.,
fiow-ra£
Chro,
peaks U'
analysis
aniline 1
t.o thosf;
to have
pyridin,o
aniline, '
data.
When
fied by (
authenti.
phic pea
of an au
chromat
compou:
RESULT!
Until
oils proe
determh
constitu
and the
Theg.
211
combined extracts of nitrogen bases represented 3.0 weight percent of the
light oil.
These bases were then analyzed by combined gas chromatography-mass
spectrometry (GC-MS), performed with a DuPont 490 mass spectrometer
interfaced to a Varian 1700 series gas chromatograph, equipped with an
80:20 splitter and a hydrogen flame ionization detector. A Hewlett-Packard
2100A computer was used for spectrometric data storage and reduction. The
mass spectrometer was operated at a resolution of 600 with an ionizing voltage of 70 eV. The ion source, jet separator and glass line from the chromatograph to the spectrometer were held at 250° C. The chromatographic effluent was continuously scanned by the mass spectrometer at a rate of four
seconds per decade.
Gas chromatographic separations were achieved using a 10 ft. X 1/12 in.
LD. glass column containing acid-washed, dimethyldichlorosilane treated
100-120 mesh Supelcoport coated with 3% Carbowax 20M. The column
was operated isothermally at room temperature for 4 min, then temperature
programmed at 4°C/min to 230° C and held at that temperature. The injector
and detector of the chromatograph were operated at 250°C while the helium
flow~rate was 30 ml/min.
Chromatographic results were quantitated by electronically integrating the
peaks using a Varian Chromatographic Data System-101. Immediately after
analysis of the sample a standard solution of 2,4-dimethylpyridine and
aniline was injected and separated using chromatographic conditions identical
to those under which the sample was analyzed. All pyridines were assumed
to have the same flame ionization detector response factor as 2,4-dimethylpyridine, and all anilines were assumed to have the same response factor as
aniline. The assumption may introduce a small error into the quantitative
data.
Where specific isomeric identifications are made, identification was verified by co-chromatographic experiments using the nitrogen base sample and
authentic samples of those compounds. In each case, the gas chromatogra~
phic peak assigned to a specific isomer had a retention time identical to that
of an authentic sample. Additionally, the mass spectrum obtained on those
chromatographic peaks was consistent with the spectrum of the authentic
compound.
RESULTS AND DISCUSSION
Until now, little has been known about the nitrogenous constituents of
oils produced by coal hydrogenation. This investigation has resulted in the
determination of 26 components in the nitrogen base fraction. The light oil
constitutes approximately 4 weight percent of the total Synthoilliquid produet,
and the nitrogen base fraction comprises 3.0 weight percent of the light oil.
The gas chromatographic profile obtained from analysis of the nitrogen
;1. '_,
212
(
~WMW~~~W~
TIME, minul~s
_
_
__
~
~~~_~_~~
TI:.MPEHATUR-E :'C
Fig. 1. Gas chromatographic profile of the nitrogen base fraction of a light oil produced
hy the hydrodesulfurlzation of coal. The identity of the variou&ly numbered chromatographic peaks appears in Table 1.
base fraction appears in Fig. 1, and the numbered chromatographic peaks are
identified in Table 1. The order of elution of the pyridine, aniline and quinoline homologs is essentially the same as that observed by others analyzing
similar materials using the same chromatographic conditions [4] . We have
observed that pyridines possessing substituents in close proximity to the ring
nitrogen are eluted preferentially.
To our knowledge, this constitutes the first reported quantitative determination of pyridines and anilines in a material produced by the catalytic
hydrogenation of coal. The presence of anilines in coal hydrogenation products is not unreasonable, as those aromatic.amines are known to occur in
light oils produced by the carbonization of coal [4J . Anilines are also present in other fuels such as hydrocracked shale oil naphtha [12J. Further, it is
reasonable that nitrogen occurring exo to benzenoid systems in the coal macromolecule would produce aromatic amines upon cleavage of the macromolecule during the liquefaction process. There are many possible origins of
anilines. They could arise from the hydrogenation of quinolines or indoles
followed by hydrocracking, depicted in eqns. 1 and 2, respectively. The sug·
~ gested precursors to these anilinesare known to exist in the liquefaction product.
Quinoline and tetrahydroquinoline were identified in this study, and are
also known to occur in higher boiling fractions of this coal liquefaction product [1]. Alternately, anilines could arise by hydrogenolysis or pyrolysis of
the coal macromolecule.
213
(1)
(2)
TABLE 1
GC-MS results of the analysis of the nitrogen base components
No. of
peak
1
:e
1)-
2
3
4
5
6
7·
8
9
10
d-
. is
rraof
ugprod-
11
12
13
14
15
16
17
18
19
20
21
22
23
Compound
2-Methylpyridine
2,6-Dimethylpyridine
2-Ethylpyridine
3-Methylpyridine and
4-methylpyridine
2-Methyl-6-ethylpyridine
2,5-Dimethylpyridine
2,4-Dimethylpyridine
2,3-Dimethylpyridine
2,4,6-Trimethylpyridine
2,3,6-Trimethylpyridine
3,5-Dimethylpyridine
2-Methyl-5-ethylpyridine
Aniline
o-Met.hylaniline
p-Methylaniline
m-Methylaniline
2,6-Dimethylaniline
2,4-Dimethylaniline
2,5-Dimethylaniline
Weight percent in
the base fraction*
0.1
0.5
0.4
0.3
1.2
1.2
2.2
1.7
2.4
3.5
1.5
1.5
6.4
12.8
8.1
15.1
4.9
5.7
2,3~Dimethylaniline
5.3
3.2
3,5-Dimethylaniline and
quinoline
C. ·aniline and isQquinoline
1,2,3,4-Tetrahydroquinoline
7_6
2.4
2.2
90.2
r.e
)rodof
*These values are based on the assumption that all pyridines have the same response
factor as 2,4-dimethylpyridine and all anilines have the same response factor as aniline.
The assumption may introduce a small error into the quantitative data.
214
N-Alkylated anilines appear to be absent or in l<;>w concentration in the
nitrogen base fraction oBhe light oils. This prompted us to consider the
possibility of side reactions with hydrogen chloride. The HofimumMartius [13,14] rearrangement, eqn. 3, converts hydrohalides of N-alkylanilines
to ring-alkylated aniline derivatives when heated.
T
OR
NH2
250·C~ Q
(3}
R
R =CH 3 , CzH s , etc.
I
~
f
J
f
!
I
In order to determine if this reaction was occurring, N-ethylaniline (1 g)
was dissolved in n-pentane (200 ml) and precipitated with hydrogen chloridie
gas, back titrated with sodium hydroxide and recovered.
GC-MS analysis of the recovered base showed that no rearrangement reaction had occurred. The sole component present was the starting material,
N··ethylaniline. Therefore, under the conditions employed to isolate these
nitrogen bases, the Hefina11n-Martius rearrangement does not occur, and the
absence or low concentration ofN-alkylated anilines appears to be a
function of the sample.
CONCLUSIONS
A light oil produced during the catalytic liquefaction of coal was found to
consist of approximately 3.0 weight percent nitrogen bases. Analysis of these
bases by GC-MS showed that the material is comprised of alkylpyridines,
anilines, and to a lesser extent quinolines; 26 compounds were identified .
. Approximately 90 weight percent of the nitrogen bases from the light oil
were identified, and, of this amount, 16 percent were pyridines and 71 percent anilines_
ACKNOWLEDGEMENT
We are indebted to Nestor J. Mazzocco and Sayeed Akhtar forprovidinl:
the necessary samples from their operations of the one-half ton per day
Synthoil Unit. We also appreciate the generosity of Amos Bartoli of the
United States Steel Corporation Research Center in Monroeville, Pa., who
provided many of t..1").e alkylated pyridines used in this investigation, and the,
helpful technical discussions with Paul Rivers of Reilly Tar and Chemical
Company.
1.
2
215
REFERENCES
1 Schweighardt, F.K., White, C.M., Friedman, S. and Shultz, J.L., 1977. Heteroatom
species in coal liquefaction products. In: Meeting of the American Chemical Society,
Preprints of the Fuel Division, 22: 124.
2 Shultz, J.L., White, C.M., Schweighardt, F.K. and Sharkey, A.G., Jr., 1977. Characteriza·
tion of the heterocyclic compounds in coal liquefaction products. Part I: Nitrogen
Compounds. U.S. Energy Research and Development Administration, Pittsburgh Ener·
gy Research Center, PERC/RI-77 /7, 25 pp. Available from the National Technical
Information Sexvice, U.S. Department of Commerce, Springfield, Va. 22161, U.S.A.
3 Hughes, M.A., 1962. Composition of ammoniacal liquors. TIL Analyses of the organic
bases by gas chromatography. J. Appl. Chem., 12: 450.
4 Bark, L.S., Cooper, R.L. and Wheatstone, K.C., 1972. The determination of organic
bases in carbonization effluents. Water Res., 6: 117.
5 TesaHk, K. and Ghyczy, S., 1974. Separation of pyridine bases of coal tar light oil by
means of capillary gas chromatography. J. Chromatogr., 91: 723.
6 Vymetal, J., 1972. Die Rektifikationstrennungder Pyridinbasen aus den Produkten
der Hochtemperaturverkokung der Steinkohle. Erdoel Kohle Erdgas Petrochem.
vereinigt mit Brennst. Chern., 25: 537.
7 White,C.M. and Newman, J.O.H., 1976. Chromatographic and spectrometric investiga·
tion of a light oil produced by the Synthoil process. U.S. Energy Research and
Development Administration, Pittsburgh Energy Research Center, PERC/RI·76/3, 20
pp. Available from the National Technical Information Service, U.S. Department of
Commerce, Springfield, Va. 22161, U.S.A.
8 Drushel, H.V. and SommerS, A.L., 1966. Isolation and identification of nitrogen compounds in petroleum. Anal. Chern., 38: 19.
9 Bhattacharya, R.N., 1968. Chromatographic separation of tar bases. Indian J. Techno!.,
6: 279.
10 Akhtar, S., Mazzocco, N.J., Weintraub, M. and Yavorsky, P.M., 1975. Synthoil process
for converting coal to nonpoJ1uting fuel oil. Energy Communications, 1: 21.
11 Sternberg, H.W., Raymond, R. and Schweighardt, F.K., 1975. Acid-base structure of
coal-derived asphaltenes. Science, 49: 188.
12 Brown, D., Earnshaw, D.G., McDonald, F.R. and Jensen, H.B.;1970. Gas-liquid chromatographic separation and spectrometric identification of nitrogen bases in bydrocracked shale oil naphtha. Anal. Chern., 42: 146.
13 Hart, H. and Kosak, J.R:, 1962. Mechanism of rearrangement of N-alkylanilines. J.
Org. Chern., 27: 116.
14 Ogata, Y., Tabuchi, H. and Yoshida, K., 1964. The hydrogen halide·catalyzed rearrangement of N-methylaniline to 0- and p-toluidiries. Tetrahedron, 20: 2717.
Chromatography
~®wD®w !;I
GCIMS Analysis of Trapped lC Peaks
F. W. Karasek, Chemistry Department,
University of Waterloo, Waterloo,
Ontario
The most positive identification of
the compound present in either a GC
or LC peak is provided by obtaining
its mass spectrum.
Although the
instrumentation and techniques for
directly interfaced GC/MS analysis
are well developed, the same situation is not true for LC/MS analysis.
The few instrumental systems now
available for directly interfaced
LC/MS analysis are complex, expensive and relatively untried.
Both the sample quantities and sensitivities of the HPLC and GC/MS
syste ms are in the proper range to
permit trapping of selected LC peaks
for direct injection of the trapped
solution into a GC/MS system. The
type of results possible are illustrated
in Figs. 1 to 3.
'RRAHETER SET: 1
I~JECT
t5f.2
EH~
2]
OF RUN
HHPHTHALEHE
1
a9:e~
21:19:83
YOLo 19 UL
CONe: 167.9 HG-UL
3J STEWART MAR-22-73 23:99
FLO~ RATE: 2
FLOW HODE: QI
NO: 1
PRESSURE: 1921
COLUMN: SPHERISORB ODS, 19 UM
CHAN 1: SP8219. 9. 16 AIJFS
MOBILE PHASE: TERNARY
A solution containing 168 ng/ III of
naphthalene in methanol was run on
the SP 8000 HPLC instrument to give
the LC peak shown in Fig. 1. To
determine the lag time interval between the detector response for the
LC peak and the arrival of the peak
at the fraction collector, successive 1
ml fractions of the eluent were
\rapped every 30 seconds throughout
the run. Each fraction was analyzed
for naphthalene by injecting 5 III
samples into the GC/MS system. The
presence of naphthalene was first
detected in the 120-150 second fraction, indicating a lag interval of
approximately 10 to 15 seconds for
the LC peak.
t1ClSILE PHASE FILE 2
TH1E
WATER HEOH
e. e
lB. B
45. B
45.0
CH1CN
1B. e
10:. B
45. Ij
45.9
Fig. 1. This LC peak was obtained for a naphthalene solution containing
168 ngAlI using the SP 8000 HPLC instrument. The 120-150 second
fraction was trapped for GC/MS analysis.
SPEctruM
ij
353
File tYF='€ =
2
B(\$E" Peo,k = 128.2
SQJ~~l~ Identification #
Nl.lfi lb€l- of peaks dete'etect
B('I.se F"2.:tk Abl.mdo.t"IcE' =
------------------,--------"
..
393
43
247
Retention TiM>? (MIn.)
Seo.flned fr')I"j
4(1 to
T>:.t!J.1 Abl..mdctnce =
=
2.6
5(11)
735
_----------------------------------------------------
The mass spectrum shown in Fig. 2
was obtained for the GC peak in a
GC/MS run for a 5 III injection of
this fraction without concentration.
The naphthalene was easily detected
and identified via this mass spectrum
obtained at the 1.7pg/lllconcentration
level present in this trapped fraction.
I
·1···'. •·
I:
Higher sensitivities can be achieved
using the selected ion monitor (SIM)
mode of the GC/MS system, since the
mass spectrometer continuously monitors only a single ion rather than
scanning through a broad spectrum.
The data for a SIM analysis of this
same fraction using the base ion for
Fig. 2. This mass spectrum was obtained for a 5 !-II injection into a GC/MS
system of the trapped fraction indicated in Fig. 1, using the normal
repetitive scan mode of the GC/MS system.
10
Chromatography
role 128.1 ION
Steve Bakalyar Senior
Scientist at
Spectra-Physics, has done extensive
studies on solvent interactions in LC
systems during the development of
the SP 8000 Liquid Chromatograph.
John Rock now LC Marketing Specialist at Spectra-Physics, previously
headed the development of the
SP 8000 Liquid Chromatograph.
3. The selected ion monitor (SIM) mode of the GC/MS system using the
m/e 128 ion of naphthalene gives picogram detectability for the
120-150 second trapped fraction.
at m/e 128.1 is shown in
From the integrated value of
area counts achieved for this
it can be estimated that
no."H·ho'an
is detectable in the LC
pff'l""nt at the 1. 7 pg/ III level. Since
instrument uses a 10 III
injection loop, this corresto a concentration of 170 pg/1l1
original sample. With such
sensitivity even minor compoin LC peaks can be successfully
iI".niHfi,.iI via their SIM mass spectral
this procedure.
working with complex organic
this procedure may be used
to pre-separate groups
cplnp,oUllds for subsequent GC/MS
An analytical method can
for both speed and
in analyzing for selected
which are suitable for
a GC/MS system. This
mp'aslses a large number of com-
Instrumentation Used
The instrument employed for the
HPLC work was the Spectra-Physics
SP 8000, a microprocessor-controlled
high performance liquid chromatograph.
The column was 'a 25 cm
Spherisorb ODS (10 Ilm)column. The
detector was an SP 8210 UV absorbance unit.
The GC/MS/Calculator system used
was the Hewlett-Packard HP 5992A
equipped with a flexible disk data
storage unit. This instrument can
operate in each of two modes. The
peak finder mode has the mass spectrometer scan the 40 to 500 amu
range repetitively, and only the mass
scan occurring at the top of each GC
peak is stored. In the SIM mode the
instrument is tuned to the diagnostic
ion specific to the compound of interest, giving a very high detection
sensitivity.
,...,....
Rod Mcllwrick
Technical Support
Chemist for Spectra-Physics located
in Darmstadt, now works with customers in Scandinavia, Eastern Europe,
and the U.S.S.R.
Dr. Frank Karasek
Professor of
Chemistry at the University of
Waterloo, works with a wide variety
of spectroscopic instruments to conduct environmental analyses.
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C2~0.'\ /?;;.~r;:.
\l.
.
o Price
IO:-irzED
energy of the electron hcam i~ dl..'krmincd hy 'tile P(li..::i;!;;]
drop between the exit grid of the Fu:'\ sy~tt:m ~It ,2;ro\;nd
potential and the filament, which is continuOl.sly \-ariai)k
can he ,eparated according to their
PAHTICLES
nlass/chargc ratio by 111('41ns of dctlcxion in a n1agnctic field in
conventional mass spectrometry.1 Other techniques have
been developed over the past decade for the separation of
charged species,' including non-magnetic linear time-of-flight
mass spectrometry.'
Wiley and McLaren' showed that the velocity of a charged
particle under the influence of an accelerating voltage is
inversely proportional to the square root of its mass/charge
ratio. This is the basis of operation of the non-magnetic
linear pulsed time-of-flight mass spectrometer, which can be
described with reference to Fig. 1.
The ion source has a five-grid electron-beam system of
Fox-type' in which the electron beam is collimated by a
series of narrow slits at the centre of the. grids. The energy
of the beam can be controlled by retarding potentials on
the grids. The operation cycle is started at 100 fLS or 10 fLs
intervals, depending on the operational frequency, by a stable
oscillator. The electron beam is normally kept from entering
the ionization region by a negative bias on the first electron
control grid above the filament.
At the beginning of a cycle, a very short (0.25 J1-s) positive
pulse is applied to the first electron-control grid, thus permitting the electron beam to pass· through this grid. The
between
a and -100 V.
A magnetic field created by simple
bar magnets collimates the electron heam to impinge upon
the trap anode across the source. A sample of gaseous
molecules, introduced via one of the various inlet systems
available, are ionized in the electron beam. During ionization, the elements surrounding the ion chamber arc at ground
potential. Immediately after the decay of the electron-beam
pulse, the ion-focus grid is pulsed to about -270 \'; the
duration of this pulse (2.5 fLs) is sufficient for all ions of
interest to be drawn out of the ionization region and to pass
into the accelerating region beyond the focus grid. A
2.8 kV potential on the first ion energy grid accelerates the
ionized sample along the length of the flight tube which is
nominally a field-free drift region.
Inside the flight tube there are pairs of horizontal and
vertical deflexion plates that position the most intense part of
the ion 'beam' on the cathode of the detector. Two tubular
ion lenses assist this focusing action. In the latest instruments, the distance between the ion source and the electron
multiplier detector is 200 em.
Ions separate according to their various rIl/e ratios as they
drift from the source to the detector under equal energy pulses.
Ion
[}] focus
grid
I I ,
anodbI I
Trap,
molecules
beam
grids
=
=
filament
~!
1
Ml
I
I
I
I
I
I
I
1'---:'
1
I
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I
I
I
Sample of gaseous
Electron
I
2..W
Horizontal
aeflexion
0
0
I
l
\ I1
\
lon
r
energy
Pumping
grid
sy~tel1l
I
I
Ion lenses
Detect.or
:.;:;.
I
Ycn.icai
dcti(;x:on
Coll,;,mating
magnets
Dr Price is a lecturer in the Chemistry Department, University of
Salford, Salford, Lanes.
Fig. 1.
meter.
The basic mode of operation oj the: time-oJ-Jljg/ll masS spectro-
255
Trigger
pu:sc
COincider"
G,-,tc :L
?:..:sc
J
c:rCJi(
-,
With ion
accelerating
pulse in
G:l':C
j
pulse
circuit
Channel I
ion sourCe
!
m
1
Oscilloscope
anoce
~I F i\'
Q
I
'
I
I
~
,"","
Cnannel 1.
.,..jl'------'-
.L.
d
Recor er channels
~
I
Ions
,
I
~l
Oscilloscope
j
Cathode
Fig. 2.
o
0
Multiplier magnets
A magnetic electron 11Iultiplier with a
multi~c1zanllel
time-gated output.
The lighter ions arrive at the detector first and as successively
heavier ions reach the detector the time between bunches
becomes less because their velocity is inversely proportional
to the square root of their mass.'
If the ions before pulsing were found at rest and if they
were all in a plane parallel to the ion-focus grid and ionenergy grid, then almost any method of drawing the ions out
of the source would provide infinite resolving power.' In
practice the ions vary in initial position and velocity, so that
the resolving power of the time-of-flight mass spectrometer
is dependent upon all the ions produced in a given cycle
simultaneously traversing the accelerating grid of the ion
source. The effect of variations in the initial positions of the
ions is reduced since the ions farthest away from the ion-focus
grid fall through a greater potential during the ion draw-out
period than do the ions nearer to it. By careful adjustment of
the ion-focus-grid pulse, it is possible to arrange that ions of
the same m/e ratio come together just as they reach the electron
multiplier detector.
Resolution can be further improved by making the final
velocities of the ions large compared to their initial velocities
and keeping the distance from the electron beam to the ionfocus grid small compared with the flight path-length. The
source can also be used for continuous ion generation or for
accepting a contimlOlIi:. bl:arrl. of externally-prod. iced. ions
whilst retaining pu]~ell ion w~thdrawal. hnprov~d resolulion
and sensitivity have been ohtaincd by moJil1c:ltion of the
source in the continuous ionizai..ion mode.
'The current
development of the tin1c-of-Hight n1ass spcctroDlcter is such
l
)
that resolving powers' of the order of 1000 have heen claimed
for some instruments with essentially mono-energetic ion
withdrawal.
The vacuum system associated with the time-of-flight mass
256
\
\... _,--------.----
--~--,-,---------------.------,----
spectrometer has been designed to provide efficient operation;
reducing the background pressure increases the usable
sensitivity. High sensitivity in a mass spectrometer requires
that extraneous gases are eliminated. High pumping speed
is needed for the large gas throughputs required in fast
reaction studies or molecular beam work with differential
pumping for continuous sampling from high pressures.
Detector and readout systems
With the high rate of scan of the time-of-flight mass spectrometer the requirements for the detector system are extremely
exacting. The detector must have a very high gain capable of
detecting single ion impacts. It must have very fast rise-time
,which responds to closely spaced ion bunches. Frequency
responses beyond 100 Mc/s are required.
The magnetic electron multiplier< with a multi-channel
time-gated output shown in Fig. 2 operates in a magnetic
field; electron paths are well defined by the combination of
magnetic and electric fields. Ions strike the cathode, with the
energy given them by the accelerating voltage (usually about
2.8 k V) and dislodge secondary electrons which are directed
to the plates of the multiplier. These electrons describe
cyc10idal motion and the fields are adjusted for optimum
cycloid dimensions. Voltages are adjusted so that equipotenti::.:.l lin.es. ;;u.; ..... i:'l..x:ted down inH) the: h)\\'cf Jynod\! st:ip_
Electrons :-:>uikiag the glass strip d~slo.lg,> on averag-e 1.35 :::
0.05 ~cconJary l..'kctfons for each prinlclry su +0 to 60 cyclollis
pnH:ucc a gair:. ::;n:;ltCl' than lOG.
The stability a;1J gain o~ the mu1ti;;1icr is. a function of the
voltages appli-.:d to the elclncnts Dl till.: Inuitiplil:r. '1'11(:
stability of the multiplier power supply is rated at 1 part in
100 000 thus ensuring constant multiplier gain, which gives
stability and accuracy to the time-of-flight mass spectra.
_~·\t tIle output end of the ml;]t.ipl;cr. the cllrrent rllJ~('S P;l%
into th~ gating region. If a pul~e i~ not divcr:.cu il1i.() dliC utthe <lnaloguc gate ch;lnncls it proceeds to the O:';;C;'i]o:-;cope
anode which dl,:liVl.:rs (;IC current to tIlt_: ():;c~jll).'~C(il,J\..". Tll~
load rcsi:.::tancc across thc oscilloscope is 100 ohm
ground,
thus it produces a n1 V output signal. If one ot" the pn.:ccJing
t(;
gate electrodes is pulsed negatively by approximately 150 Y,
it diverts the elcctron beam into that channel. The gate is
maintained at the negative potential only long enough to
collect the current pulse due to a single Inass peak tinring each
cycle of the instrument. The gating is rcpeated on each
cycle and the series of current pu1scs due to a particular mass
are delivered to the input of an electrometer circuit. The
current pulses arc then integrated, amplified and finally passed
to a recorder in analogue form.
Scanning is achieved by changing the time at which the
gate electrode is pulsed relative to the ion-focus pulse Juring
cach cycle. Several aspccts of this gating feature of the
multiplier arc important. Up to six channels arc usually
fitted. This means that up to six peaks may be monitorcd
simultaneously, or up to six scans of the mass spectrum may be
performed simultancously, or any combination of thesc with
the limitation that the same peak cannot be gatcd into any two
channels concurrently.
To permit spectrum scans faster than 1.5 s the anal~gtie
scanner units can be set to scan up to six parts of the mass
spectrum concurrently. For instance, the scanners can be set
to scan the ranges 1-12, 12-35, 35-90, 90-150, 150-250,
250-400 atomic mass units and can be started simultaneously
with one switch. This finds useful application in monitoring
gas chromatographic peaks which frcquently last about 1 s.
It must be pointed out that the maximum usable scan rate is
limited only by thc statistical variation in the number of ions
of a particular mass reaching the detector during the period in
which that mass peak is being scanned. The smaller the
peak the lower the scan rate must be to obtain a usable peak
shape. Unlike the magnetic deflexion mass spectrometer, the
rcsolution of the time-of-flight mass spectrometer is not
degraded during very fast scans within the rise-time capabilities
of the detector circuits and recorder system.
A peak-eliminator technique has been developed to cope
with cases, such as gas-chromatography effiuent-identification,
\\']1l'rc one brg-c r~".:k d,l\.' to 11-,(" C;\rTJ(>r :'::;1::' \\"0'0:1d ~:ltllratc the
l1;ldtipli~:r anti give a non-JiiH:ar g~.in. ;\ pldsing circuit has an
:ldjust;lhic rime-deby cap:lcili\'cly cOl~pl\'\l 1() thl.: Z pkltc at
t:;c j'ront end oj thl" llHillip;ivr. A Il('g:l:I\'\..'-glJing pllIsc is
~pplicd to the Z pl,lt"c, clitting oiT tlH.: c1c:ctron transit to the
n";l:ltipiicr for the width of a Ina;.;:-:; peak. The sckctisity is
such that it can attenuate by 1(J-- 1; mass 40 (i.e. argon) without
affecting masses 39 or 41. This extcnds the practical dynamic
r.mgc of sensitivity in gas chromatograph ic cfHllent identifica-
tion hy X 10 to X WO. Tilc development of a total-output
integrator' cnables the time-of-flight mass spectrometer to be
uscd as the detector as \Yell as the identifier for gas chromatography.
The mass spectra obtained from a time-of-flight mass
n:-.cil\(J~c(iP~ ~Cr'ctn, or
the signals from the analoguc SC;l1"'.Iwrs C;in he ;-vcnrckJ. usir.g a
potentiometric or oscillnt::raphic recorder. The oscilloscope
output has sufficient sensitiyity and frequency rc:-,ponsc for the
mass spectrum produced on each cycle to llc rcaJ as a separate
spectrometer arc either displayed on an
entity.
The oscilloscopc output is used for short-time du ration
studies, total spectrum monitoring, and for quaEtatin~ work.
The oscilloscope trace is photographed for permanent records.
In the case of fast reactions a Jrlln1 C;1ITICra is used in order to
photograph successive mass spectrum traccs as little as 50 fLs
apart. A less expensive technique for following fast reactions
has been developed by Lincoln. 8 The analogue scanner
output has a longer response time than the oscilloscopc but has
greater sensitivity and accuracy. The ol.,ip"t is recorded
using an oscillographic recorder with a frequency response
greater than a 1000 cis. Normally six galvanometcrs are
used in the oscillographic rccordcr so that it is advisable to use
a model that gives easy trace identification. Several galvanomcters can be set up to scan the same spectrum at different
levels of sensitivity simultaneously by placing a suitable
preamplifier between the analogue system and the recorder.
This is very useful in survey work. The time taken to scan
a complete spectrum can be varied from 1 s upwards" the
longer the scan time the more accurate the spectrum.
Applications
The applications of a mass spectrometer are determined by the
range of sam pIc inlet systems that can be uscd. All the
/
_:_ •..1
The time-oj-flight mass ~-peClrollleler at the University of Salford, a Model 14/107 supplied by Bendix international, l.Ve,u ':Iorh.
_ _.-1I
:'l)r -::'>\.' \::~:I~::,1:i\'(' l",ami;1a:inl'l
of"
111;111\·
c()mpnt!l111-".~I.ll):;1
~;l:: ,,~rl iH': u~;..'ll ill ~kl'Tml1lt' tkCOJlljlO>HHI:i
1t'lll!,ITatllrt.'S
;.1lld pnJdilCi:--, (;0111kc 1 :, u~cJ it for mass f-p<..'ctroHH,:tric tLl.:rll1<ll
1;
an;';y:.;j",.
Saiiii)lv:-; of ,Qa~('s ~lnd \'olatik liquid~ can hc in~rndllc<..'d yi;j
Ll",t n..';h;LioJ) chamber inlet (F,:r.: . .3). :\ IlCaLcJ Ill0kcul:.ir-lcak
l11lct-syskm:) is ~l\"aiJahle for direc't coupling to the tlrnc-ofHight. mas~ spectrometer. rrhis enahle:-- t h~ analysis of gas
sampks;lt 1 TdlT cm 3. An inkt system ha::. al.::.o bl~cn deyelopeJ.
for continuous sampling and analy~is of gases from seH.-ral
atmospheres pressure to 10~'1 'rorr. a 'fhi::; system has found
application in puhnonary research and atlnospheric analysis in
<l
simulated space cabins.
The tin1c-of-flight 111ass spectrometer has been used to
m~a~un:: flrp(,!1ri1nc~ and innj~mion por{::Dtiz.1s or many spcci~:s.
'Typical examples are difluoroc(lrh~nl'~.lj; ihiois,Ji' cth\-l"ll:..:'
sulphide and dhylenilninc.1 8 The instrument can al;o h::
used for nq,!;:Itivl' ion studicsY~ IOI1--mokclllc rcactions . .';In
be stlldied \\'ith the CO!l\'l'lllional limc-of-ili,:.!hl Ill;\:-.;.; :--p,'l';r,)111ctCi" ion sOllrce. 1H Danloth:) h,lS dc\'c!oj1\· . . l a rnodilj,'d
source which permits accurate mcaSUrl':il1cnt of the prc.sSllrc
in the interaction region. Among the rt.'acti(Ji1s inVl'stig;1;C.J
1\';' +I-1 z,:w and Dt + CH 1.21 Schllier ~lnd Stl~hcr:!:! h.l\'c
investigatcd ionization by meV-range pr()lOn~ and k,,\'-rallL(e
electrons. rrhis gives a cornparison of thc ionization proccsst.'s
produced by dissirnilar mass particles at silnilar yc1ocitics.
Hunt~:1 has used a time-of-flight IT',ass spectrometer, I11o\litll'J
by several sets of retarding grids in the drift tuhe, to stlldy
D1etastahle ions~ By retarding the ions at different time'S in
their tr;]nsit from the source to the multiplier, the n1eta~table
arc
Kry
A. Quartz viewing window
B. Shuth.'f
-
C. lon source
D. Fast reaction chamber
E. Gold foil
F. Shutter
Fig. 3.
G. Fixed molccular beam baffle
H. High vacuum valve
r. Kn'udscn cell
J. Heat shields
K. \Vatcr-cooled heat sink
L. Pumping systcm
The fast reaction chamber and Knudsen effusion ce"fl.
samples must be converted to a gaseous state before they can be
analysed. Due to its open geometry, the time-of-flight mass
spectrometer can easily accept a wide range of inlet systems
and can consequently be used to study a large range of different
problems. Several inlet systems can be simultaneously
coupled to the spectrometer, although they must be used
individually.
The time-of-flight mass spectrometer can be used for
conventional mass spectrometric studies, e.g. qualitativeD,lo
and quantitativel l analysis, measurement of ionization and
appearance potentials.12 The resolving power is between
300 and 400 for currently available instruments. Improvements in instrument design may raise the resolving power to
the region of a 1000.
Solids and liquids of low volatility are analysed using a direct
inlet system." This operates at temperatures from ambient
to 750°C at pressures from 10-· to 10-' Torr*. The spectral
cracking patterns obtained are in general agreement with those
published in the API tables.l3 Most of the patterns in the
API tables were obtained using magnetic mass spectrometers
with ion sources operated at a nominal 250°C. Thc time-offlight mass spectrometer ion source usually operates at a
much lower temperature (50°C) and some slight diJ-ferences
i:l tile' patterns will he notic~. :d .1101.: to this factor. Or'1.,'!'aLiojj of
the iO:1 source at 50 "C has t;"c <'l.I,iYantagc that iila~s :--pt'C~ ;-;11
crackin~
patterns may he o;)taincd from compount.~~ of lo\y
thl'rIl1al stability. If dc~irn;, ;;~l~ ion SOUit,.:e can lw 0"i)era;:cl.l
at temperatures up to 200 'C by placing a hake-out o\'ca
arounu the source. ~rhc direct inlet system ha~ ~h,Tn uscJ
.. In SI units, pressure is defined in nc'wtons per square metre,
= kg m-1 s -2 = J D1- J • A 'I'orr is defined by the
expression (101 325/760) N m-'.
i.e. N m- 2
25S
lifetimes can he bracketed if they arc in a suitable range.
IVlany workers have used a time-of-flight Inass spectrometer
to monitor the species present in plasmas and nan1cs.
O'Halloran and Fluegge 2' have sampled ions and!or neutrals
from plasmas at 11 000 K. iVIilne,25 using line-of-sight
sampling of neutral species from flames, reported the presence
of polymers of the common air constituents. Scheurich,
sampling positive and negative ions from flames, has obtained
profiles of ions from different areas of the flames.'"
Special techniques
The main stimulus for the development of the time-of-flight
mass spectrometer is the value of its extremely rapid scanning
rate to study fast reactions; and the special inlet systems which
can be fitted to it.
.
Fast Reactions. Because of the highly reactive character of the
species being analysed, several requirements must be fulfilled
for the satisfactory mass spectrometric analysis of fast reactions.
The sample must be introduced in such a fashion as to minimize wall collisions and also collisions between the sample
particles before ·ionization. Reactions are studied in the fa"t
reaction chamber shown in Fig. 3. Continuous samplirg
takes place via the small pinhole at the centre of the thin golLi
foil separating the reaction chamber from the ionization
region. This alignn1cnt Iuinin1izes the time lag between
s'll1'p;in,o: ;,\1id inilii";1tion, thus rcdllci;i,~ 1:.(' 'Pf)SS;; . . ;~;\\" of ~'l~e
Spl'~I'_'S
;-'",1 11 t),;,,'1..;
unlkr~()ing-
Dacl\gTo'I.~d L!:;iSI::-- <lr~
ing
SY:->;,,'ilL
FIJI'
k('pt to a
rl.'~l(l!l)i~'
;r::li"\;j,ilil.1
;)C:'lllt'
i . y;:
J'bsh pIlotoiy:.;.i::-- ;-':t1iliil..'s,
;,>I,;Z.::il\;:.
\','r:' j';\::-l
;hc
\'L<;
Pll:j','()i..:;1';)
i~
~';~("r~\, <l.h:::nj';li.i{l~; \);'
<ill1i
I\.~<:d-'; u;nl ;;;i~
in:--crtn: j;',;u 1h(' I.+,;liiibcr to illlnr(l\I..' li;l,'
the rcaelill;,.!; sysknl.
Kistiak(;\\si\.}
system to study the flash' photolysis oj" k1."tcne anLl nitr(),~t.Tl
dioxide.
Meyer2S has published the descriptIOn of an improved
system for studying flash photolysis. The system permits
6()-70 per cent absorption of the flash energy by the reactants.
Bimolecular reactions with rate constants approaching
10' m" mol- 1 S-l can bc studied. The detection of the
hydroxyl radical in the nitrogen-dioxide-sensitized reaction
between hydrogen and oxygen was the first time that the
combination of flash photolysis and time-resolved mass
spectrometry had been used for the detection of a transient
reactive free radica1. 29
Several workers have investigated flash pyrolysis of solids.
Lincoln 30 describes a technique that has been used to study
flash pyrolysis of various polymers and cadmium sulphide.
Friedman 31 has investigated the flash pyrolysis of phenolformaldehyde. The technique has also been used to identify
species vaporized by the action of laser beams on solids.""
Analyses of shock waves can be facilitated by making the
shock tube an extension of the fast reaction chamber. \Vith
such an arrangement the pinhole inlet samples the centre of
the shock wave sO that the disturbing effects of the shock tube
"'alls arc largely eliminated. Modica33 has evaluated the gas
sampling methods behind reflected shock waves. Bradley and
Kistiakowsky have studied the thermal decomposition of
N,03' and also the polymerization and oxidation of acetylene. 35
The thermal decomposition of chlorine molecules in shock
tubes has been investigated by Diesen and Felmlee."·
The scanning rate required to follow the kinetics of a chemical reaction will depend upon the reaction rate. The time-offlight mass spectrometer can be used to monitor reactions
with lifetimes greater than 1 s by ch~ice of suitable scan rates.
Heiklen and Johnson37 have studied the photo-oxidation of
ethyl iodide and Waeks38 the production of methylamine from
azomethane.
Knudsen cell. Rauh39 and \Nhite 40 pioneered high-temperature thermodynamic studies using a Knudsen effusion cell
coupled to a time-of-flight mass spectrometer. The cell
(Fig. 3), now commercially avail~ble, can be used for temperatures as high as 2500 K. Volatile species from the sample
under investigation effuse from the Knudsen cell and are
immediately ionized as they pass through the electron beam.
The ions thus formed are detected and used to identify the
volatile species. Heats of vaporization at high temperature
can be determined by measuring the dependence of ion
intensity on the Knudsen cell temperature, since the ion
intensity is dependent upon the partial pressure in the IOn
source. l l
Ackerman and Rauh41 have studied the tungsten-oxygen
system in the temperature range 1300 K to 1500 K and
showed that the thermodynamically important vapour species
are W,O'2' W a0 9 , WaOs and W 20 •. Other systems studied
are uranium-oxygen,42 beryllium fluoride 44 and boron oxide.40
Gas chromatographic effluent identification. The coupling of
a gas chromatograph, which can separate complex mixtures,
to a mass spectrometer, which can identify the components in
the effluent stream, has obvious advantages. 4s The time-offlight mass spectrometer is suited for this application by virtue
of its extremely rapid scan rate; the predynode gating, which
preYents electron multiplier saturation by carrier gas ions; and
its ability to handle high molecular weight species. Modification of the mass spectrometer to measure wtal ion current?
enables the spectrometer to be used as a quantitative as well
,.
as a qualitative detector for the chromatographic cmuentstrcam.
Eberr" has designed an inlet system that permits detection and
identification of components in concentrations as low as
0.1 p.p.m. even if they are only partially separated by gas
chromatography. Capillary chromatographic columns have
also been sllccessfuliy couplcJ to a time-oF-flight mass
spectrolnetcr.4i
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!.
2.
3.
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.
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.. I'"
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t
I,:·
t.
t.\
i
259
r!
/
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