PB 200268
IMPROVED INSTRUMENTATION FOR DETERMINATION OF EXHAUST
GAS OXYGENATE CONTENT
Marvin L. Vestal, et al
Scientific Research Instruments Corporation
Baltimore, Maryland
March 1970
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DISTRIBUTED BY:
National Technical Information Service
U. S. DEPARTMENT OF COMMERCE
5285 Port Royal Road, Springfield Va. 22151
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CRC-APRAC-CAPE-ri-68-3
);; TECHNICAL REPORTS
fifle and Suotule
Improved Instrumentation for Determination of Exhaust Gas Oxygenate
Content — SRIC
'/. AuthorTs)
Marvin L. Vestal; Allan G. Day III; and William H Johnston
.
3. Recipient s Catalog No
5. Report Date
January 1971
6. Performing Organization Code
8. Performing Organization Rept. No.
SRIC 70-6
9. Performing Organization Name and Address
Scientific Research Instruments Corporation
6707 Whitestone Road
Baltimore, Maryland 21207
12. Sponsoring Agency Name and Address ~
Coordinating Research Council, Inc.
30 Rockefeller Plaza, New York, N.Y, 10020; and
Environmental Protection Agency
.., Durham, North Carolina,
10. Project/Task/Work Unit No.
TT. Contract?Gra'nFNo"
13. Type of Report & Period Covered
Final ~ March 1970
15. Supplementary Notes
14. Sponsoring Agency Code
CRC-APRAC-CAPE-11-68
16. Abstracts
This report deals with the prototype aldehyde analyzer built by Scientific Research
Instruments Corporation (SRIC). With the instrument, called the AUTOSPECT, SRIC has been able
to simultaneously analyze engine exhaust for 9 different oxygenates. Alothough the AUTOSPECT can
be calibrated to determine almost any desired oxygenate, the current instrument is set to determine th
9 most prevalent aldehydes believed to be present in exhaust gas. Work on the AUTOSPECT is aimed
at procedural improvements and reductions to a practical portable oxygenate analyzer. In addition
to unburned hydrocarbons, carbon monoxide, and oxides of nitrogen, the exhaust of automotive
vehicles contains small amounts of partially combusted materials known as oxygenates. The
most prevalent class of oxygenate components are known as aldehydes. Aldehydes are odoriferous,
in some cases irritating to the eyes, and potential sources of photochemical reactions conducive to
the formation of smog.
17. Key Woros and Document Analysis, (a). Descriptors
Pollution Control
Exhaust Gas
Oxygenates
odor
smog
photochemical
17b. IJc-iti'lors/O,- n Cnti'id Terms
AUTOSPECT
7c. COSATI Field/Group
21E; 13B; 21D
8. Distribution Statement
Re leasable
l
to the public
19. Security Class(This Report)
UNCLASSIFIED
^.Security Class. (This Page)
UNCLASSIFIED
21. No. of Pages
56
22. Price
$3.00
NDS-b67(1-70)
-------
Report No. SRIC 70-6
IMPROVED INSTRUMENTATION FOR
DETERMINATION OF EXHAUST GAS
OXYGENATE CONTENT
Contract Nos. CPA 22-69-40 and CAPE 11-68
Annual Report March 1970
Marvin L. Vestal, Allan G. Day III,
and Wm. H. Johnston
With contributions from
C.E. Waring, F.W. Lampe, Gordon J. Fergusson,
J.H. Futrell, Austin Wahrhaftig, M.S.B. Munson,
Wm. F. Biller, Edward Kratfel, and Melvin Vanik
Prepared for the National Air Pollution Control Ad-
ministration, Durham, North Carolina, 27701,and
the Coordinating Research Council, Inc., 30 Rock-
efeller Plaza, New York, New York.
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TABLE OF CONTENTS
Page
TABLE OF CONTENTS ii
ABSTRACT iii
INTRODUCTION 1
EXHAUST GAS ANALYSIS BY CHEMICAL IONIZATION
MASS SPECTROMETRY 3
General 3
Apolication to Engine Exhaust Gas Analyses 9
THE PROTOTYPE SYSTEM 13
Chemical lonization Source 19
Direct Reading Mass Spectrometer 21
Sample Inlet System 23
Calibration 25
RESULTS AND DISCUSSIONS 27
APPENDIX 54
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ABSTRACT
Chemical ionization mass spectrometry is shown to be a
powerful general tool for the analysis of trace oxygenates in ex-
haust emission; it provides an analytical tool for trace components
over a dynamic range from 1 parts per billion ( ppb) to 10,000 parts
per million ( ppm) .
A prototype mass spectrometer employing the double focus-
ing principle of Mattauch and Herzog and featuring fixed ion-collec-
tors at integral masses over the mass range 1 to 128 has been con-
structed and is described. The fixed collectors make possible sel-
ection of any nine mass numbers in this range and provide full duty
cycle for each measured peak with instrumental time constant signi-
ficantly shorter than 5 seconds.
Theoretical aoplication of the method and experimental applica-
tion of the apparatus to the following oxygenated species in the con-
centration range of 30-1300 ppm has been made: formaldehyde,
acetaldehyde, acrolein, propionaldehyde, acetone, crotonaldehyde,
butyraldehyde, 2-butanone, n-valeraldehyde, 2-pentanone, 3-pentanone,
ethyl-butyraldehyde, and benzaldehyde. It is shown that in all cases,
analysis is feasible via the M + 1 ion ( protonated parent compound)
using reagent ion CH5 except that, with this reagent, interferences
occur for acrolein and crotonaldehyde necessitating the use of a
different reagent for these species.
-111-
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Calibration of the mass spectral apparatus for all the above
compounds has been made. Sensitivity of response for the various
compounds range from 5-108 millivolts per ppm of oxygenated com-
pound. With the exception of the larger aldehydes this response is
practically all due to the M + l ion. In the case of the larger aldehydes
loss of H2O from the protonated molecule, forming the M-17 peak,
is the dominant process.
Preliminary measurements were sufficiently encouraging that
the breadboard stage was skipped and a prototype research unit was
designed and built. Further work requires the redesign of one por-
tion of electronics, optimization of operational parameters, tests
in cooperation with Scott Laboratories on exhaust from a current
model automobile under cyclic conditions, tests for other compon-
ents such as NOX, exploration of negative ion reactions and design
of a simplified unit for optimum utilization by untrained personnel
and at minimum cost.
- iv-
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INTRODUCTION
The goal of this research is the development of an instrumental
method of analysis for oxygenated hydrocarbons in engine exhaust.
This method is based on the combination of the chemical ionization
technique developed by Field and coworkers combined with the direct
reading mass spectrograph developed at Scientific Research Instru-
ments Corporation ' . A bibliography of publications on chemical
ionization mass spectroscopy is given in the Appendix.
The theory of analysis by chemical ionization is presented in
Chapter II of this report. This theory shows the enormous enhance-
ment in sensitivity and specificity which is possible with chemical
ionization as compared to electron impact ionization.
The prototype system combining chemical ionization with the direct
reading mass spectrograph for the rapid, continuous analysis of oxy-
genated hydrocarbons in engine exhaust is described in Chapter III.
This prototype system employs a chemical ionization source which
may be operated at source pressures as high as 10 torr. Mass analysis
is performed using a. small Mattauch-Herzog type double-focusing mass
spectrometer. Fixed collectors are provided for each mass in the range
from 1 to 128 amu. Any nine of these masses may be monitored contin-
uously using nine independent electrometers and readout channels.
( 1) This method is covered by patent rights of Esso Research and Engi-
neering Company with exclusive license to Scientific Research In-
struments Corporation, Baltimore, Maryland.
-1 -
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Chemical ionization measurements have been conducted on a
series of the aldehydes and ketones identified by Oberdorfer as
the more abundant oxygenated species occurring in engine exhaust.
The compounds studied include formaldehyde, acetaldehyde, pro-
pionaldehyde, n-butyraldehyde, iso-butyraldehyde, n-valeraldehyde,
iso-valeraldehyde, ethyl butyraldehyde, acrolein, crotonaldehyde,
benzaldehyde, acetone, butanone, 2-pentanone, and 3-pentanone.
These experimental results show that the technique should be cap-
able of detecting most of the oxygenated hydrocarbons at levels from
300 parts per million down to 10 parts per billion in engine exhaust using
methane as a chemical ionization reagent. With the exceptions of acro-
lein and crotonaldehyde no interference from other compounds in engine
exhaust is indicated by our data taken in conjunction with the results of
other investigations described in the literature. In the case of acrolein
the crotonaldehyde some interference from hydrocarbons is encountered
when methane is used as the reagent.
(2) P.E. Oberdorfer, "The Determination of Aldehydes in Automobile
Exhaust Gas, " Paper 670123 presented at the SAE Automotive
Engineering Congress, Detroit, Michigan, January, 1967.
-2-
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Chemical ionization measurements have been conducted on a
series of the aldehydes and ketones identified by Oberdorfer as
the more abundant oxygenated species occurring in engine exhaust.
The compounds studied include formaldehyde, acetaldehyde, pro-
pionaldehyde, n-butyraldehyde, iso-butyraldehyde, n-valeraldehyde,
iso-valeraldehyde, ethyl butyraldehyde, acrolein, crotonaldehyde,
benzaldehyde, acetone, butanone, 2-pentanone, and 3-pentanone.
These experimental results show that the technique should be cap-
able of detecting most of the oxygenated hydrocarbons at levels from
300 parts per million down to 10 parts per billion in engine exhaust using
methane as a chemical ionization reagent. With the exceptions of acro-
lein and crotonaldehyde no interference from other compounds in engine
exhaust is indicated by our data taken in conjunction with the results of
other investigations described in the literature. In the case of acrolein
the crotonaldehyde some interference from hydrocarbons is encountered
when methane is used as the reagent.
(2) P.E. Oberdorfer, "The Determination of Aldehydes in Automobile
Exhaust Gas, " Paper 670123 presented at the SAE Automotive
Engineering Congress, Detroit, Michigan, January, 1967.
-2-
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EXHAUST GAS ANALYSIS BY CHEMICAL IONI/ATION MASS
SPECTROMETRY
General
Chemical ionization mass spectrometry produces ionic derivi-
tives of the compound whose analysis is required by a sequence of
reactions involving:
1. Electron impact ionization of a reagent gas.
2. Ion-molecule reactions of the reagent gas to produce a set of
reagent ions that are stable with respect to further reaction with the
reagent gas .
3. Reaction of the reagent ions with the compound whose analysis is
required to produce ions characteristic of this compound.
By contrast, conventional mass spectrometric analysis involves direct
ionization of the unknown by electron impact at pressures sufficiently
low that collisions of the resulting ions with molecules do not occur.
Thus if we denote the reagent gas by M, the stable reagent ions by
R , and the substance to be analyzed by X, we have the following
ionization mechanisms:
A. Chemical Ionization M + e —> M^ + 2e ( l)
Mi+ + M —> R+ + Ni (2)
R++ X —> X+ + R (3)
-3-
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B. Direct lonization X + e —> X: + 2e (4)
In these ionization mechanisms, Mj represents the set of reactive
ions of reagent gas M, N- is a set of neutral products that need not
concern us further, R is a reagent ion (or set of reagent ions) , X.
is the set of ions characteristic of the unknown and the reagent gas,
and X. is the set of ions characteristic of electron impact ioniza-
tion of the unknown X.
Note that in the case of chemical ionization the set of ions
X^ is produced without the simultaneous ejection of an electron that
occurs in the direct ionization of X to form the set of ions X- .
This is a very important difference in the two types of ionization
since the absence of a free electron in ( 3 ) imparts much more severe
energy restrictions to this reaction than is the case for (4 ). The
net result is that one expects the set of ions Xj to be a much simpler
set than X- ; i n fact, one can envision systems in which X^ is a single
ion, a situation practically never obtaining for the set X; produced
by (4). These expectations have been confirmed by many experi-
ments utilizing a number of reagent ions, so that there is no doubt
that chemical ionization mass spectrometry provides the analyst with
a much simpler mass spectrum to interpret. This greater simplicity,
in turn, leads to more reliable identification of the unknown compounds
present in any system to be analyzed.
-4-
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Sensitivity
In addition to simpler mass spectra, chemical ionization mass
spectrometry is a considerably more sensitive method of analysis for
compounds present at the part-per-million (ppm) level in a gaseous
mixture. Consider, for example, an unknown X present at a concen-
tration level of 1 ppm in some carrier gas. If we were to analyze for
X with conventional electron impact mass spectrometry we would en-
vision the following conditions:
Total ion-source pressure, P-y < 10 5 torr .
Concentration of sample in ion source, n-p = 3 x 10l
(molecules/cm3, for P in torr) .
Electron currents Ie < 1 ma = 10~3 amperes.
Electron impact ionization cross-section »|fl{,10 15 cm2.
( nominal value for 70 ev electrons)
Electron path in ion source, LiefSi 1 cm.
Detection efficiency, EsJl% (efficiency with which ions produced in
the source are transmitted through the mass analyzer and detected
at the ion collector) .
Under these conditions the total ion current detected is given by
IT= <^i nT IeLeE (5)
= 10"15( cm2) • 3 x 1011 ( cm"3) • 10"3 ( amperes) • 1 ( cm) . 10"2
= 3 x 10 9 amperes
-5-
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The total ion current for the unknown X present at a concentration
level of 1 part per million is given by
I - 10"6 IT = 3 x 10"15 amperes (6)
3v -L
This current is normally distributed among a number of fragment ions
produced by electron impact ionization.
The total ionization efficiency in the electron impact ionization
can, in principle, be increased by increasing the electron current, the
ion source pressure, or the detection efficiency. However, the max-
imum ion current which can be handled in most mass analyzers without
introducing non-linearity due to space charge effects is about 3 x 10 9
amperes. Therefore, little additional enhancement in sensitivity can be
obtained by increasing the total ionization efficiency.
In chemical ionization mass spectrometry the total pressure in the
ion source is typically about 1 torr. The intensity of the primary ioniza-
tion to produce the ions M^ according to reaction ( 1) is still given by
Equation ( 5) . As can be seen from Equation ( 5) above, much smaller
values of either electron current or detection efficiency can be employed
while still realizing the maximum total ion beam that can be utilized in
the mass analyzer. As a result, the small orifices required in the
chemical ionization source to obtain high pressures do not limit the sen-
sitivity of the technique since more than adequate electron beams and ion
beams can be transmitted through the necessary orifices.
-6-
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In chemical ionization the reagent ion intensity is given by
i) (7)
where Im is given by Equation ( 5) , n-p is the concentration of reagent gas
in the ion source, 0"^ is the cross-section for the ion-molecule reaction
> 10 15 cm2), Lj is the average ion travel distance in the ion source
sfcl cm) . The exponent in equation (7) may be expressed by
= P2nT Li = 3 x 10<> PT L.
(8)
> 3 x 1016 ( 10"15) (1) PT = 30 P
where Pnn is in torr. Thus the ions initially produced by electron impact
(M^ ) are quite completely converted to reagent ions ( R ) if the ion source
pressure is a few tenths of a torr or more, and l( R ) f& Irp .
If X is at a sufficiently low concentration that its ionization by re-
action ( 3) does not significantly deplete the concentration of reagent ions,
it is easily shown that the ion current produced from the species X is
given by
(9)
Where n-^ is the ion source concentration of X ( molecules /cm3) , ff"3 is
the cross -section for reaction ( 3) , and Lj is the ion path length.
At low sample concentrations , ff"3 n,,.L^ « 1 , and equation ( 9) reduces
to
-7-
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Ix = IT .
10)
Experimentally, the value oftf^ Lj. _> 10 15 cm3 for many of the ion
molecule reactions involved. For an ion source pressure Pry,, and
a concentration of X of 1 ppm, Equation ( 10) becomes:
Ix = IT . 3 xlO16 PT . 10"6 (molecules/cm3). 10"15 (cm3) ( 11)
= 3 x 10~5 PT . IT
Comparing with Equation 6 we see that
Iv (CHEMICAL IONIZATION)
A
Ix (ELECTRON IMPACT)
•? ™
for the same total ion current, at the maximum operating pressure of the
prototype chemical ionization system ( 10 torr) chemical ionization is
at least 300 times as sensitive as electron impactionization in terms of
the ion current produced from a trace component for a given total ion
current.
Linearity
The ion current produced from reactive species X in the reagent
gas is given by Equation (9). At low concentrations the ion signal is
directly proportional to concentrations of X as given in equation ( 10) .
For Equation ( 10) to be valid within 5%, it is required that
-8-
-------
(J^ nx L,i <_ 0.05
(13)
nX < 5 x 1013 molecules/cm3
at higher concentrations a significant fraction of the reagent ions are
destroyed by reaction, and the exponential equation ( 9) applies rather
than the linear equation ( 10) . The implications of this analysis of
sensitivity and linearity is summarized in Table I.
Application to Engine Exhaust Gas Analyses
The specific problem to be solved in these analyses is to measure
oxygenated hydrocarbons at low concentrations in a complex mixture of
gases, the principal component of which is nitrogen. Nitrogen, the
natural carrier gas of this complex mixture, is not itself a suitable re-
agent gas for the analysis of oxygenated hydrocarbons. A suitable re-
agent gas must, therefore, be added to the mixture. At first sight this
might seem to involve a serious dilution of the sample. However, the
fact that molecular nitrogen ions will undergo charge transfer reactions
with practically any reagent gas we would envision easily circumvents
this difficulty. For example, suppose methane, having reagent ions
CH5 and C2H5 , had been added to the mixture as the reagent gas.
Any electron impact ionization of the nitrogen will be followed rapidly by
N2 + + CH4 > CH4+ + N2 (14)
and
N2 + CH4 —-> CH3 + + N2 + H (15)
-9-
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TABLE I. SENSITIVITY AND LINEARITY OF CHEMICAL IONIZATION
ANALYSES AS A FUNCTION OF TOTAL ION SOURCE PRESSURE
Pressure Minimum Detectable Concen- Maximum Allowable Concen-
(torr) tration ( parts per billion) ( 1) tration ( parts per million)( 2)
0.2
0.5
1.
2.
5.
10.
20.
50.
100
200
500
1,000
( 1) Corresponding to 3 x 10 15 amperes mass analyzed ion current for
trace species and 3 x 10 9 total mass analyzed ion current.
(2) For deviation from linearity < 5%.
150
60
30
15
6
3
1.5
0.6
0.3
0.15
0.06
0.03
7, 500
3,000
1, 500
750
300
150
75
30
15
7.5
3
1.5
-10-
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thus producing the same principal active ions of methane that will
follow electron impact ionization of methane. These ions will not
react with molecular nitrogen but will react rapidly with methane to
produce the normal reagent ions CH5 and C2H5 . Therefore, with
the exception of production of a slightly different ratio of reagent ions,
ionization of the nitrogen is equivalent to ionization of reagent gas and
there is no loss of sensitivity.
The basic nature of the oxygenated compounds to be analyzed
suggests that a particularly appropriate chemical ionization reaction
is that of proton transfer from a suitable reagent ion to the oxygenated
hydrocarbon, viz:
XH+ + C = 0 — >C-OH + X (16)
This reaction will occur readily for any hydrogen containing reagent
ion provided that
>
P(X)
where P (X) is the proton affinity of compound X. Of course, if the pro-
ton affinity difference is too great the exoergicity of the reaction may
TD ^ I
result in considerable fragmentation of the ion .^C-OH . Some frag-
mentation may be desirable for clarity of identification when mass inter-
ference occurs, but, in general, extensive fragmentation is to be avoided.
The choice of reagent gas is thus directed towards one having reagent ions
i
XH such that P(X)< P( . C = O) but withAP being as small as possible.
-11-
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Ideally, the reagent gas should produce reagent ions that
would not protonate other compounds in the gas mixture, such as
olefins, aromatics, paraffins, carbon monoxide, etc. Although
some selectivity in this regard is possible, complete selectivity of
protonation of oxygenated hydrocarbons will not, in general, be
possible. Hence, we must choose a reagent gas whose reagent ions
and product ion produced by reaction with other species present do
not by mass interference obscure the protonated forms of the oxygenated
hydrocarbons .
These considerations lead to the conclusion that methane, with
reagent ions CH5 and C2H5 (M/e 17 and 29, respectively) , is a reason-
ably good initial choice for a reagent gas for the oxygenated hydrocarbons.
The pertinent chemical ionization reactions are the following:
CH5 + C = 0 — ^C-OH + CH4 (3A)
+ C2H4 (3B)
+ + C2H6 (3C)
where, depending on the specific energetics, dissociation of the ionic
products may occur to some extent. Chemical ionization spectra for
methane reagent reacting with a number of aldehydes and ketones are
presented in Chapter IV.
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THE PROTOTYPE SYSTEM
The prototype system for analysis of oxygenated hydrocarbons in
engine exhaust gases consists of a chemical ionization source coupled with
a direct reading mass spectrometer. A heated gas inlet system is pro-
vided for interfacing between the exhaust gas sampling point and the inlet
of the mass spectrometer. A schematic diagram is given in Figure 1 of
the ion optical system for the combination of the chemical ionization source
with the direct reading mass spectrometer.
The mass analyzer employs the Mattauch-Herzog double focusing
geometry with a 6" radius electrostatic analyzer and a fixed collector for
each mass from 1 to 128 amu. This system was designed to be operated
normally in the direct reading mode in which a selected set of masses are
monitored continuously. However, provisions for electrically scanning to
mass 250 are included in the design and scanning has been used during the
initial exploratory research.
The chemical ionization source and differential pumping systems are
designed to operate at source pressures as high as 10 torr for efficient
chemical ionization of the oxygenated hydrocarbons of interest. With these
high ion source pressures, the pumping system maintains the pressure in
the ion source housing at a suitably low pressure (typically 10 4 torr) to
avoid electrical breakdown. The analyzer section is isolated from the
source section by a differential pumping slit and maintained at sufficiently
low pressure (typically < 10 torr) to avoid degradation of performance by
ion-molecule collisions in the analyzer. Sufficient testing has been accom-
plished to assure that the differential pumping requirements are met by the
-13-
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-14-
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present design. Heaters are provided for the chemical ionization
source and the glass inlet lines to allow operation at temperatures from
100°C to 350°C to minimize the anticipated problems in accurately sam-
pling the raw engine exhaust.
In Figure 2 a pictorial sketch is given showing the relative locations
of the principle components of the system. The ion source housing and
electrostatic analyzer housing are separately pumped by high speed
4 inch diffusion pumping systems located as shown in Figure 2. The
mechanical pumps backing the diffusion pumps are located in the bottom
of the cart supporting the mass spectrometer. This cart is mounted on
heavy duty casters and contains the complete mass spectrometer inlet
system and pumping systems. The electronics for the system are loca-
ted in a separate caster-mounted cabinet. The complete system is designed
to be readily mobile.
A photograph of the principal components of the chemical ionization
direct reading mass spectrometer is shown in Figure 3. These compo-
nents are contained within the vacuum housing shown schematically in
Figure 2 in the relative locations within the system shown in Figure 1.
A photograph of the completed prototype system is shown in Figure 4.
The electronics cabinet on the left in Figure 4 contains all of the readout
and control systems for the prototype unit. The cart on the right contains
the direct reading mass spectrometer with chemical ionization source and
the associated vacuum systems and sample inlet system.
-15-
-------
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-17-
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Figure 4. A Photograph of the Completed Prototype System.
The electronics cabinet on the left contains all of the readout and
control systems and the cart on the right contains the direct reading
mass spectrometer with chemical ionization source and associate
vacuum systems and sample inlet systems.
-18-
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Chemical lonization Source
The chemical ionization source was designed to allow both
chemical ionization and electron impact ionization with the prototype
system. A detailed diagram of the chemical ionization source is
shown in Figure 5. This source design is very similar to the con-
ventional Nier type ionization source for a magnetic mass spectro-
meter with several detailed modifications to allow its use for chemical
ionization.
The sample flows into the source through a hole in the repeller.
The repeller and electron trap are sealed to the source block by cera-
mic washers to minimize extraneous leaks of gas from the source.
The only substantial gas flows out of the source are through the orifices
provided for the entrance of the electron beam and the exit of the ion
beam. The electron beam entrance is a 0.020" diameter hole and the
ionization exit is a slit 0.002" x 0. 125". The electron beam is mag-
netically collimated using a 200 Gauss source magnet built up from
Alnico V bar magnets and soft ion pole pieces . The electron emitting
filter is a Rhenium ribbon 0.001" thick by 0.030" wide welded to 0.040"
diameter molybdenum. The other metal parts in the ionization source
are fabricated from Inconel 600 alloy and the ceramic insulators are
high alumina ceramic.
Heaters for the source and sample inlet lines are provided and
the system is bakeable to 350°C. The final slit in the ionization gun
is sealed to a baffle separating the source and the electrostatic analyzer
housings . This slit is 0.005" x 0.125" and constitutes the only sub-
stantial gas flow orifice between the two housings. These two housings
are separately pumped by 4" oil diffusion pumps backed by 140 litre
-19-
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SAMPLE
ION SOURCE BLOCK
MOUNTING PLATE
MAGNET POLE
DIFFERENTIAL
PUMPING SLIT
ION BEAM
MAGNET POLE
ELECTRON ENTRANCE
ELECTRON BEAM
BAFFLE BETWEEN SOURCE AND ELECTROSTATIC ANALYZER HOUSINGS
5.
Schematic Diagram Of The Chemical lonizarion
Source For The Prototype System For Analysis Of
Oxygenated Hydrocarbons In Eng ine Exhaust.
-20-
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per minute mechanical pumps. This differential pumping system
allows operation with source pressures as high as 10 torr while
maintaining the source housing at pressure below 10 3 torr and
the electrostatic analyzer housing below 10 torr. Total ion
beams in excess of 10 9 amps have been obtained with this ioniza-
tion source.
Direct Reading Mass Spectrometer
The mass spectrometer is based on the original Mattauch-
Herzog design of a double focusing mass spectrometer. The electro-
static analyzer is a 6" radius cylindrical condenser providing a de-
flection angle of 31.8°. This assembly is constructed of
Inconel 600 and high alumina ceramic insulators and is bakeable
to 350°C. An Alnico V permanent magnet provides a uniform mag-
netic field of 3500 Gauss across a half-inch gap. The Mattauch-
Herzog geometry was chosen because it is double focusing for all
masses simultaneously. The foci for individual masses fall in a
precisely determinable plane and the location along the plane is prop-
ortional to the square root of the mass. This view allows the precise
a priori determination of the relative locations for collectors for spe-
cific individual masses.
The principal unique feature of the mass analyzer is the multiple
collector array used as a substitute for the traditional photo-plate
detector in the Mattauch-Herzog geometry. This collector assembly
consists of separate detectors for each unit mass in the range from
mass 1 to mass 128 amu. A close-up photograph of the detector
assembly is shown in Figure 6. An electrical connection from each
collector is brought out through a vacuum feedthrough to a connector
-21-
-------
-------
panel. This connector panel, which is similar to a miniature
telephone switchboard, provides the means for selecting the masses
which are monitored. Ten output channels are provided each con-
sisting of a solid state electrometer mounted on the switchboard panel
and a meter readout located on the front panel of the electronics cab-
inet as shown in Figure 3. The full scale sensitivity of these channels
is continuously adjustable over a 1000:1 range from the front panel.
Individual mass collectors selected for monitoring are connected to
the input of one of the electrometers through a short jumper wire.
The unused collectors are grounded by the use of special grounding
clips provided. By merely rearranging the jumpers on this switch-
board the set of masses monitored can be changed to correspond to a
particular measurement.
Sample Inlet System
The sample inlet system for chemical ionization measurements on
exhaust gases is shown schematically in Figure 7. This system is
designed both for monitoring of actual exhaust gas samples directly
from vehicle exhaust and for calibration of the system using artificial
exhaust gases. The entire system through which exhaust gases pass is
surrounded by a heating jacket which will maintain the system at approx-
imately 150°C. An infusion pump and syringe is provided for controlled
injections of calibration samples into the gas flow. The gas flow rate
is monitored by a mass flow meter. A glass manifold is provided for
coupling between the main flow of exhaust gas and the chemical ioniza-
tion mass spectrometer. The glass manifold is designed to operate at
a nominal absolute pressure of 1/2 atmosphere. A small sample of
the exhaust gas stream is drawn off into this manifold through a fixed
-23-
-------
MASS
FLOW
METER
VENT OR
PUMP
I
MUST GAS-—*-
I
LJCATlMf* 1 A f* L* CT
nt-A 1 Irib JAOK tT
1
1
I
FIXED LEAK
• SYRINGE
INFUSION PUMP
CHEMICAL
IONIZATION
MASS
SPECTROMETER
/
/
PRESSURE 6AUOE- 0-760 MM.
SUB-ATMOSPHERIC PRESSURE REQ.
REAGENT GAS
VARIABLE CONDUCTANCE VALVE
MECHANICAL VACUUM PUMP
7
Schematic Diagram Of The Sample Inlet And Calibration
System For The Prototype System For Analysis Of
Oxygenated Hydrocarbons In Engine Exhaust-
-24-
-------
leak. A second fixed leak connects the glass manifold to the
chemical ionization mass spectrometer. The reagent gas is
metered into the glass manifold through a sub-atmospheric pres-
sure gas regulator. The total pressure in the glass manifold may
be adjusted by using a variable conductance valve between the mani-
fold and a mechanical vacuum pump. This valve serves as a fine
control on the pressure in the manifold and hence the pressure in
the chemical ionization mass spectrometer.
Calibration
The inlet system was designed to allow rapid calibration of the
entire prototype system. A sample of artificial exhaust gas or inert
gas is passed through the main exhaust gas line at a high flow rate,
typically 15 litres per minute. A sample of aldehyde or ketone, or
other compound for which a calibration measurement is required, is
placed in the syringe and fixed in position on the infusion pump. With
a known infusion rate and a known flow rate of the exhaust gas, precise
mixtures at the ppm level can be obtained. A sample of this mixture
is drawn into the glass manifold where the reagent gas is added in a
proportion which can be read from the absolute pressure gauge pro-
vided and the resulting mixture analyzed in the chemical ionization
mass spectrometer. The entire inlet system is operated hot (typically
150°C) during calibration measurements in an attempt to avoid errors
due to condensation or polymerization on the walls.
-25-
-------
This same inlet system is also used in the actual exhaust
measurements. With the syringe and infusion pump in place, the
calibration for a particular compound can be checked at any time
by injecting that compound at a known rate into the exhaust gas
stream.
-26-
-------
RESULTS AND DISCUSSIONS
Chemical ionization mass spectral measurements have been
carried out on a number of aldehydes and ketones. In most of this
work methane was used as the reagent gas but experiments have also
been carried out using propane, iso-butane, neo-pentane, and water
as reagents. The mass spectra obtained in a series of measurements
using methane are given in Figures 8 through 25. The relative sensi-
tivities observed in these measurements are summarized in Table II.
These measurements were carried out using a cold inlet system
and some of the variations in the sensitivities observed are probably
due to difficulties with the inlet system. Also some of the spectra
show residual contamination from compounds run in the previous
measurements. When high concentrations of the sample were used,
for example, Run No. 2 on crotonaldehyde, Run No. 1 on benzaldehyde
and Run No. 13 on acetaldehyde, some reduction in the sensitivity is
observed as a result of using up a significant amount of the methane
reactant ions. Below concentrations of about 300 parts per million
this effect is not observed and it is expected that the signal should be
linear with concentration at lower concentrations. With the exception
of Run No. 17 this series of measurements was conducted at a source
pressure of 1 torr, source temperature of 150°C, and electron em-
ission of 400 microamps.
The aldehyde-methane mixtures were prepared on a stainless steel
vacuum line. A sample of liquid reagent grade aldehyde was expanded
into the evacuated line and the pressure measured with a mechanical
manometer. A small sample of the vapor (0.5 cc) was isolated and
then further expanded into an evacuated 500 cc glass bulb. The glass
-27-
-------
TABLE II: RELATIVE SENSITIVITIES MEASURED IN A SERIES
OF CHEMICAL IONIZATION MEASUREMENTS ON ALDEHYDES
AND KETONES USING METHANE REAGENT.
Sensitivity
Run
No.
13
15
6
14
12
2
7
5
11
10
9
8
3
4
1
Compound
Acetaldehyde
Acrolein
Propionaldehyde
Acetone
Crotonaldehyde
Crotonaldehyde
n-Butyraldehyde
iso-Butyraldehyde
2-Butanone
n-Valer aldehyde
iso-Valeraldehyde
2-Pentanone
3-Pentanone
Ethyl butyraldehyde
Benzaldehyde
Molecular
Weight
44
56
58
58
70
70
72
72
72
86
86
86
86
100
106
Concentration
(ppm)
350
140
270
270
80
40,000
200
35
55
50
40
60
100
30
1300
mv / p pm
M + l
24
69
27
36
29
0'.3
6
80
33
5
2
20
10
8
4.5
)
Total
24.4
70
28
37
31
0.4
19
108
34
19
15
21
11
33
5
-28-
-------
bulb was then filled with methane to 1 atmosphere of pressure. By
this technique the concentration of the aldehyde in the methane is
given by
V2 • P2 1000 • 14.7
Where Vj is the volume before expansion, V2 is the total volume, in-
cluding the 500 cc bulb, Pj is the measured pressure of the aldehyde
vapor in ps ia and P2 is the methane pressure. In using this static
sample preparation it is possible that the actual final concentration of
aldehyde may be somewhat lower than calculated if sufficient time for
equilibration is not allowed. However, the concentration should not be
higher than calculated.
The measurement on formaldehyde used Baker analyzed reagent
grade formalin solution containing 37.8% HCHO stabilized with 13. 1%
methanol. The methane chemical ionization spectrum for this mix-
ture is given in Figure 8. The concentration of formaldehyde in this
sample was approximately 1000 ppm. The protonated formaldehyde
ion is at mass 31 and the protonated methanol ion is at mass 33.
The only interference with the protonated formaldehyde that is ex-
pected to occur is the double C13 isotope peak from C2H5 ; this may
limit the detection limit for formaldehyde to about 0. 1 ppm.
The chemical ionization spectrum for 1300 ppm benzaldehyde in
methane reagent is given in Figure 9. The principal benzaldehyde
ion is P+1 at mass 107. Impurity peaks from the earlier run on formalin
i
solution are visible at masses 31 and 33. It may be noted that the mass
17 peak from methane is rather small compared to the other peaks in
-29-
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XI
Sensitivity
X10
•H—I—h
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CLEVITE CC
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20
30
40
MASS NUMBER
(Atomic Mass Units)
Figure 8. Chemical lonization Mass Spectrum for
Formalin Solution with Methane Reagent
at 1 torr and 150°C.
-30-
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the spectrum, indicating that a substantial fraction of the reagent
ions have been used up by reaction at this concentration of benzaldehyde.
The chemical ionization spectrum for 4% crotonaldehyde in methane
reagent is given in Figure 10. The major crotonaldehyde peaks in this
result are the P+1 at mass 71, and the P-l at mass 69. A significant
amount of the protonated dimer ion is observed at mass 141. At this
relatively high sample concentration, it may be noted that the methane
reagent ions have virtually disappeared.
The chemical ionization spectrum for 100 ppm of 3-pentanone in
methane reagent is given in Figure 11. The only significant peak pro-
duced from the 3-pentanone is the P+l ion at mass 87. Impurity peaks
from the early runs of benzaldehyde and formalin solution are visible at
masses 107, 71, 33, and 31.
The chemical ionization mass spectrum in 30 ppm of ethyl butyr-
aldehyde in methane reagent is given in Figure 12. The principal ions
oroduced from the ethyl butyraldehyde are the P+l ion at mass 101 and
the fragment ion produced by loss of water from the protonated parent
mass 83. The ion at mass 59 may be due either to loss of C3H(, from
the protonated parent or due to a significant impurity of either propion-
aldehyde or acetone in the sample. The other ions in the spectrum are
due principally to impurities residual from the previous samples.
The chemical ionization mass spectrum for 35 ppm of iso-butyral-
dehyde in methane reagent is given in Figure 13. The principal ion
produced from the iso-butyraldehyde is the P+l at mass 73. Fragment
ions due to loss of H2 and loss of water from the protonated parent are
observable at masses 71 and 55 respectively.
-32-
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The chemical ionization mass spectrum for 270 ppm of propion-
aldehyde in methane reagent is given in Figure 14. The principal
ion produced from the propionaldehyde is the P+1 at mass 59. A
small amount of the protonated dimer ion is observed at mass 117.
The chemical ionization mass spectrum for 200 ppm of n-butyral-
dehyde in methane reagent is given in Figure 15. The protonated par-
ent ion occurs at mass 73 and in this case the fragment ion due to loss of
water at mass 55 is larger than the protonated parent ion. It is un-
likely that the large peak at mass 59 i s due to fragments of the benzalde-
hyde fragment ions but is probably either propionaldehyde residue left
from the previous sample or due to a significant impurity of either
oropionaldehyde or acetone in the butyraldehyde sample.
The chemical ionization mass spectrum for 60 ppm of 2-pentanone
in methane reagent is given in Figure 16. The principal ion produced
from the 2-pentanone is the P+l at mass 87, The other ions in the
spectrum are probably due to residual impurities in the previous
samples.
The chemical ionization mass spectrum for 40 ppm of iso-valeral-
dehyde in methane reagent is given in Figure 17. The P+l ion occurs
at mass 87 but the fragment ion due to loss of water from the protonated
parent at mass 69 is larger in intensity.
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The chemical ionization mass spectrum for 50 ppm n-valyralde-
hyde in methane reagent is given in Figure 18. The protonated parent
ion is at mass 87 and again the fragment ion at mass 69 due to loss of
water from the protonated parent ion is more intense than the P+1 ion.
The chemical ionization mass spectrum for 55 ppm for 3-butanone
in methane reagent is given in Figure 19. The only significant ion pro-
duced from the 3-butanone is the P+1 ion at mass 73.
The chemical ionization mass spectrum for 80 ppm of crontonaldehyde
in methane reagent is given in Figure 20. The only ion produced from
the crotonaldehyde is the P+1 ion at mass 71. This measurement is a
repeat of the earlier measurement at higher concentrations of the croton-
aldehyde given in Figure 10 and represents the mass spectrum obtained
under analytic conditions.
The chemical ionization mass spectrum for 350 ppm of acetaldehyde
in methane is given in Figure 21. The principal ion produced from the
acetaldehyde is the protonated parent at mass 45. A small amount of
the protonated dimer ion is observed at mass 89.
The chemical ionization mass spectrum for 270 ppm of acetone in
methane reagent is given in Figure 22. The protonated parent ion occurs
at mass 59 and no significant fragment ions are observed. A small
amount of the protonated dimer ion is observed at mass 117 and a
fragment due to loss of water from the protonated dimer at mass 99.
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The chemical ionization mass spectrum for 140 ppm of acrolein
in methane reagent is given in Figure 23. The principal ion in the
spectrum is the P+l at mass 57. A very small amount of the dimer
ion at mass 113 is also observed. No significant fragment ions are
observed from the acrolein.
The chemical ionization mass spectrum for a methane blank run
immediately following the preceding runs is given in Figure 24. This
measurement was done virtually under the same conditions as the pre-
ceding runs on the aldehydes. The bottom trace on Figure 24 corres-
ponds to increasing the sensitivity by both increasing the electron beam
current by a factor of six and increasing the recorder sensitivity by a
factor of ten. This result shows the noise level of the instrument and
indicates that a few parts per billion of the aldehydes should be detect-
able at the highest sensitivity.
Run No. 17 (Figure 25) was carried out to assess the sensiti/ity
of the chemical ionization method. The source pressure was increased
for this run to 5 torr and the electron emission was increased to 2 milli-
amps . In this measurement a sample consisting of 2 ppm of benzalde-
hyde in methane was used. The signal for the M+l ion (mass 107) was
600 millivolts per ppm of benzaldehyde. The noise level was two milli-
volts rms. This measurement indicates that a concentration of 10 parts
per billion of benzaldehyde would give a signal-to-noise ratio greater
than 3.
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-51-
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In the absence of interference and fragmentation it is expected
that the sensitivity for the other aldehydes and ketones should be
comparable. The larger saturated aldehydes; butyraldehyde, valer-
aldehyde, and ethyl butyraldehyde, all show substantial fragmentation
of the M-f-1 ion produced by proton transfer from methane reagent ions.
These fragmentations, which involve loss of hydrogen molecule and
loss of water molecule as the principal reactions, do not occur to any
appreciable extent for the isomeric ketones studied. The fragment
ions corresponding to loss of H2O and H2 may be used to distinguish
between the aldehydes and ketones. These fragmentations cause the
sensitivity, as determined from the M+l ion intensity, to be somewhat
lower for the larger saturated aldehydes than for the other compounds
studied.
The saturated aldehydes and ketones do not suffer from inter-
ference with the hydrocarbons present. The oxygenated compounds
give ions orincipally at the parent mass plus 1, while the hydrocarbons
of the same molecular weight yield principal ions at the parent mass
minus 1. The protonated alkanes larger than ethane are apparently
not stable and the protonated species have not been observed in the
mass spectrometer. On the other hand, the unsaturated aldehydes,
e.g. acrolein and crotonaldehyde.are interfered with by the C4 and C5
hydrocarbons respectively. Butyl ions at mass 57 may be produced
by proton transfer to the butenes and by hydride ion transfer from the
butanes. These reactions produce ions of the same nominal mass
as the protonated acrolein.
-52-
-------
It appears likely that a different reagent will be required for
the measurement of the unsaturated aldehydes in engine exhaust.
Additional experimental data is needed to choose an appropriate re-
agent. One possibility is ammonia which will produce NH4 as the
principal reagent ion. This ion may transfer a proton to acrolein
and not to the butenes. However, NH4 will certainly not react with
formaldehyde. Thus it may be necessary to do the analysis in two
steps; using methane to measure the saturated aldehydes and using
a different reagent to measure the unsaturated aldehydes. Further
experimental work will be undertaken to establish the reagent and
operating conditions with which acrolein can be analyzed in engine
exhaust by using the chemical ionization technique.
The results of the present work indicate that the chemical ioniza-
tion direct reading mass spectrometer can be used to monitor individual
saturated aldehydes and ketones in engine exhaust in the concentration
range from 10 parts per billion to 300 parts per million. Further work
is required to fully establish the sensitivity, accuracy, and specificity
of this technique in the application to routine exhaust measurements.
Additional work is also required to extend the technique to unsaturated
aldehydes and to other components of interest.
-53-
-------
A PPENDIX
SELECTED CHEMICAL IONIZATION BIBLIOGRAPHY
-54-
-------
SELECTED CHEMICAL IONIZATION BIBLIOGRAPHY
1. F. II. Field, "Chemical loni/ation Mass Spcc-
trornelry, " Accounts of Chemical Research 1,
42(1968).
•2. M. S. 15. Munson and F. II. Field, "Chemical
loni/ulion Mass Spectromelrv. I. General In-
troduction,"]. Am. Chem. Soc. 88, 1621 (1966).
3. M. S. B. Munson and F. H. Field, "Chemical
lonization Mass Spectromelry. Jl. Esters,"
,1. Am. Chem. Soc. 88, 4337 (1966).
4. F. II. Field, M. S. B. Munson and D. A. Becker,
"Chemical lonization Mass Spectrometry, Pa-
raffin Hydrocarhons," Advances in Chemistry
Series, No. 58, American Chemical Society,
Washington, D. C. 1966, p. 167.
5. M. S. B. Munson and F. II. Field, "Chemical
lonizalion Mass Spectrometry. IV. Aromatic
Hydrocarhons," J. Am. Chem. Soc. 89, 1047
(1967).
6. F. H. Field and M. S. B. Munson, "Chemical
lonization Mass Speclrometry. V.Cycloparaffins,"
J. Am. Chem. Soc. 89, 4272 (1967).
7. F. H. Field. "Chemical lonization Mass Spec-
trometry. VI. CY HJJ Isomers. Toluene, Cyclo-
heptalriene, and Norbornadiene," J. Am. Chem.
Soc. 89, 5328(1967).
8. F. H. Field, Peter Hamlet and W. F. Libhy,
"Chemical lonization Mass Spectromelry, VII.
Reactions of Benzene Ions with Benzene,"
J. Am. Chem. Soc. 89, 6035 (1967).
9. F. H. Field, "Chemical lonization Mass Spec-
trometry. VIII. -\lkencs and Alkynes," J. Am.
Chem. Soc. 90, 5649 (1968).
10. F. II. Field, "Chemical lonizalion Muss Spec-
Irometry. IV Temperature and Pressure Studies
with Benzyl Acetate and l-Amyl Acetate,"
J. Am. Chem. Soc. 91., 2827 (1969).
II.F. 11. Field. "Chemical lonizalion Ma.-s Spec-
lromelr\. \. Temperature Simile* with Sub-
stituted Ben/\l Acetates," J. Am. Chem. Soc.
9J, 6331(1969).
12. I). P. Weeks and F. II. Field, "Chemical loni-
y.alion Mass Speclronielr). \l. Reaction* of
Melho\\ mclli\ I Formate and Melho\\ methjl
Acetate \\ilh Methane and Isoliutane," J. Am.
Cliem.Soe.92. 1600(1970).
13. F. II. Field, "Chemical loni/ation Mass Spec-
tromelry. \ll. Mcohols," J. Am. Chem. Soc.
92.2672(1970).
14. F. II. Field and M. S. B. Munson, "Reactions of
Gaseous Ions. XIV. Mass Speetromelric Studies
of Methane at Pressures to 2 Torr," J. Am.
Chem. Soc. 87, 3289 (1965).
15. M. S. B. Munson and F. II. Field, "Reactions of
Gaseous Ions, XV. Methane +1% Fthane and
Methane + 1% Propane," J. Am. Chem. Soc.
87,3294(1965).
16. M. S. B. Munson and F. H. Field, "Reactions of
Gaseous Ions, XVI. Effecls of Additives on Ionic
Reactions in Melhane," J. Am. Chem. Soc. 87,
4242 (1965).
17. M. S. B. Munson and F. H. Field, "Reactions of
Gaseous Ions. XVII. Melhane + Unsaturaled Hy-
drocarhons," J. Am. Chem. Soc. 9]., 3413
(1969).
18. J. H. Fulrell and T. 0. Tiernan, "Ionic Re-
actions of Unsaturaled Compounds. I. Poly-
merizalion of Acetylene," J. Phys. Chem. 72,
158(1968). ~~
19. T. 0. Tiernan and J. H. Futrell, "Ionic Re-
actions in Unsaturaled Compounds. II. Eth-
ylene,"J. Phys. Chem. 72, 3080 (1968).
20. F. P. Abramson and J. H. Futrell, "Ionic Re-
actions in Unsaturated Compounds. III. Pro-
pylene and ihe Isomeric Butenes." J. Phys.
Chem. 72, 1994(1968).
21. B. M. Hughes, T. 0. Tiernan and J. F. Fulrell,
"Ionic Reaclions in Unsaluraled Compounds.
IV. Vinyl Chloride," J. Phy. Chem. 73, 829
(1969).
22. J. H. Fulrell, T. 0. Tiernan, F. P. Abramson,
C. D. Miller, "Modification of a Time-of-Flight
Mass Spectrometer for Investigation of Ion-
Molecule Reactions at Elevated Pressures," Rev.
Sci.Instr. 39, 340(1968).
23. E. Gelpi and J. Oro, "Chemical lonizalion Mass
Speclromelry of Pristane, " Anal. Chem. 39,388
(1967).
24. II. M. Fales, G. W. A. Milne and M. L. Vestal,
"Chemical lonizalion Mass Speclromelry of
Complex Molccuh-s," J. Am. Chem. Soc. 91,
3682 (1969).
25. II. M. Fales, II. Lloyd and G. W. A. Milne
"Chemical lonizalion Mass Speclromelry of
Complex Molecules. II. Alkaloids," J. Am.
Chem. Soc. 9>2, 1590(1970).
(conl.)
-------
26. H. Ziffer, H. M. Fales, G. W. A. Milne and F.
II. Field, "Chemical lonizalion Mass Speclro-
metry of Complex Molecules. 111. Structure of
the Photodimers of Cyclic a—/3 Unsaluraled
Ketones," J. Am. Chem. Soc. 92, 1597 (1970).
27. J. H. Futrell, F. P. Abramson, A. J. Bhatta-
charya and T. 0. Tiernan, "Proton Transfer
Reactions in the Simple Alkanes: Methane,
Ethane, Propane, Butane, Pentane, and Hex-
ane," J. Chem. Phys. 52, 3655 (1970).
28. H. M. Fales, G. W. A. Milne and T. Axenrod,
"Identification of Barbituates by Chemical
lonization Mass Spectrometry," Anal. Chem.
42, 1432 (1970).
29. G. W. A. Milne, T. Axenrod and H. M. Fales,
"Chemical lonization Mass Spectrometry of
Complex Molecules. IV. Ainino Acids," J. Am.
Chem. Soc. 92, 5170(1970).
30. T. 0. Tiernan and J. H. Futrell, "Studies in
Chemical lonizalion Mass Spectrometry. I. Re-
actions of Selected Proton and Hydride Trans-
fer Reagents with Hexane Isomers," Rev. Sci.
Instrum. (in press).
31. L. Wojcik and J. H. Futrell, "Studies in Chem-
ical lonization Mass Spectrometry. II. Aliphatic
Alcohols," Biochem. Biophys. Res. Commun.
(in press).
32. D. M. Schoengold and M. S. B. Munson, "A
Combination of Gas Cbromalography and
Chemical lonizalion Mass Spectromrtry,"
Anal. Chem. J_2, Dec. 1970 (in press).
33. D. Beggs, H. M. Fales, G. W. A. Milne and
M. L. Vestal, "A Chemical lonization Mass
Spectrometer Source," Rev. Sci. lustrum.
(in press).
34. A. A. KiryushUn, H. M. Fales, T. Axenrod, E.
J. Gilbert and G. W. A. Milne, "Chemical loni-
zation Mass Spectrometry of Complex Mole-
cules. VI. Peptides," J. Am. Chem. Soc.
(in press).
35. D. P. Beggs and F. H. Field, "Reversible Re-
actions of Gaseous Ions. I. The Methane-Water
System," J. Am. Chem. Soc., to be published.
36. D. P. Beggs and F. H. Field, "Reversible Re-
actions of Gaseous Ions. II. The Propane-Water
System," J. Am. Chem. Soc., to be published.
37. F. H. Field and D. P. Beggs, "Reversible Re-
actions of Gaseous Ions. III. Studies with
Methane At P = 0. 1 - 1.0 Torr and T = 77-
300° K," J. Am. Chem. Soc., to be published.
-------
THE FOLLOWING PAGES ARE DUPLICATES OF
ILLUSTRATIONS APPEARING ELSEWHERE IN THIS
REPORT. THEY HAVE BEEN REPRODUCED BY
A DIFFERENT METHOD SO AS TO FURNISH THE
BEST POSSIBLE DETAIL TO THE USER.
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The electronics cabinet on the left contains all of the readout and
control systems and the cart on the right contains the direct reading
mass spectrometer with chemical ionization source and associate
vacuum systems and sample inlet systems.
-18-
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