the remainder of the discharge tube. Reactions 4 and 5 also account for the steep slopes of the calibration curves of many elements and the enhancement effect of potassium chloride on spectral intensity. When a sample solution is dried on the filament, the solute crystallizes and is deposited on the surface of the filament. As the concentration of the salts in the sample increases, the crystals grow larger, and the fraction of salt directly in contact with the filament decreases. In turn, this increases the fraction of salt that volatilizes as molecular vapor with the result that emission intensity increases rapidly because of reactions 4, 5, and 2. If no extraneous salt is added to the sample, for metals whose salts are decomposed easily, the slope of the calibration curve will be steep. In the presence of potassium chloride, its crystals will include the metal salts and prevent both the dissociation of the chloride molecule and the formation of the refractory oxides as the sample is volatilized , from the filament. Potassium chloride is not always efficient either in enhancing spectral intensity or changing the slope of the calibration curve, probably due to fractional volatilization of salts from the filament. In this case, a suitable salt should be chosen as an additive, e.g., calcium chloride when barium or strontium is to be determined, as described earlier. Addition of potassium chloride may strongly affect the excitation conditions of the helium plasma through the population of metastable helium atoms and the electron temperature. The fact that the position of the’ emission maximum in the discharge tube shifts toward the cavity ( I )
reflects these effects of potassium chloride. Since the intensity of helium lines and the electron temperature increase as the distance from the cavity decreases, the variation of the excitation conditions due to the addition of potassium chloride may be partly compensated for by the shift in the position of the emission maximum. The proposed mechanism described here fits the observed data. Other factors, such as changing impedance of the microwave cavity upon introduction of KC1, could also enter into the excitation mechanism and the enhancements observed. However, because of the transient nature of the sample introduction process, optimization of the cavity tuning is not practical.
LITERATURE CITED (1) H. Kawaguchi and B. L. Vallee, Anal. Chem., 47, 1029 (1975). (2) D. S. Auld, H. Kawaguchi, D. M. Livingston, and 8. L. Vallee, Proc. Natl. Acad. Sci. USA, 71,2091 (1974). (3)H. Kawaguchi and D. S.Auld, Clin. Chem. ( Winston-Salem, N.C.), 21,591
(1975). (4) K. W. Busch and T. J. Vickers, Spectrochim. Acta, Part 6, 28, 85 (1973). (5)P. Brassem and F. J. M. J. Maessen, Spectrochim. Acta, Part 6, 29, 203
(1974). (6)R. Avni and J. D. Winefordner, Spectrochim. Acta, Part B, 30, 281 (1975). (7) H. Kawaguchi, M. Hasegawa, and A. Mizuike, Spectrochim. Acta, Part 6, 27, 205 (1972). (8)Juan Ramirez-Mfinoz, “Atomic-Absorption Spectroscopy”, Elsevier Publishing Company, New York, 1968.p 271. (9)A. Ando, K. Fuwa, and B. L. Vallee, Anal. Chem., 42,818 (1970).
RECEIVEDfor review September 3,1976. Accepted November 11,1976. Work supported by Grant GM-15003 from the National Institutes of Health of the Department of Health, Education, and Welfare.
Determination of Impurities in Gases by Atmospheric Pressure Ionization Mass Spectrometry Hideki Kambara” and lchiro Kanomata Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo, Japan
An Atmospheric Pressure Ionization (API) mass spectrometric technique in which primary ions are produced by corona discharge at atmospheric pressure is successfully applied to detection of small inorganic molecules in highly purifled gases. The Ions, which result from a complex series of ionlzatlonreactions, are continuously supplied through two small apertures (0.1 mm and 0.2 mm) into the mass analyzing region. A total A is obtained at the mass analyzer Ion current of 1 X entrance sllt by using a two-stage differential pumping method. A collision-Induced dissociation method is employed to determine the impuritles. As a result, many impurltles are detected sensitively, Le., NO, COP, H20, and O2 in nitrogen gas and CH4, H20, and C2Hs in oxygen gas. The concentrations of NO in nitrogen gas and CH4 in oxygen gas are determined to be 250 ppb and 1.8 ppm, respectively.
The analysis of trace quantities of compounds has become increasingly important in various fields in addition to organic chemistry. For example, highly purified nitrogen gas is used as the standard for optical fluorescence analysis of air pollution. A t times, trace quantities of impurities in the standard gas have a critical influence on investigation results. It has been noted that high purity nitrogen gas produces a back270
ground interference ( I ) . However, the lack of an adequate analytical instrument has prevented comprehensive studies of this problem to date. The need for improved gas analysis techniques has also become vital in semiconductor technology. This is because it is considered that impurity concentrations should be kept below lo-’ mol parts. In addition, it is necessary to control and monitor process gas quality (2). This requires the detection of inorganic as well as organic compounds. One of the most effective gas analysis techniques involves a combination of mass spectrometry and gas chromatography. It can detect organic compounds as small as 10 pg. The sensitivity of a mass spectrometer depends on the background spectra, the number of ions reaching the ion collector, and the ionization rate of nondissociated samples. Therefore, there are many other difficulties involved in detecting small molecules than with large organic compounds. One is that, in the low mass region, many kinds of background ions can be observed which disturb the detection of the impurity trace quantities. Another is that these molecules cannot be concentrated with any available interface, e.g., jet separator, between the gas chromatograph and the mass spectrometer. No practical solution to these problems has been reported yet. Recently Homing developed a new mass spectrometric system, the so-called “Atmospheric Pressure Ionization Mass
ANALYTICAL CHEMISTRY, VOL. 49, NO. 2, FEBRUARY 1977
Spectrometer" (3, 4 ) , which is sensitive for organic compounds. It is equipped with a corona discharge ionizer as well as a P-ray ionizer. His work was concerned only with organic compounds; however, it has been found that this chemical ionization method may be applied to the detection of small inorganic molecules. One difficulty in applying the instrument to the analysis of inorganic molecules is the formation of many cluster ions. The formation of various cluster ions from a simple sample obscures the origin of the ions. It is also inconvenient for making an accurate calibration curve representing a sample. This investigation presents a new mass spectrometer which is slightly different from that reported by Homing. This device utilizes corona discharge a t atmospheric pressure and twostage differential pumping. Ion-molecule collisions are utilized to distinguish cluster ions from parent ions. It is successfully applied for the detection of impurities in nitrogen and oxygen gases. EXPERIMENTAL Apparatus. The apparatus used is shown in Figure 1. I t consists of three parts, an ionization region a t atmospheric pressure, a differential pumping region, and an analyzing region. Two aperture electrodes separate these regions. They are made of stainless steel and ion-plated with gold to avoid contamination. The diameter of the aperture is 0.1 mm for the first one and 0.2 mm for the second. The ionization region contains a needle electrode for corona discharge from which primary ions are produced. A sewing needle is employed as the discharge electrode and is positioned 7 mm from the first aperture electrode. In the experiments, a voltage of 3 kV was supplied to the needle electrode through a 20-MQ resistor, which gave a total discharge current of 5 PA. An ampere meter was put between the needle electrode and the resistor to monitor the total discharge current. The ions created by the corona discharge react rapidly with neutral impurities. Primary ions and the ones produced enter the analyzing region after passing through the intermediate pressure region, where the ions are focused on the second aperture with a centripetal electric field. The intermediate region was evacuated to 1Torr with a 250 l./min rotary pump. The four electrodes mounted in the intermediate region focus the ions on the second aperture. They are made of stainless steel and ion-plated with gold. The distance between the first and the second aperture is 20 mm. The drift voltage supplied between the aperture electrodes was changed from 10 to 30 V in order to observe changes in mass spectra. The voltage supplied to each electrode was adjusted so that many ions pass through the second aperture a t 30 V of the drift voltage. The ratios between the supplied voltages were fixed throughout the experiment. For instance, they were 30,25,20, 15, and 14 V from the first aperture electrode to the final focusing electrode, respectively. A voltage of 2 V was supplied to the second aperture electrode. The distance between the final focus electrode and the second aperture was 0.5 mm, so that the electric field strength near the second aperture was 240 V/cm. One focusing electrode was used to monitor the ion current. The ion current introduced into the intermediate region was 6 X lov9 A. T h e ion current stability was adequate with a 5%/h drift for this investigation. The ion current becomes 1 X A after passing through the second aperture. The ions were focused on the entrance slit of the mass spectrometer and half of them entered the mass spectrometer. The analysis region was evacuated by a 6-inch oil diffusion pump with a water cooling baffle and a liquid nitrogen trap, having an effective pumping speed of about 200 U s . The measured pressure in the analysis region was 2 X Torr. A quadrupole mass spectrometer which covers a 2 to 150 amu mass range was employed. A 16-stage electron multiplier model 474 from Hamamatsu Television Corp. was employed and the operational amplifier used was an Aikoh Electric Corp. Model 6343M. Operation. Impurities in nitrogen gas (99.999%pure) and oxygen gas (99.99% pure) were investigated. After maintaining the heat in the reaction chamber and the inlet system a t about 250 "C for several days with flowing nitrogen gas, the investigation was carried out. The ionization region was kept a t about 150 "C during the experiment. Mass spectra were measured for various electric fields in the intermediate region. Dry and wet nitrogen gas were introduced into the ionization region. A water bottle was plugged into the gas stream to investigate the de-
I Torr
4I
1 2 T
SampleGas
1
I
5
6
~
I
7
Figure 1. Schematic diagram of the experimental apparatus (1) Needle electrode, (2) aperture slit (0.1-mm i.d.), (3)focus electrodes, (4) aperture slit (0.2-mm i.d.), (5) calibration ion source, (6) quadrupole mass analyzer, (7) electron multiplier, (8) molecular sieve trap (13X)
pendence of H30+intensity on water concentration. Six water bottles with different volatilization speeds were prepared. Wet nitrogen gas was mixed with dry nitrogen gas in various ratios. The water concentration in the nitrogen stream was determined by measuring the weight of volatilized water and the flow rate of the wet and dry nitrogen gases. Another nitrogen gas containing 5.43 ppm NO was mixed with the sample nitrogen gas in order to estimate NO concentration in the sample nitrogen gas. The usual gas flow rate was 2 l./min. Oxygen gas from the supply system in the laboratory was introduced into the ionization region through a 13X molecular sieve trap. A quartz heating region and another trap were mounted behind the molecular sieve trap as shown in Figure 2. The heating region was heated to 900 "C to oxidize impurities in the oxygen gas and the latter trap cooled with liquid nitrogen to investigate changes in the mass spectra. The oxygen flow rate was 1 l./min.
RESULTS A N D DISCUSSION Ion-Molecule Reactions. Many investigations of ionmolecule reactions including Nz+ and Oz+ have been reported to date (5-8). The following sequences of ion-molecule reactions occur after corona discharge in the ionization region at 760 Torr and the intermediate region at 1Torr: discharge
-
Nz+,0 2 +
N2++ 2N2 ---* N4+ + N2
+ HzO HzO+ + Ha0 H30' + HzO + Nz Nz+, N4+ + COz
+
H20+
-
+ NO 0 2 + + N2
Nz+, N4+
NO+ + N2,2N2
-+
----*
02+
+ 2Nz H30+ + OH H+(HzO)z + Nz C02+ + Nz, 2Nz
--
N4+
NO+
+ H20 + 0 2
+ NO +0 2
02+*H20
Oz+.HzO + HzO ----* H30+ + OH
+ 2H20 02++ CH4
02+
H30+
+0 2
+ OH + 0 2
+
CH302+ H
etc. Impurities in gases are ionized by these ion-molecule reactions as well as by photons emitted by the discharge. Ion-Molecule Reactions in the Intermediate Region. Ion-molecule reactions in the intermediate region are complicated because a strong electric field is present. Dissociation of ions occurs in the intermediate region in addition to ionmolecule reactions. The ion intensities of all ion species would change in the s a m e way if all the same types of ion-molecule ANALYTICAL CHEMISTRY, VOL. 49, NO. 2, FEBRUARY 1977
271
70
-
60 -
ap Y
> 50-
5 cn z
i
2
3
W 40-
t-
4
Flgure 2. Schematic diagram of the oxygen gas supply system (1,3)Molecular sieve trap (13X), (2) quartz combustion tube, (4) liquid nitrogen
i5
s W
2
reactions occur in the intermediate region as in the atmospheric region. However, some ions changed drastically when the drift voltage was varied, while others did not change much. This indicates that other types of ion-molecule reactions are present. If the electric field is stronger than 10 V/cm.Torr, ions are accelerated and sometimes have enough energy to dissociate through collisions with neutral molecules (9). Generally, the bond dissociation energies of cluster ions are small. For example, they are 14 kcal/mol for D(Nz+-Nz), 70 kcal/mol for D(N+-N2), 36 kcal/mol for D(HsO+-HzO), and 22.3 kcal/mol for D(H+(HzO)z-H20) (IO,11).The field strength critical for dissociating cluster ions is given by D/1, where D is the bond dissociation energy in eV and 1 the mean free path of ions. The mean free path of molecules is about 0.08 mm at 1 Torr and 150 OC. The critical field strength is estimated to be about 100 V/cm for N4+ and 230 V/cm for H+(Hz0)2, assuming the mean free path of these ions to be 0.08 mm. Recently, a collision-induced decomposition method has been extensively studied for characterizing the ion structure of organic molecules (12). This method has been carried out a t kinetic energy greater than 1 keV. The decomposition phenomena described in this paper occur at low kinetic dissociation threshold energies and are useful for characterizing the ion structure. Drift voltage dependence of ion intensities in trapped and untrapped nitrogen gas streams is shown in Figures 3 and 4, respectively. Drastic changes in N2+ and N4+ ion intensities are shown in Figure 3. The reaction forming N4+ occurs very rapidly so that almost all Nz+ ions become N4+ before they enter the intermediate region. However, N2+ ions were observed a t drift voltages larger than 10 V, which corresponds to a field strength of 80 V/cm near the second aperture. This is because N4+ dissociates through collisions with Na. The critical field strength of 80 V/cm approximates the estimated value. The ion intensities of H+(H20),, where ( n = 1,2), change drastically at the drift voltages larger than 15 V as shown in Figure 4.This indicates that H+(H20)2 begins to dissociate at 15 V which corresponds to a field strength of 130 V/cm near the second aperture. This is about a half of the estimated critical field strength of 230 V/cm for H+(H20)2 dissociation. The discrepancy is caused by a recombination reaction and uncertainty as to the mean free path. Collision-induced dissociation makes the mass spectra more complex. However, this can be used for classification of ions into two groups. Ion intensities for the first group, which includes COz+, 0 2 , NO+, etc., stay nearly constant as drift voltage changes. Ion intensities change drastically for the second group, which includes cluster ions and dissociation products. It is possible to determine which group any given ion belongs to by observing changes in mass spectra at various drift voltage. This process 272
ANALYTICAL CHEMISTRY, VOL. 49,
NO. 2, FEBRUARY 1977
30-
20-
% -I W
l0-
IO 20 30 DRIFT VOLTAGE ( V I
Figure 3. Relation between ion intensity and drift voltage (trapped nitrogen gas stream)
L
301
-. IO
20
30
D R I F T VOLTAGE ( V )
Figure 4. Relation between ion intensity and drift voltage (untrapped nitrogen gas stream)
was used for assigning some of the ion species in this work. Impurities in Nitrogen Gas. Mass spectra for 30-V drift voltage between the two aperture electrodes are shown in Figure 5. Strong ion peaks originating from water were observed as depicted in Figure 5a. This water was considered to be mainly residual water left on the wall of the inlet tube. Ions such as 0 2 + , NO+, and N3+ were also observed. After passing through the molecular sieve, most of this residual water was removed and nitrogen ions became observable. Other remarkable changes in the mass spectra were the decrease in NO+ and the appearance of CO2+. Nitrogen oxide, which has a fairly strong absorptive character, can be trapped in the molecular sieve. The residual signal of mass number 30 in Figure 5b is deduced to be NO+ produced by a reaction of 0 2 + with Nz or some other impurities such as hydrocarbons. Carbon dioxide, having an ionization potential higher than HzO,
(b)
H30+
I
x IO 44
-
No+
30
li
x 10
L
'
M /e Figure 5.
Mass spectra of nitrogen gas at 30 V of drift voltage
(a)Untrapped nitrogen gas stream, (b) trapped nitrogen gas stream
cannot be observed in Figure 5a. Carbon dioxide can also be captured in a molecular sieve trap. However, an absorptive equilibrium between COz in the gas stream and COz on the molecular sieve is established soon after the gas begins to flow. Actually, CO,+ was not observable immediately after the trap was thermally purified. Mass spectra for a drift voltage of 20 V are shown in Figure 6. Ions were classified from these spectra as listed in Table I. Water creates cluster ions of M = 19, 37, and 55. Nitrogen creates ions of M = 28,42, and 56. Ion intensities of M = 30, 32, and 44 did not change appreciably by varying the drift voltage. Therefore, they were considered stable and classified as NO+, Oz+, and COz+. The ion for M = 46 decreased as the drift voltage increased. This indicates that the ion has a small dissociation energy and was classified as H,O.N,+ not NOz+. The ion intensity of M = 18decreased as the drift voltage increased, which indicates that it was an unstable cluster ion or an ion formed mainly in the intermediate region. No dissociative product of M = 18 could be observed, thus it is concluded that the ion must be H,O+ or NH4+ created in the intermediate region by the reaction of reactant ions with out gases. To estimate the concentration of water in the nitrogen gas, the relation between ion intensity and water concentration for a drift voltage of 30 V was investigated for water concentration up to 4 ppm and the results are shown in Figure 7. It is neither linear nor quadratic because of the complex process involved in forming H30+. Possible processes are as follows;
m/e Figure 6.
Table I. Impurity Ions in Nitrogen Gas and Their Origins Mass No.
Ion species
Origins
18 19 28
HzO+ or NH4+
H20+ or NH4
H30+ Nz+
29 30
NzH+ NO+
HzO+ Nz Nz NO 0 2
HzO N? 44
co2+
CO,
46 56
H,O*Nz+
HzO
N4+
Nz
V =3 0 ~ F l o w Rate 2.0Ilrnin.
/
+ HzO N,H+ + OH + Nz NzH+ + HzO H30+ + Nz N4+ + HzO H,O+ + 2N2 HzO+ + H20 + Nz H30+ + OH + Nz N4+ + 2Hz0 H30+ + OH + 2N2 N4+
Mass spectra of nitrogen gas at 20 V of drift voltage
(a)Untrapped nitrogen gas stream, (b) trapped nitrogen gas stream
4
+c
-+
+ 0.2
+
-+
L B
+
On the other hand, NO+ is produced through a simple charge transfer reaction of N4+ with NO. This gives a linear relation confirmed experimentally between the concentration of NO
OO
1
2
3
4
H20 CONCENTRATION ( PPM) Figure 7.
Calibration curve for water concentration in nitrogen gas
ANALYTICAL CHEMISTRY, VOL. 49, NO. 2, FEBRUARY 1977
273
(a 1 (a
1
.
0; 32
0; 32
x 100 C47 H3'0;~~
'7
0
M/e
1
M /e
Flgure 8. Mass spectra of oxygen gas at 20 and 30 V of drift volt-
Flgure 9. Change in mass spectra as the sample is heated
ages
(a) Before heating, (b) after heating
(a) 30 V, (b)20 V 10-
and NO+ ion intensity. The relative ion intensity of NO+ was closely approximated by (1.6C 0.4)%, where C is the concentration (ppm) of NO added to the nitrogen gas stream. From these results, the NO concentration in the untrapped nitrogen gas and residual water in the trapped nitrogen gas stream were estimated to be 250 and 200 ppb, respectively. Impurities in Oxygen Gas. Mass spectra for two different drift fields are shown in Figure 8. Ions for M = 27 and 30 decreased as the drift field became weaker, however, ions for M = 29,43,47, and 50 increased. This indicates that the ions for M = 27 and 30 are products of collision induced dissociation or products of ion-molecule reactions in the intermediate region. The ions for M = 43,47, and 50 increased so drastically that they appear to have low dissociation energies. In order to observe the origin of these ions, mass spectra were observed after various treatments, including heating, liquid nitrogen trapping, and trap removal as shown in Figures 9 and 10. The ion signals were assigned as listed in Table 11. Impurities such as C02, which has an ionization potential higher than 02, could not be observed. When the heater was turned on, most hydrocarbons were oxidized and disappeared from the mass spectra. Strong ion signals of H30+ appeared instead of these hydrocarbons. Drastic changes in the spectra were observed when the cold trap was plugged into the passage and then removed. Almost all impurities disappeared during use of the cold trap. The ion peak for M = 47 rapidly increased as the cold trap was being removed, and then decreased again in a short period. The ion peaks for M = 29 and 30 varied in a similar manner. Their increase and decrease rates were not as fast as that of M = 47. The varying speed for M = 43 and 27 was lower than those for M = 29 and 30, but faster than that for &Of. This difference in speed is considered to be the difference in freezing points. The freezing point ordering is deduced to be M = 47, M = 29 and 30, M = 27 and 43, and HzO from the lowest one. The classifications of ion species and origins were estimated from ion-molecule reaction data (6) in addition to the above facts. In order to estimate quantities, the impurities were first oxidized into H20 and C02 and then a relation between ion
+
274
ANALYTICAL CHEMISTRY, VOL. 49, NO. 2, FEBRUARY 1977
5 v)
8-
3
-
2%
3
;9C2Hi
Oi
19 HsO'
x 100
0
M/e Figure 10. Change in mass spectra as the sample is cooled (a) Being cooled, ( b )just after removing cold trap
intensity of H30+ and water concentration in 0 2 was investigated. The relation curve is shown in Figure 11. The ionization potential for H2O is higher than for 0 2 , therefore H30+ is deduced to have been produced through the following ionmolecule reaction with 0 2 + : 02+
+ 2H20
-
H30+
+ 0 2 + OH
The calibration curve is closely approximated by a quadratic equation for water concentration C as expected from the above reaction. These impurities gave 3.6 ppm of water after oxidi-
~~~
~
Table 11. Impurity Ions in Oxygen Gas and Their Origins Mass No.
Ion species
Origins
32
0,'
37
(Hq0)2H+ 7 + or C9H30+
34
-
02 1fi0180
-
l60180+
H2O C3H8 or C2H4 CHI H20
50
V=3 0 ~
Flow Rate 1.0 IImin.
P - I
/
= I
P
0 4
d 2
6
4
LITERATURE CITED
8
H20 CONCENTRATION ( P P M )
Figure 11. Calibration curve for water concentration in oxygen gas c: water concentration in ppm
zation. Methane has the smallest reaction coefficient for the impurities so that the main component must be CH4. Assuming most water comes from CH4, the concentration of CH4 in 0 2 was estimated to be 1.8 ppm. The concentration of other impurities can be estimated, if the following relation holds: -=klCl
k2CZ
I1
I2
CONCLUSION This study has proved that collision-induced dissociation at various drift voltages, which can take place in the intermediate region, provides a great deal of information about observed ions and is very useful for classifyingion species.This method has also been shown highly sensitive for detecting small inorganic as well as organic molecules. It can detect quantities in oxygen gas as small as 10 ppb for water, 20 ppb for CH4, and 0.1 ppb for CzH6. Calibration curves have been obtained; however, they depend on the ion-molecule reaction type, the ionization region temperature, and/or coexisting components. Many factors remain to be solved before this method can be applied to quantitative analysis. Nevertheless, this method offers many advantages in application to various systems. One is for the analysis of purified process and dopant gases used in semiconductor processes. Another is in combination with gas or liquid chromatography (13-15).
0.2C2+ 0.15
/
OO
respectively. The suffixes 1and 2 denote different species. The concentration of C2Hs was estimated to be lower than 2 ppb by taking into account the dissociation of CH302+ in the intermediate region. The k and I values used for the estimation cm~/molecule~s and 1.3% for CH4 and 1.6 X are 7.8 X 10-9 cm~/molecule~s and 0.28%for CzHs, respectively. Since ion signals as small as Io2can be detected, 20 ppb of CH4 and 0.1 ppb of C2H6 would be detectable (SIN 10).
where ki, Ci, and Ii (i = 1,2) are ion-molecule reaction coefficients, sample concentrations, and relative ion intensities,
(1) T. Motooka, Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo, personal communication, 1974. (2)E. A. Irene, J. Electrochem. Soc., 121, 1613 (1974). (3) . , E C.Hornina. M. G. Hornina. D. I Carroll, I.Dzidic. and R N. Stillwell. Anal Chem., 45,-936(1973). (4)D. I. Carroll, I.Dzidic, R. N. Stillwell, M. G. Horning, and E. C. Horning, Anal. Chem., 46,706 (1974). (5) V. Cermak, A. Dalgarno, E. E. Ferguson, L. Friedman, and E. W. McDaniel, "Ion Molecule Reactions", John Wiley & Sns, New York, N.Y., 1970. (6)F. C.Fehsenfeld, A. L. Schrneltekopf, D.B. Dunkin, and E. E. Ferguson, ESSA Tech. Rep. ERL 135-AL3, September 1969. (7)A. Good, D. A. Durden, and P. Kebarle, J. Chem. Phys., 52,212 (1970). (8)B. R. Hollebone and D. K. Bohme, J. Chem. SOC.,Faraday Trans. 2, 69,
-
1569 (1973). (9)M. M. Shahin, J. Chem. Phys., 45,2600 (1966). (10) R. K. Asundi, G. I. Shulz, and P. J. Chantry, J. Chem. Phys., 47, 1584
11967). - - , \
(11) P. Kebarle, S. K. Searles, A. Zolia, J. Scarborough, and M. Asshadi, J. Am. Chem. SOC.,69,6393 (1967). (12)F. W. McLafferty, P. F. Bente 111, Richard Kornfeld, Shih-Chuan Tsai, and Ian Howe, J. Am. Chem. SOC.,95,2120(1973). (13)E. C. Horning, D. I. Carroll, I. Dzidic, K. D. Haegele, M. G. Horning, and R. N. Stillwell, J. Chromatogr. Sci., 12, 725 (1974). (14)E. C. Horning, D. I. Carroll, I. Dzidic, K. D. Haegele, M. G. Horning, and R. N. Stillwell, J. Chromatogr., 99,13 (1974). (15)D. I. Carroll, i. Dzidic, R. N. Stillwell, K. D. Haegele, and E. C. Horning, Anal. Chem., 47,2369 (1975).
RECEIVEDfor review December 31,1975. Accepted November 10, 1976.
ANALYTICAL CHEMISTRY, VOL. 49, NO. 2, FEBRUARY 1977
275
