Mass Spectrometric Gas Analysis UtiIizing Selective Photoionization W . P. Poschenrieder and Peter Warneck
G C A Technology Dicision, Bedford, Mass. 01 730 Photoionization in a narrow energy band makes feasible the selective ionization of only a few components of a gas mixture. This technique simplifies the resulting mass spectrum and eliminates to some extent the overlap and interference of peaks from the individual constituents. Application of gas filters and thin film metal filters reduces residual ionization caused by second-order radiation and scattered light. I n this manner, a sensitivity limit of about 100 ppm has been achieved for the detection of the constituent with the lower ionization potential in binary gas mixtures with overlapping parent mass peaks. Gas filters were found to show greater versatility and convenience of handling than did the thin film metal filters. The effectiveness of selective photoionization is demonstrated for several simple gas mixtures.
THESPECIFIC ADVANTAGES of photoionization sources in analytical mass spectrometry when compared with the commonly employed electron impact ion sources are slowly gaining recognition. One important aspect is that photoionization eliminates the need for a heated filament which often causes delicate samples to undergo thermal decomposition in the hot ionization box, in addition to pyrolysis directly at the filament. Brion ( I , 2 ) recently discussed this point in detail. The absence of the filament also minimizes outgassing and memory effects. Another aspect concerns the considerable reduction in fragmentation when photoionization replaces electron impact ionization. This feature has also been discussed previously (I-5). The present paper deals with yet another aspect of photoionization which follows from the use of a UV monochromator as a convenient, narrow band energy selector. With energy selection applied in the analysis of complex gas mixtures, a selective ionization of only a few of the involved components can be achieved in contrast to the ionization of all the components as is customary with electron impact sources. Accordingly, the mass spectrum is simplified, and the overlap and interference resulting from individual components can be minimized. An example may illustrate this principle. For a mixture of small amounts of CO in air, common electron impact ionization results in a complete overlap of the CO mass spectrum by the other components of air, and only high resolution instruments can separate the nearly identical parent masses of CO and NB. Since the ionization potential of CO lies about 1.5 eV below that of N2,the contribution of N? to the mass spectrum can be removed by the choice of an ionizing energy falling in between the two ionization potentials. A low resolution mass spectrometer will then suffice for the
(1) C. E. Brion, ANAL.CHEM., 37, 1706(1965). (2) C.E. Brion, ANAL.CHEM., 38, 1941 (1966). (3) W. P. Poschenrieder and P. Warneck, J. Appl. Phys., 37, 2812 (1966). (4) R. F. Herzog and F. F. Marmo, J . Chem. Phys., 27, 1202 (1957).
(5) R. M. Elliott, in “Mass Spectrometry,” C . A. McDowell ed., McGraw-Hill, New York (1963).
analysis. Clearly, the same effect can be accomplished with monoenergetic electron impact ion sources that provide the necessary energy resolution, but the ion yield from electron impact near threshold is low and it rises only slowly with Increasing electron energy. For photoionization, however, the yield near threshold rises sharply, attaining maximum values rather quickly. Herein lies the advantage of applying photoionization in gas analysis by selective ionization. While the basic feasibility of this approach has been demonstrated previously with the photoionization mass spectrometer in this laboratory (3), it has also been shown that there is an inherent experimental problem in that the principal optical radiation selected by the employed grating monochromator is contaminated with light of shorter wavelengths from contributions of the second-order optical spectrum and scattered light. As a consequence, one observes a small amount of ionization even at wavelengths above the ionization threshold. For CO in nitrogen, therefore, it was not possible to discern less than 1% of CO on mass number 28 against the background of nitrogen ionization in the 796 to 885 A wavelength range. The background ionization was most pronounced when an argon spark light source was employed. This source generates an intense emission spectrum in the 400 to 450 A wavelength region, and it is this emission which is responsible for the second-order contribution to the 700 to 800 A region. In view of the potential value of selective photoionization for gas analytical applications, we have investigated suitable filtering devices which would eliminate or at least reduce the short wavelength contribution to the total photoionizing radiation. The results of this investigation are reported here. Two basic types of filters were studied: (a) thin metal filters and (b) gas filters. The former were found to be difficult to handle, whereas the latter showed versatility and convenience of application. For the subsequent discussion, it is appropriate to state the basic sensitivity and detection limit of the employed photoionization mass spectrometer. The instrument showed weakly the 15N2peak on mass number 30 when nitrogen was introduced to the io? source at a pressure of about torr, and when the 788 A line from an argon spark source was used for ionization, In this case, the number of photons entering the ionization chamber was close to lo9 photons/sec. The corresponding ion current of the 15N2 peak was, approximately, 1 X A at the input of the multiplier. Accordingly, a sensitivity of A/torr is obtained. The noise of the detection system corresponds to about 5 x lO-lQ A and has two main sources, namely, the multiplier dark current and pickup from the spark source. Since the natural abundance of ‘jN2 is 16 ppm, a detection limit of about 10 ppm is deduced. Presently, this detection limit cannot always be utilized when selective photoionization is applied to discriminate gases with coinciding mass peaks, even though the use of filters does provide a considerable improvement over the previous attainable detection levels. Following a short description of the apparatus, we shall VOL. 40, NO. 2, FEBRUARY 1968
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first discuss the effectiveness of the gas filter and the thin film metal filter; then report on results obtained for a variety of gas mixtures. EXPERIMENTAL
The photoionization mass spectrometer used in these experiments was similar to that employed previously (3). The basic instrument involved a cylindrical ion source with ion extraction in axial direction, perpendicular to the ionizing light beam; a 180-degree magnetic analyzer with wedgeshaped airgap ; a 20-stage electron multiplier detector; and associated electronics. The mass spectrometer was attached to the exit arm of a l/Z-meter Seya monqchromator. Its slits were adjusted to provide a bandpass of 5 A. The ionizing light intensity was monitored with a sodium salicylate-coated photomultiplier placed behind the ion source. Two types of light sources were used: either a repetitively pulsed spark source, or a dc-cold cathode discharge source. The following changes to the basic instrument were incorporated: 1. The McLeod manometer was replaced by a thermocouple gauge to monitor the pressures in the ion source. Keeping the pressures at about 3 microns reduced the occurrence of ion-molecule reactions. In previous experiments, the pressure range was from 10 to 100 microns. Use of a short pathlength of ion extraction further minimized ion-molecule reactions. 2. To compensate for the decreased ion production at the lower pressures, it was necessary to increase the diameter of the ion exit orifice to 2.5 mm. The resulting appreciable penetration of the accelerating field into the source provided a rather efficient ion collection. To increase this effect, the repeller plate was given a spherical shape, and the exit orifice disc was made slightly conical. 3. A differentially pumped gas absorption cell 20 cm long was installed in the entrance arm of the monochromator. On one end, the absorption cell was terminated by the disc carrying the monochromator entrance slit, on the other end by a pair of baffles, spaced 5 cm apart and provided with a 1 cm diameter aperture. The pressure of this cell could be raised to approximately 400 microns before the capacity limit of the four-inch diffusion pump serving the monochromator was reached. The pressure in the monochromator chamber was then about 25 microns. 4. At these high pressures, it was observed that photoelectrons originating near the monochromator exit, either from photoionization of the gas or from light striking a surface, underwent multiplication due to secondary collisions in the electric field of the exit slit system. A portion of the electrons thus generated was accelerated into the ion source where it contributed to the ionization of the sample gas. To eliminate this undesirable effect, an electric field perpendicular to the light beam was placed in front of the monochromator exit slit, so that any electrons entering this region were diverted before reaching the exit slit. The field was provided by a pair of 5 cm long parallel plate electrodes which also collected the electron current. Toward the monochromator section, the field was shielded by a grounded baffle plate. This arrangement essentially eliminated ionization of the gas sample in the ion source due to electrons originating from the monochromator. 5. A thin film holder was positioned between light source and entrance slit of the monochromator. Unbacked thin fiIms of indium were fabricated by the method described by Hunter, Angel, and Tousey ( 6 ) , and supported by a piece of 80z transparent mesh. Several attempts were required to produce a film of adequate UV transmittance, yet free from tears and holes. The supporting mesh was fastened to an arm which could be actuated magnetically to swing the film into or out of the light beam.
(6) W. R. Hunter, D. W. Angel and R. Tousey, Appl. Optics, 4, 891 (1965).
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PHOTOMULT IPI ER SIGNAL
985
945
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875
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Figure 1. Photomultiplier signal from argon spark light source and Nf+ collector current (electron multiplier anode current) from nitrogen ionization
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Upper two traces: no filter, light source pressure 25 microns. Lower two traces: argon gas filter, light source pressure ,- 100 microns. Threshold for N*+ formation is 796
RESULTS AND DISCUSSION
Gas Filter. From its absorption spectrum (7), argon appeared the most suitable gas to suppress the background ionization of nitrogen at wavelengths above 796 A. The absorption limit of argon occurs at 788 &only a few angstroms below the ionization threshold of nitrogen. Between 778 and 480 A, the absorption is continuous with a rather uniform absorption cross section of 3.5 X 10-17cm2. At shorter wavelengths, a few transmission regions appear, caused by the Ml edge, and the absorption cross section gradually decreases. Since at wavelengths greater than 788 A argon is entirely transpment (except at the resonance wavelengths 1048 and 1066 A), it provides a rather unique cut-off filter suitable for the present purpose. The efficiency of the argon filter in reducing the N2+background at wavelengths above the nitrogen ionization onset is demonstrated in Figure 1 . The mass spectrometer signal observed on mass number 28 and the photomultiplier signal were recorded simultaneously as a function of wavelength in the 760 to 985 A spectral region. (7) J. A. R. Samson, J . Opt. Soc. Am., 54,420(1964).
The two upper traces shown in Figure 1 were obtained with the unfiltered argon spark source (operated at 25 microns pressure) to show the appreciable Nz+ currents generated throughout the entire wavelength region. The two lower traces demonstrate the effect of the argon filter. The mass spectrometer collector current is decreased significantly in the wavelength region above the onset of N Zionization, although signals are still observed at isolated wavelengths. An average reduction of N2+ background by a factor of 100 is achieved. The ion current is also effectively reduced in the argon absorption region at wavelengths below 780 A. As a further point of interest, note the change of the intensity distribution in the principal optical spectrum caused by running the argon spark source at a higher pressure (100 microns). Thin Metal Filter. From the summary of Hunter, Angel, and Tousey (6), of the transmission characteristics of thin metal films in the vacuum ultraviolet, indium was selected as one of the few materials suitable for the present application. Its highest transparency occurs in the 780 to 900 A wavelength region. The short wavelength cut-off is rather sharp, occurring near 740 A; at long wavelengths, the transmission limit is approximately 1080 A. The filter used in the present experiments transmitted about 10% of the incident light in the 800 to 900 A spectral region, which by comparison with the data presented by Hunter, et a/., indicates a film thickness of about 0.2 micron. The wavelength dependence of the transmission was in accord with the data by Hunter, et al. The effect of the indium filter upon the Nz+ ion yield from nitrogen is shown in Figure 2 as a function of wavelength. As with the argon gas filter, a considerable reduction of the Nzf background at wavelengths above the nitrogen ionization onset is realized, although again a complete elimination of this background was not achieved. A comparison of signal levels in this wavelength region with and without light reveals some residual ionizaton due, most probably, to pin-holes in the indium film. Since a complete elimination of residual nitrogen ionization apparently cannot be achieved, the indium filter must be considered inferior to the argon gas filter, as it transmits only 10% of the incident light in the wavelength region of interest, whereas the argon filter transmits 100%. The difficulty of handling is another disadvantage of the indium filter. Investigation of Gas Mixtures. The following mixtures of gases were studied using the selective photoionization method: carbon monoxide in nitrogen, carbon monoxide plus ethylene in nitrogen, nitrous oxide in carbon dioxide, acetone in butane, methane in oxygen, and a mixture of ethane, ethylene, and acetylene in nitrogen. The mixtures were prepared in 12liter flasks making use of a high vacuum gas handling system. Research grade gases were introduced to the system via metering valves, except for the acetone-butane mixture, for which the acetone was introduced by a syringe through a rubber septum. The results for the individual mixtures are discussed separately. CARBON MONOXIDE-NITROGEN. Mixtures containing small amounts of CO in nitrogen were studied using the argon filter in conjunction with the argon spark light source operated at 200 microns pressure. The group of emission lines centered near 879 A was utilized because of its favorable intensity and because the photoionization cross section of CO has a peak near this wavelength (8, 9). Several mass spectra (8) R. E. Huffman,J. C. Larrabee and Y.Tanaka, J . Chem. Phys., 40, 2261 (1964). (9) G. R . Cook, P. H. Metzger and M. Ogawa, Can. J . Phys., 43, 1706 (1965).
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WAVELENGTH ( h Figure 2. N y +collector current as a function of wavelength for nitrogen ionization by argon spark light source, with and without thin film indium filter
obtained with these conditions are shown in Figure 3. These include a spectrum of nitrogen containing no CO, a spectrum of nitrogen with 0.05% CO admixed, and a spectrum of nitrogen containing 0.5% CO. Pure nitrogen still shows a small amount of ionization at the mass 28 peak due to incomplete filtering of light from short wavelengths, but the limit for detection of CO in the presence of nitrogen is now much lower than before ( 3 ) , as 0.05% of CO is detected without difficulty. From the extent of residual nitrogen ionization and the sensitivity for CO at mass number 28, one can deduce a detection limit of 100 ppm for CO in nitrogen. Similar results were obtained when the gas filter was replaced by the indium filter. The argon spark source was run at low pressures, and since for this condition the group of lines emitted at 835 A was more intense than that at 879 A, the 835 A emission was utilized to ionize the gas mixture, even though the photoionization cross section of CO at this wavelength is somewhat smaller. A detection limit of 500 ppm of CO in nitrogen was deduced from these experiments. Comparison with the value given above for the argon filter indicates again the superiority of the gas filter. It was also of CARBON MONOXIDE-ETHYLENE-NITROGEN. interest to investigate the influence of a third interfering component on the amount of detectable CO. Ethylene is such a component, as its parent ”peak occurs on mass number 28. At 879 A, ethylene shows some fragmentation, yielding signals also on mass numbers 27 and 26. The ratio of signal strength was determined as 10053.1 :21.5 for mass numbers 28, 27, and 26, respectively. The mass specVOL. 40, NO. 2, FEBRUARY 1968
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I
Figure 3. Mass spectrum of small amounts of additives to nitrogen Ionizing wavelength 879 b. From bottom: nitrogen; 0.05 % CO in nitrogen; 0.5% CO in nitrogen; 0.19% CO and 0.10% C2HI in nitrogen. Note impurities of O2 and H 2 0in nitrogen trum obtained with a mixture of 0.19% CO, 0.10% C2H4,the remainder being nitrogen, is shown in Figure 3. The experimental conditions were the same as those for the other spectra shown. The amount of ethylene present in the mixture can be assessed from the size of the mass 26 peak, as this number is free from interference. For the employed mixture, the amount of CO can then be calculated from the mass 28 peak by subtracting the contribution due to ethylene. It is clear that this procedure will limit the amount of detectable CO to a certain fraction of that of ethylene. It appears that the detection limit of CO will be about 10% of the amount of ethylene, but not less than 100 ppm due to the additional presence of N,. The detection limit for CzH4 is about 50 ppm if the mass 26 peak is utilized for detection. NITROUSOXIDE-CARBON DIOXIDE.A mixture of a small amount of N 2 0 in carbon dioxide presents another example for the discrimination of two interfering parent peaks by selective photoionization. The threshold for ionization of COZoccurs at 900 A (IO,I]), whereas that for NzO photoionization (1.2) lies at 961 A. The wavelength range available for the discrimination of NzO from CO, thus has a width of 60 A. Selective ionization of NzO was achieved by use of the spark source operated with nitrogen at 25 microns pdessure. This provided a strong group of emission lines at 923 A. The argon filter was employed. As was the case for nitrogen, a residual background of COz photoionization was observed which limits the detection of NzO. By comparison of signals produced on mass number 44 of pure CO, and a mixture of 0.1% NzO in COz, respectively, a detection limit of 400 ppm was determined. This limit is higher than that established for CO in nitrogen because photoionization of N2O at 923 A is not so efficient as that of CO at 879 A. In addition, a com(10) G. R. Cook, P. H. Metzger and M. Ogawa, J . Chem. Phys., 44, 2935 (1966). (1 1) R. S. Nakata, K. Watanabe and F. M. Matsunaga, Science of Light, 14, 54 (1965). (12) K. Watanabe, J. Chem. Phys., 26, 542 (1957).
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paratively higher residual background signal may have occurred due to a larger contribution of scattered light from the wider wavelength interval available for CO, ionization above the cutoff of the argon filter. To reduce the contribution of scattered light, we tried krypton as gas filter, which shifts (13) the first absorption edge up to 885 A. No improvement was noticed, presumably because argon is a better absorber than krypton in the 400 to 500 A spectral region where the second order interference originates. ACETONE-BUTANE. Mass spectrometric investigations of the photoionization of acetone and butane were reported by Inghram and collaborators (14, 15). The ionization potentials are: 9.69 eV for acetone, 10.57 eV for isobutane, and 10.63 eV for normal butane (16). The wavelengths corresponding to these values are 1279, 1173, and 1166 A, respectively. However, at these wavelengths, photoionization does not cease abruptly due to the presence of thermally excited molecules. The ionization usually extends a few tenths of an electron volt toward lower energies. From Inghram’s work (14, 15), one can deduce that practical ionization limits are 1200 A for the butanes and 1340 A for acetone. To achieve selective photoionization of acetone, use was made of the dc light source operated with a mixture of hydrogen and argon. The monochromator was adjusted to transmit the strong Lyman-a line at 1216 A. Pure hydrogen in the light source would result in a spectrum rich in molecular lines in the spectral region 900 to 1600 A, but the admixture of argon enhances the atomic Lyman-a line, whereas the molecular line spectrum is subdued. Since there is no emission in the 600 A wavelength region, interference by a second-order optical spectrum should be absent. Nevertheless, some residual photoionization of butane was observed which clearly indicates the importance of scattered light originating from the 900 to 1200 region. This contribution to the total ionizing radiation could be eliminated by the use of a COP gas filter. The COz absorption spectrum (11) has a relative minimum near 1216 A, Toward shorter wavelengths, the absorption increases steeply, whereas toward longer wavelength, the absorption increase is less pronounced. Therefore, COS serves as a suitable bandpass filter for Lyman-a: radiation. The effect of this filter on the discrimination between acetone and butane at mass number 68 is shown in Figure 4, where the mass spectrum of a mixture of 0.16% acetone in butane is compared with pure butane. The elimination of the butane parent peak by application of the CO, filter provides an estimated lower limit of 100 ppm for the detection of acetone in butane. Also of interest is a peak at mass number 56 which is not reduced by the COzfilter. This is due to a butene impurity in butane. The various butene isomers have ionization potentials (16) below that of acetone, so that they are subjected to photoionization by Lyman-a: radiation. Peaks at mass numbers 59 and 60 occur only in the presence of acetone. They may be attributed to reactions of acetone ions with butane. METHANE-OXYGEN, The requirement for methods to determine small quantities of impurities in oxygen was recently discussed by Roboz (17). He also pointed out the various problems encountered in this respect when using
A
(13) R . E. Huffman, Y . Tanaka and J. C . Larrabee, Appl. Optics, 2, 947 (1963). (14) B. Steiner, C. F. Giese and M. C. Inghram, J . Chem. Phys.,34, 189 (1961). (15) E. Murad and M. G. Inghram, J . Chem. Phys.,40,3263(1964). (16) K. Watanabe, T. Nakayama and J. Mottl, J . Quant. Spectrosc. Radiat. Transfer, 2, 369 (1962). (17) J. Roboz, ANAL.CHEM., 39, 175 (1967).
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Figure 4. Mass spectrum of a mixture of 0.16 acetone in butane (upper trace) and butane only (lower trace) ionized at 1216 A, with and without CO, gas filter
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Figure 5. Mass spectrum of a mixture of 0.145z CHain oxygen (upper trace) and oxygen only (lower trace) Ionizing wavelength 923 b obtained from nitrogen spark source. No filter was applied
spectrometers equipped with electron impact ion sources. The difficulties will not be reviewed here, but it appears that the photoionization method would provide some unique advantages. As an example, we consider here the determination of small amounts of methane in oxygen. Ordinarily, the methane parent peak at mass number 16 coincides with the O+ fragment peak of oxygen. However, photoionization at wavelengths above the onset of O+ formation (18) (- 658 A) eliminates the M = 16 peak due to oxygen and permits the assessment of methane. Figure 5 shows the mass spectrum obtained for a mixture of 0.145% CHI in oxygen and, for comparison, pure oxygen. The nitrogen spark light source was used and the monochromator was adjusted to 923 A. N o filter was applied. As a consequence, a small oxygen fragmentation signal occurs at mass number 16. However, even in the presence of this background signal, a detection limit of 75 ppm is realized for the detection of methane in oxygen. With the application of the argon gas filter, the residual O2 fragmentation disappeared entirely so that, for methane in oxygen, the basic detection limit of the instrument (10 ppm) can actually be utilized. ETHANE-ETHYLENE-ACETYLENE-NITROGEN. As a final example of the versatility of the selective photoionization technique, the analysis of a mixture consisting of ethane, ethylene, and acetylene in nitrogen may be discussed. Electron impact ionization of such a mixture leads to a complete overlapping of the individual component mass spectra; thus mass spectrometric analysis becomes very difficult. The overlap can be eliminated when photoionization is employed. Table I shows appearance potentials for interfering parent and frag-
ment ions derived from electron impact data (19), as well as observed photoionization thresholds (14,20-22). For ethane, no photoionization threshold for the C2H4+ fragment was available during the present work. Therefore, threshold wavelengths for C2He+ and C2H4+ were estimated in the present work from the cessation of the ion currents when scanning from lower to higher wavelengths. The derived values are also entered in Table I. Very recently, however, Chupka and Berkowitz (23) reported AP(C2H4+)= 12.08 eV. Reasonable agreement is obtained for the various threshold observations. According to the electron impact data (19), the minimum energy required to ionize ethane, ethylene, and acetylene simultaneously is 11.6 eV; the maximum energy allowed in order to avoid fragmentation is 12.1 eV. Use of photoionization lowers these values slightly because of the occurrence of ionization tails caused by thermally excited molecules and also because electron impact values often do not correspond to the adiabatic ionization potentials. Table I indicates that the limits are 11.54 eV and 11.94 eV, respectively, corresponding to the wavelength range 1074 to 1030 A. Since the spark light source does not provide a sufficiently (18) G . L. Weissler, J. A. R. Samson, M. Ogawa and G. R. Cook, J . Opt. SOC.A m . , 49,338 (1959). (19) F. H. Field and J. L. Franklin, “Electron Impact Phenomena,” Academic Press, New York (1957). (20) R. Botter, V. H. Dibeler, J. A. Walker and H. M. Rosenstock, J . Chem. Phys., 44, 1271 (1966). (21) B. Brehm, 2. Nuturforsch., 21a, 196 (1966). (22) R. Botter, V. H. Diebeler, J. A. Walker and H. M. Rosenstock, J . Chem. Phys., 45, 1298 (1966). (23) W. A. Chupka and J. Berkowitz, J . Chem. Phys., 47, 2921 (1967).
Table I. Threshold Energies and Wavelengths for Ionization by Electron and Photon Impact of Ethane, Ethylene and Acetylene Electron impact Photoionization Gas Ion Mass unit Energy (eV) Wavelength (A) Energy (eV) Wavelength (A) Investigator Acetylene CzHz+ 26 11.4 1088 11.40 1088 Brehm (21) 11.40 1088 Dibeler (20) Ethylene CzHa+ 28 10.5 1179 10.50 1179 Brehm ( 2 2 ) CzHz+ 26 13.5 918 12.96 957 Brehm (21) 13.12 945 Dibeler (22) Ethane C&+ 30 11.6 1069 11.55 1075 Inghram (24) 11.54 1074 present C2Haf 28 12.1 1025 12.03 1030 present 12.08 1026 Chupka (23) ~~
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intense emission line in this wavelength region, the dc discharge source was again utilized. When this source is operated with argon, it emits mainly the strong argon doublet at 1048 and 1066 A and some weaker lines at longer wavelengths due to hydrogen, oxygen, and nitrogen impurities. No emission occurs at shorter wavelengths, so that this source does not require a filter. Ethane, when photoionized with the argon dc source, produced a small peak at mass number 28 in addition to the mass 30 ethane parent peak. However, gas chromatographic analysis indicated that the ethane contained a 0.4% impurity of ethylene, which explains the mass 28 contribution. The mass spectrum of a mixture consisting of 7.36% ethane, 0.165% ethylene, 0.33% acetylene, the remainder being nitrogen is shown in Figure 6, together with the spectrum of pure ethane. The ionizing wavelength was 1048 A. From the spectrum, the relative sensitivity of the gases is found to be 1:3.3:4.4 for ethane, ethylene, and acetylene on mass numbers 30, 28, and 28, respectively. Also apparent from Figure 6 is the relatively sizable contribution of the M 1 peaks presumably caused by ion-mclecule interactions.
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CONCLUSIONS
The results presented in this paper demonstrate the feasibility of selective photoionization in the mass spectrometric analysis of gas mixtures. The use of a gas filter in the optical path has improved the detection limit to very useful levels. For binary gas mixture with overlapping component parent peaks, this limit is about 100 ppm for the constituent with the lower ionization potential. The accuracy of measurement is comparable to that of conventional mass spectrometry and is governed mainly by the stability of the light source and the detection system. Since the light intensity is monitored, the effect of fluctuations in the light intensity may be cancelled by use of a proportional readout system, if high accuracies are desired. The major drawback of the present technique appears to be the comparatively long signal integration times required (1 sec) due to the relatively low signal levels, so that the possibility of rapid scanning is excluded. Further improvement of the detection limits appear possible, but it would require an increase in the basic detection limit of the instrument as well as a decrease of the residual signals due to unwanted ions. The first requirement can be met by electronically increasing the signal to noise ratio of the detector system, but the second hinges on a redetermination of the predominant cause of the residual ionization-i.e., the higher order spectrum, scattered light, and accelerated electrons. A further complication caused by the use of comparatively high pressures in the ion source is the occurrence of ion-molecule reactions. As previously (3), high ion source pressures are employed deliberately to make the photoionization method suitable for analytical applications. While the conversion of ions from one type to another can be a benefit in the identification of molecules in some cases, it generally counteracts the desired simplification of mass spectra for the purpose of gas analysis. The ion source extraction geometry employed in the present experiments was designed to minimize ion-molecule reactions, Nevertheless, these were
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M A S S NUMBER Figure 6. Mass spectrum of a mixture of 7.36% C2Hs, 0.165% C2H4,0.33% C2Hzin nitrogen (upper trace) and ethane (lower trace) ionized at 1048 A generated by a dc argon source. No filter was applied
observed, even though for the samples discussed, they did not cause any interference. The general significance of ion-molecule reaction interference is as yet difficult to assess, but it is clear that this can be a problem, particularly if a further improvement of the present technique is sought. ACKNOWLEDGMENTS
Credit is due to Dr. A. E. Barrington who initiated the present program by suggesting the selective photoionization technique to detect CO contamination in a closed environment; to Dr. H. Liebl who first proposed use of the special 180-degree mass analyzer; and to Mr. G. Wood for continuous encouragement and support. RECEIVED for review August 21, 1967. Accepted December 14, 1967. Supported by the National Aeronautics and Space Administration under Contract Nos. NAS1-4927 and NASl6335.
