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P. J. Todd, A. J. H. Boerboom, M. Durup, J. Durup, and S. H. Bauer for very helpful advice and consultation.
LITERATURE CITED (1) Devienne, M. F. M. C . R . Seances Acad. Scl., Ser. 8 1988, 267, 1279-1281. (2)Gray, J.; Tomllnson, R. H. Int. J. Mass Spectrom. I o n Phys. 1974, 75,121-131. (3) McLafferty, F. W.; Todd, P. J.; McGilvery, D. C.; Baldwin, M. A. J. Am. Chem. SOC. 1980, 702, 3360-3363. (4) Todd, P. J. Ph.D. Thesis, Cornell University, Ithaca, NY, May 1980. (5) Gellene, G. I.; Porter, R. F. Acc. Chem. Res. 1983, 76,200-207. Wesdemiotis, C.; McLafferty, F. W. J. Am. Chem. SOC. (6) Danis, P. 0.; 1983, 705,7454-7456. (7) Burgers, P. C.; Holmes, J. L.; Mommers, A. A,; Teriouw, J. K. Chem. Phys. Lett. 1983, 702,1-3. (8) Burgers, P. C.; Holmes, J. L.; Mommers, A. A.; Szulejko, J. E.; Terlouw, J. K. Org. Mass Spectrom. 1984, 79,442-447. (9) Geilene, G. I.; Porter, R. F. Int. J. Mass Spectrom. I o n Processes 1985, 64,55-66. (IO) Clalr, R.; Holmes, J. L.; Mommers, A. A,; Burgers, P. C. Org. Mass Spectrom. 1985, 20,207-212. (11) Burgers, P. C.; Holmes, J. L.; Mommers, A. A. J. Am. Chem. SOC. 1985, 707,1099-1101. (12) Terlouw, J. K.; Kieskamp, W. M.; Holmes, J. L.; Mommers, A. A,; Burgers, P. C. I n t . J. Mass Spectrom. I o n Processes 1985, 64,
245-250. (13) Danis, P. 0.;Feng, R.; McLafferty, F. W. Anal. Chem. 1988, 58, 346-354. Feng, R.; Tso, J.; McLafferty, F. W. J. (14) Wesdemiotis, C.; Danis, P. 0.; Am. Chem. SOC. 1985, 707,8059. (15) Kim, M. S.; McLafferty, F. W. J. Am. Chem. SOC. 1978, 700, 3279-3282. (16) Laramee, J. A,; Cameron, D.; Cooks, R. G. J. Am. Chem. SOC. 1981, 103,12-17. (17) Rourke, F. M.; Sheffield, J. C.; Davls, W. D.; White, F. A. J . Chem. Phys. 1959, 37, 193-199. (18) Bowen, R. D.; Barbalas, M. P.; Pagano, F. P.; Todd, P. J.; McLafferty, F. W. Org. Mass Spectrom. 1980, 75,51. (19) Ast, T.; Porter, C. J.; Proctor, C. J.; Beynon, J. H. Bull. SOC. C h h . Beograd. 1981, 46, 135-151. (20) Bydln, Yu. F.; Bukhteev, A. M. SovietPhys. Tech. Phys. Engl. Transl. 1980, 5 , 512-519.
(21) Bukhteev, A. M.; Bydin, Yu. F.; Dukel'skii, V. M. Soviet Phys. Tech. Phys. Engl. Transl. 1981, 6, 496-499. (22) Durup, M.; Pariant, G.; Appell, J.; Durup, J.; Ozenne, J. Chem. Phys. 1977, 25, 245-261. (23) Durup, J.; Durup, M. Unlverslty Paris-Sud Orsay, private communication, May 1979. (24) Morgan, T. G.; Brenton, A. G.; March, R. E.; Harris, F. M.; Beynon, J. H. I n t . J. Mass Spectrom. I o n Processes 1985, 64, 299-314. (25) Danis, P. 0.Ph.D. Thesis, Cornell University, Ithaca, NY, Aug 1985. (26) Rabrenovlc, M.; Trott, G. W.; Kim, M. S.; Beynon, J. H. Org. Mass Spectrom. 1984, 79,203-204. (27) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J. Phys. Chem. Ref. Data Suppl. 1977, 6. (28) Franklin, J. L.;Diilard, J. G.; Rosenstock, H. M.; Herron, J. T.; Draxi, K.;
Fleld, F. H. Natl. Stand. Ref. Data Ser. (U.S. Natl. Bur. Stand.) 1969, No. 26. (29) Schram, B. L.; Boerboom, A. J. H.;Kleine, W.; Kistemaker, J. Physlca (Amsterdam) 1988, 32, 749-761. (30) Christodouiides, A. A.; McCorkle, D. L.; Chrlstophorou, L. G. I n "Electron-Molecule Interactions and Their Applications"; Christophorou, L. G., Ed.; Academlc Press: Orlando, FL, 1984; Vol. 2, pp
423-641. (31) Siuyters, Th. J. M.; DeHaas, E.; Kistemaker, J. Physica (Amsterdam) 1959, 25, 1378-1388. (32) Todd, P. J.; McLafferty, F. W. I n t . J. Mass Spectrom. Ion Phys. 1981, 38, 371-378. Feng, R.; Wesdemlotls, C.; McLafferty, F. W., in prepara(33) Danis, P. 0.; tion.
(34) McLafferty, F. W.; Bente, P. F., 111; Kornfieid, R.; Tsai, S. 4.;Howe, I. J. Am. Chem. SOC. 1973, 95,2120-2129.
RECEIVED for review August 9, 1985. Accepted October 1, 1985. This work was supported by the National Science Foundation (Grant CHE-8406387). Construction of the tandem double-focusingmass spectrometer was supported by the National Institutes of Health (Grant GM-16609) and the Army Research Office (Grant DAAG29-82-K-0179). Much of this work was presented at the 32nd Conference On Mass spectrometry and Applied Topics, San Antonio, TX, May 1984.
Determination of Additives in Polypropylene by Selective Chemical Ionization Mass Spectrometry Patrick Rudewicz' and Burnaby Munson* Department of Chemistry, University of Delaware, Newark, Delaware 19716
A method for the determination of additives in polypropylene without prlor separation is described. The additives are vaporized from polypropylene samples in a heatable glass probe under chemical ionization (CI)conditions uslng a 1.1% NH, in CH4 reagent gas mixture. The dominant Ion in this mixture, NH,', is a low-energy reagent ion that reacts wlth the addltives to give very simple spectra of (M H)' or (M NH,)' tons and little fragmentation. The selectivity of the ammonium ion for the detectlon of addltives In a hydrocarbon rnatrlx is demonstrated.
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Various compounds are added to polymers to retard freeradical-induced photolytic and thermal degradations. These additives include UV absorbers, which reduce the rate of photolytic oxidation; sterically hindered phenols, which act as radical chain terminators; and various sulfur and phosphorus compounds, which deactivate peroxides formed by the *Present address: Smith K l i n e a n d F r e n c h Laboratories, 620 Allendale Rd, K i n g of Prussia, PA 19406.
reactions of free radicals with oxygen. Reliable analytical methods are essential for the analysis of polymer additives, especially for those polymers used in food packaging and as medical plastics. The analysis of polymer additives is usually carried out with an initial extraction step followed by separation and identification of the additives. Extraction procedures for many polyolefins have been reviewed by Wheeler ( I ) . Extraction is often the most difficult and time-consuming step in the analysis of additives, especially when quantitation is required. The extraction of antioxidants from ground polyethylene with chloroform in a Soxhlet extractor for 6 h has been described (2). Extraction of antioxidants from 8-mesh polypropylene pellets in tetrahydrofuran takes 24 h (3). Schabron and Fenska have reported a faster extraction procedure for three antioxidants from polyethylene using decalin at 110 OC for 30 min (4). This procedure was also used for the extraction of additives from polypropylene (5). After extraction from the polymer the additives must be separated prior to identification. Several HPLC procedures have been described (3-5)) with UV detection and the use of retention times for identification. Other methods, including
0003-2700/86/0358-0358$01.50/00 1986 Amerlcan Chemical Society
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paper chromatography, thin-layer chromatography, and gas-liquid chromatography, have been reviewed by Wheeler (1). Recently, liquid chromatography/chemical ionization mass spectrometry was applied to the analysis of antioxidants and UV stabilizers in polymers (6). Because of the long extraction times, several spectroscopic methods have been employed for the in situ determination of polymer additives (7,8). The UV determination of antioxidants in hot-pressed polyethylene films has been described (7). Similarly, IR (8) and phosphorescence (9) analyses of antioxidants using double-beam spectrophotometers with some pure polymer film in the reference beam have been reported. The main disadvantage of the spectroscopic techniques is a lack of specificity. Mass spectrometric techniques have been occasionally applied to the determination of additives in polymers. Additives in polypropylene have been characterized by heating large (a few tenths of a gram) samples of polypropylene in the heated inlet system of a mass spectrometer and then analyzing the volatile products by electron ionization (EI) mass spectrometry (IO). Others have used a probe and E1 mass spectrometry to characterize additives in polypropylene ( 2 , I I ) . We wish to describe the use of selective reagents in chemical ionization (CI) mass spectrometry for the determination of additives in polypropylene without prior separation. The additives are vaporized from the polypropylene samples in a heatable glass probe and ionized under chemical ionization (CI) conditions using a low-energy proton transfer reagent.
EXPERIMENTAL SECTION A small slice of a polypropylene pellet (1-2 mg) containing a few hundredths to a few tenths of a percent of volatile additives was placed in the well of a heatable glass probe built in our laboratory (12). The probe was placed into the source of a Du Pont 492B mass spectrometer, which has been modified for CI operation (12),and heated from 30 "C to 350 "C at 20 or 30 OC/min. Spectra were obtained every 6 s with a Hewlett-Packard 21-h4X computer and a Du Pont data system. Spectra of the pure additives were also obtained by introducingthe samples with the heatable glass probe. The ion accelerating voltage was approximately 1750 V. The source repeller voltage was kept at 0 V to maximize the ionic residence times. A mixture of 1.1%NH3 in CH, (MG Scientific Gases, North Branch, NJ) was used as the reagent gas at 0.5 torr, with the source temperature kept at 225 "C for the pure additives and 240 "C for the polymer samples. The source pressure was measured with a capacitance manometer (MKS Instruments, Burlington, MA) through a hollow glass probe connected directly to the ion source. Approximately 2 g of polymer pellets was extracted with 25-50 mL of solvent, acetonitrile overnight at room temperature or boiling decalin (195 "C) for 10 min. The additives were separated and quantitated by using a Beckman Model 342 liquid chromatograph with a 4.6 X 250 mm Beckman 5-hm Ultrasphere ODS column using 955 CH30H/H20as the mobile phase and a Kratos Spectroflow 783 UV detedor at 280 nm. A good linear calibration curve was obtained from the pure additive. The polymer samples and the pure additives were obtained from the Exxon Chemical Co., Baytown, TX. RESULTS AND DISCUSSION Ammonia chemical ionization mass spectrometry (CIMS) has been used extensively in the characterization of many types of organic compounds. Because of its high proton affinity, NH3 frequently gives very simple CI spectra consisting of (M + H)+ and/or (M + NH4)+ions with very little fragmentation. We have found that NH3 CI sensitivities for oxygenated compounds with proton affinities less than that of NH3, 204 kcal/mol (13),increase with decreasing partial pressure of NH3 in CH4 (14); consequently, a 1.1% NH3 in CHI reagent gas mixture was chosen for the analysis of polymer additives. With this mixture, the ammonium ion, NH4+,is the major reactant ion (85% of the reactant ioni-
CI spectra of Ionox 330, Irganox 168, and UV-531: reagent gas, 1 % NH,/CH,; source pressure, 0.5 torr; source temperature, 225 OC.
Flgure 1.
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zation) at 0.5 torr total source pressure. If the proton affinity of the sample is greater than 204 kcal/mol, the ammonium ion will transfer a proton to give (M + H)+ ions. This (M + H)+ ion will not solvate with NH3 to any significant extent since NH3 is present in low concentration. If the sahple has a proton affiiity lower than 204 kcal/mol but contains a polar functional group (-SH, -OH, =0, etc.), the ammonium ions will react by electrophilic attachment to give (M + adduct ions. The structures, trade names, and CI mass spectra (1.1% NH3 in CH4 as the reagent gas) of three commonly used polymer additives are shown in Figure 1. Each additive gives either an abundant (M H)+ or (M + NHJ+ ion with little fragmentation. (For simplicity, the masses of the heavier ions are reported from calculations with integral masses: (M + NH4)+ for Ionox 330 = 792 from integral masses vs. Cs4HBz03N+ = 792.6 from nuclidic masses.) The antioxidant, Ionox 330, a sterically hindered phenol, gives (M NH4)+as the base peak with a low abundance of (M + H)+ ions and two low abundance ions, which may be fragment ions or which may result from lower molecular weight impurities. The phosphite, Irganox 168, a peroxide deactivating agent, and the substituted benezopheone, UV-531, an ultraviolet stabilizer, give essentially single species spectra, (M H)+. The proton affinities of these compounds are not known, and there are few model compounds that one could use to predict whether (M + H)+ or (M + NH4)+should be the dominant ionic species. However, the proton affinity of phenol is known, 196.3 kcal/mol, and is significantly less than that of ammonia, 204.0 kcal/mol (13). The proton affinities of benzophenone (210.9 kcal/mol) and (CH30),P (220.6 kcal/ mol) are both significantly higher than that of ammonia (13). From these data and the CI spectra, one would conclude that the hindered phenol, Ionox 330, is a weaker base than ammonia and the phosphite, Irganox 168, and aromatic ketone, UV-531, are more basic.
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986 r
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PROBE TEMP.
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Figure 2. (M + H)' ion profile for UV-531 vaporized from polypropylene: reagent gas, 1.1% NH,/CH,, 0.5 torr: source temperature, 240 "C; probe heatlng rate, 20 "C/min.
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168 222 276 PROBE TEMPERATURE ( C)
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Flgure 4. Heating profiles for (M + H)+ for Irganox 168, m / z 647, and (M + NH4)+ for Ionox 330, m / z 792, from polypropylene. Probe heating rate was 3 0 "C/min.
(M + H)+or (M + NH4)+ions in the NH, CI spectra. However, greater confidence in the identification would be provided by spectra obtained with a more reactive CI reagent gas, like CHI, which gives significantly more fragmentation than does NHP The CHI CI spectrum of Ionox 330 contains (M H)+ as the most abundant ion and abundant fragment ions at m / z 569, 219, 203, and 163. This compound may be readily identified in a simple mixture by noting that these ions have the same temperature profiles. Similarly, the CHI CI spectrum of Irganox 168 contains a reasonably abundant (M H)+ ion, the base peak at mlz 441 and another fragment ion at m / z 147. Similar temperature profiles for these ions indicate the presence of this additive. One could probably identify the additives from a limited set of likely possibilities by comparing temperature (or time) plots of characteristic ions, even when several additives are present. The disadvantage of methane as a CI reagent for quantitative studies in probe distillation analyses is the same as the disadvantage of EI: the spectra are more complex and it is likely that there will be interferences of ions from additives and pyrolysis products. With NH3 as the CI reagent gas, however, the pyrolysis products of polypropylene are barely detectable (if at all) and there is much less likelihood of interferences among additives. Quantitation may be achieved by integration of the ion current for a characteristic ion, (M H )', m / z 327 from UV-531, in an experiment such as that shown in Figure 2 and comparison of this area with areas obtained from vaporization of known amounts of UV-531 under very similar conditions. In this experment, mass spectral analysis gave a value of 0.054 wt % W-531 in a sample of polypropylene reported to contain 0.05 w t % additive. Similar experiments several months later (with separate calibration curves) gave values of 0.054-0.057 wt % UV-531. The good agreement between analysis and preparation indicates that there are no strong matrix effects and that most of the additive is vaporized from the polymer sample. A thorough analysis of the effects of instrumental parameters on the precision and accuracy of this analysis has not been done. The problems associated with quantitation by probe analysis have been discussed extensively (16) and apply to this procedure as well. In these experiments a nonlinear calibration curve was obtained over the range of 0.2-1 pg of additive, probably because of adsorption of small amounts of the additive on the glass probe. Silylation of the probe tip may reduce these adsorption losses. One could readily detect the UV-531 from a 0.3:mg sample containing 0.05% additive. The short-term reproducibility of peak areas was i 6 % .
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Flgure 3. Spectrum of UV-531 vaporized from polypropylene: reagent gas, 1.1% NH,/CH,, 0.5 torr; source temperature, 240 "C. Figure 2 shows the ion profile for m / z 327. When a slice of a polypropylene pellet (1.6 mg) containing approximately 0.05 wt % UV-531 is heated from a probe at 20 "C/min. The additive, UV-531, begins to vaporize rapidly from the sample when the probe temperature reaches 175-180 "C, approximately the melting point of this sample of polypropylene. The small amount of m / z 327 (scans 15-25) is perhaps due to a rapid heating of the tip of the polymer slice, which extends slightly into the reagent gas a t 240 "C. Another possible explanation is that a small amount of UV-531 is present on the surface of the polymer. The additive completely vaporizes from the sample before the onset of the polypropylene decomposition at approximately 310 "C. Since the ammonium ion does not react rapidly with hydrocarbons (1.9,spectra at scan numbers, 85-110, contain predominantly ions at m/z 327 with very little ionization from the polypropylene. Figure 3 shows that the spectrum for scan 87, the peak maximum, is very similar to the spectrum of pure UV-531 shown in Figure 1. There are some low abundance ions present in the spectrum of the UV-531 from the polymer that are not present in the spectrum of pure UV-531. The origin of these ions cannot be clearly established; however, their heating profiles begin when the polypropylene melts, indicating that they may be from trace amounts of volatile impurities present in the polypropylene. Mixtures and higher molecular weight additives can also be analyzed with this technique. Figure 4 shows two ion profiles when a 2.0-mg sample of polypropylene containing 0.15 w t % Irganox 168 ( m / z 647, (M H)') and 0.05 wt % Ionox 330 ( m / z 792, (M + NH4)+is heated at 30 "C/min. Both additives begin to vaporize rapidly at temperatures near the melting point of polypropylene, about 176 "C. The two additives, however, can clearly be distinguished by their different masses and their very different heating profiles. The lower molecular weight additive gives a relatively sharp peak with a maximum at approximately 210 "C and the higher molecular weight additive gives a broader peak that maximizes near 300 "C. It is possible to identify these additives from a small set of likely compounds from the masses of their characteristic
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Heating rates of 20-30 "C gave satisfactory results for several additives, but no effort was made to establish an optimum heating rate. Extraction of polypropylene pellets with acetonitrileat room temperature overnight gave clear evidence for the additives, but the extraction was approximately 2% efficient. Extraction with boiling decalin (4, 5) gave 0.053 wt 70 UV-531 in this sample. The samples used in this work, 1-2 mg, do not constitute either upper or lower limits on the amounts of polymer that can be used. Smaller samples allow one to study the distribution of additives within a bulk sample. This use of small samples (I1mg) allows determination of heterogeneities of additive concentrationsin bulk samples but would not produce a good average value of additive concentration in a large batch of polymer. Care must be maintained in these studies that the bulk of the decomposition products of polypropylene do not contaminate the metal surfaces in the acceleration region of the mass spectrometer. ACKNOWLEDGMENT The authors are grateful to Ron Orlando for obtaining some of the spectra. Registry No. UV-531, 1843-05-6;Irganox 168, 31570-04-4; Ionox 330, 1709-70-2;polypropylene (homopolymer),9003-07-0.
LITERATURE CITED (1) Wheeler, D. A. Talanta 1968, 15, 1315-1334. (2) Swaren, S. J.; Wims, A. M. J . Appl. Polym. Sci. 1975, 19, 1243-1256. (3) Ranfelt, F.; Lichtenhaler, R. G. J . Chromatogr. 1978, 149, 553-560. (4) Schabron, J. F.; Fenska, L. E. Anal. Chem. 1980, 52, 1411-1415. (5) Schabron, J. F.; Bradfield, D. 2. J . Appl. Polym. Sci. 1981, 26, 2479-2483. (6) Vargo, J. D.; Olson, K. L. Anal. Chem. 1985, 5 7 , 672-675. (7) Luongo, J. P. Appl. SpectrosC. 1965, 19, 117-120. (8) Miller, R. G. J.; Wiills, H. A. Spectrochim. Acta 1959, 14, 119-126. (9) Drushei, H. V.; Sommers, A. L. Anal. Chem. 1964, 3 6 , 836-840. (10) Yoshlkawa, T.; Ushimi, K.; Kimura, K.; Tamura, M. J . Appl. Polym. SCi. 1971, 75, 2065-2072. (1 1) Shimanskas, C.; Ng, K.; Kariiner, J. Paper presented at the 33rd Annual Conference on Mass Spectrometry and Applled Topics, San Diego, CA, May 26-31, 1985. (12) Spreen, R. C. Ph.D. Thesis, University of Delaware, Newark, DE, 1983. (13) Lias, S. G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. Ref. Data 1904, 13, 695-808. (14) Rudewicz. P.; Munson, B. Paper presented at the 32nd Annual Conference on Mass Spectometry and Applied Topics, San Antonio, TX, May 27-June 1, 1984. (15) Hunt, D. F. Prog. Anal. Chem. 1973, 6 , 359-376. (16) Millard, B. J. "Quantitatlve Mass Spectrometry"; Heyden: London, 1978.
RECEIVED for review June 19,1985. Accepted October 7,1985. This work was supported by the National Science Foundation (CHE-83 12954).
Ion Mobility Spectrometry in Carbon Dioxide Souji Rokushika and Hiroyuki Hatano
Department of Chemistry, Faculty of Science, Kyoto University, Kyoto, 606 Japan Herbert H. Hill, Jr.*
Department of Chemistry, Washington State University, Pullman, Washington 99164-4630
This work investigates the potential of uslng carbon dioxlde as a drift and carrler gas in ion mobility spectrometry. AIthough separations in carbon dioxlde were previously thought to be difflcult due to large Ion Cluster formatlon, this study showed that under normal operating temperatures for analytical ion moblilty spectrometry separation of both reactant ions and product ions Is possible. Comparison of reactant ions and product Ions in CO, and N, demonstrated that whlle Ion drift times are conslderably longer In CO, than in N,, the patterns of the ion mobility spectra are similar for the two drW gases. Test compounds studied in thls work were a series of straight-chaln methyl esters ranging In molecular welght from 88 to 446.
The use of carbon dioxide as a drift gas for ion mobility spectrometry (IMS) has been limited. In fact, there has been only one brief investigation of the ion transport properties of C02at atmospheric pressure (I). This study concluded that the "mobility of ions in C 0 2is largely independent of the ion species so long as the pressure is greater than about 100 torr". On the basis of this conclusion, it would seem that C02would not be useful as a drift gas in IMS. Nevertheless, recent developments in supercriticalfluid chromatography (SFC) and ion mobility spectrometry have made the continued investigation of C02 as a drift gas desirable. 0003-2700/86/0358-0361$01.50/0
The use of supercritical carbon dioxide as a mobile phase in capillary chromatography has enabled the efficient interfacing of ambient pressure ionization detection methods such as flame ionization (FID) for the sensitive detection of compounds separated by supercritical fluid chromatography (2). The highly successful nature of the SFC/FID interface suggests that other detection methods traditionally assigned to the domain of gas chromatography may also be possible with SFC. Ion mobility spectrometry has been successfully interfaced to high-resolution gas chromatography (3). Perhaps it can also serve as a sensitive and selective detection method for SFC. Since one of the most common mobile phases employed in SFC is carbon dioxide, interfacing IMS with SFC would be facilitated if C02could be used as a drift gas in the ion mobility spectrometer. When the chromatographic mobile phase is matched with the ion mobility drift gas, complications may be avoided when the two gases mix in the ionization region of the ion mobility spectrometer. The primary objective of the investigation reported in this paper was to provide preliminary information on the mobility behavior of ions in C02 under conditions similar to those that would be employed if the spectrometer were interfaced to a supercritical fluid chromatograph. Despite the absence of data on atmospheric pressure IMS in COz, there has been some interest in ions formed in C 0 2 and their mobilities at low pressures. Mobilities of 0- and 0 1986 American Chemical Society
