Atomic mass spectrometry - Analytical Chemistry (ACS Publications)

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Anal. Chem. 1990, 62. 394R-402R , ‘871987, 18-66. (N112) Dark, W. A. Int. W S (N113) Velth, C. A.; cohen.R. f?&b?nef 1989, 30, 942-8. (N114) Shekovaya. E. E.; Ershov, 0. V.; Lyubimtseva, (3. P.; Teishev, A. E.; our’yanova, V. V.; Pavlov. A. V. Vysokomd. S m W . , Ser. A 1987, 29, 2226-9 (Russ); Chem. Abstr. 1988, 708, 6731j. (N115) Wei, Y.; Hsueh, K.; Tang, X.; Sun, Y. Polym. Prepr. (Am. Chem. SOC.. Dlv. POlym. chem.)1989, 30(1), 226-7. (N116) Dawst, D. Spectra 2000 [Deux Mille] 1987, 122, 40-1 (Fr); Chem. Abstr. 1987, 107, 218378d. (N117) Chen, W. Y.; Ho, W. H.; Chiu, Y. S. Int. GFC Symp. ‘87 1987, 447-54. (N118) Vakhtb, I.A,; Metlyakova, I.R.; Petrakova. E. A.; Samoiiova, N. P. &St. MSSy 1988, (3). 50-1 (Russ); Chem. Abstr. 1988. 709, 55594~. (Nll9) Nosterm, V. V.; Chubarova, E. V.; Belen’kii. B. G. Vysdromd. Soedin.. Ser. A 1989, 37,653-7 (Rws); Chem. Abstr. 1989. 7 7 7 , 40252a. (N120) Mukherjee, A. K.; Patri, M. Angew. # a k m / . Chem. 1989, 777, 131-9. (N121) Fuller, E. N. Report 1988. BDX-6134948; Order No. DE88013226, Avail. NTIS. (N122) MeiSSm, K.; Vogel, J.; Poltersdorf, B. pleste Keutsch. 1987, 34, 449-51 (h); Chem. Abstf. 1988, 108, 132730~. (N123) Yaw, M. H.: Liaw, W. C. J . Chin. Chem. Soc. (Taipei) 1988, 35, 147-52. (N124) Altken, C.; Harrod, J. F.; Gill. U. S. Can J . Chem. 1987, 65. 1804-9. (N125) Sawan, S. P.; Tal, Y. G.; Huang, H. Y.; Muni, K. P. Powm. Repr. (Am. Chem. Soc., Div. Polym. Chem.) 1988, 29, 252-3. (N126) Fwmoy, T. R.; Semlyen, J. A. P w m . Commun. 1989, 30, 86-9. (N127) MuHer, A. J.; OpHa, R. L. Ehsctf. Conacts 1988, 34, 269-300. (N128) Ma, R.; Cheng, W. Fenxl Ceshi Tongbao 1988, 7 (5), 32-6 (Ch); Chem. Abstr. 1989, 770, 155159~. (N129) Weisskopf. K. J . Polym. Sci., Pari A : Polym. Chem. 1988, 26, 19 19-35. (N130) Sjprey, R. E. Elestomefics 1989, 727 (3). 15-17. (N131) Lee, C. H.; Mailinson, R. G. J . Appi. Polym. Sci. 1989, 37, 33 15-27. (N132) bomnicheva, N. A.; Kogan, S. I.; Kuznetsova, V. A,; Sorokin, A. Y.; Budtov, V. P. Vysokomol. Soedln., Ser. A 1989, 37, 597-601 IRuss): Chem. Abstr. 1889, 1 7 7 , 40221q. (N133) Wagenaar, P. Conw. FATIPEC 1987, 78th (Voi. 3), 491-503. (N134) Sz6ztay. M.; Laszld.edvig, 2.; Tudos, F.; Hoerhold, H. H.; Klee, J. Angsw, Makromd. Chem. 1988. 782, 149-62. (N135) Noel, D.; Cole, K. C.; Hechler, J. J.; Chouliotis, A.; Overbury, K. C. J . ChromatOgr. 1987. 408, 129-44. (N136) Larwn, F. N.; Spieker. D. A. Int. GfC Symp. ’87 1987, 336401. (N137) Wooten, A. L.; Rswltt, M. L.; Sellers, T.; Teller, D. C. J . chromatcgr. 1988, 445, 371-6. (N138) Rledl, B.; Celve, L.; Blanchette, L. tio/zforschung 1988, 42, 315-18. (N139) Husain, S.; Sastry. G. S. R.; Raju, N. P.; Narasimha, R. J . chrometom. . -I. - 1088. 454. 317-26 (N140) Dobarganes, M. C.; Perez-Camino, M. C.; Marquezduiz, G. Feft WbS Technol. 1988, 90, 308-1 1. (N141) Goncelves, D. Arq. Bid. Tecnd. 1988, 31, 469-74 (Port); Chem. Abstr. 1989. 171. 6012n. (N142) Mertin,’I.; Uk. U. J . Chromatogr. 1989, 466, 339-45. (N143) Johnson, D. K.; Chum, H. L. ACS Symp. Ser. 1988, 376, 156-66. “44) W d t O , S. A.; C w . D. J.; Kiceniuk, J. W.; Simpler, K. T. Org. GeoCIKM). 1988. 13, 273-81. (N145) Yaw, P. W.; Mantsch, H. H.; Kotlyar, L. S.; Woods, J. R. Energy 2.. 26-31... .Fuels __ - 1988. ...-, (N146) Beazley. P. M.; Hawsey, L. E.; Plummer, M. A. Transp. Res. Rec. 1987. 7775. 46-50. (N147) ‘Cy, N:; McIntyre, D. D.; Toth, G.; Straw, 0. P. Fuel 1987, 66, 1709- 14.

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(N148) Yamada, Y.; Koinuma, Y. SekiyuGakkaLshi1989. 32, 7-10 (Japan); Chem. Abstr. 1989, 710, 9843ld. (N149) Glover, C. J.; Bullin, J. A.; Button, J. W.; Davison, R. R.; Donaldson. G. R. Report 1987. Order No.,PB88-128566, Avail. NTIS. (N150) Leite, L.; Camillo, M.; Deane, G. H. W.; Cintra. R. H.; Vieira. R.; Araujo, N.; Brandao, L.; Ferraz de Carvalho, J. R. Bd. Tec. F€lROBRAS 1989, 32, 41-50 (Port); Chem. Abstr. 1989, 7 7 1 , 1797892. (N151) Garrick. N. W.; Wood, L. E. Asphalt Paving Techno/. 1988, 57, 26-40. (N152) Buchanan, D. H.; Warfel, L. C.; Bailey. S.; Lucas, D. Energy Fuels 1988, 2, 32-6; (N153) Pandey, R. N. Report 1987, MICROLOG-86-03145, Avail. Energy Mines Res. Can. 555 Booth St.. Ottawa, Ont., Can., K1A 001. (N154) Changming, 2.; Aiying, L.; Furong, X.; Zengmh S. Prepr.-Am. Chem. SOC.,Div. Pet. Chem. 1989, 34, 247-50. (N155) Mulllgan, M. J.; Thomas, K. M.; Tytko, A. P. Fuel1987, 66, 1472-80. (N156) Cerny, J.; Miera, J.; Vavrecka, P. Fuel 1989. 68, 596-600. (N157) Philip, C. V.; Moore, P. K.; Anthony, R. G. ACS Symp. Ser. 1987, 352, 183-200. (N158) Ftzgerald, J. J.; Hoffman, J. A.; Nebo, Chinedo, 0. J . Chromtogr. SCi. 1989, 27, 186-92. (N159) Nakahara, H.; Kobayashi, E.; Hattori, S.; Kamata, T. N/ppon Kagaku Kaishi 1987, (12), 2308-14 (Japan); &em. Abstr. 1988, 108, 138758h. (N16O) Takata, E.; Okada. Y.; Shkai. K. Waku Kagaku 1988, 34, 117-22 (Japan); Chem. Abstr. 1989, 110, 215112~. Fraser, S. B.; Hollinshead, C. Int. GfC Symp.‘87 1987, (N161) Chappeii, I.; 370-87. (N162) Strasak, M.; Bystricky, S. J . Chfomatcgf. 1988, 450, 284-90. (N163) Kashima, M. Kenkyu Moku-KaiIo Hoan Da@akko, Dai-2-bu 1987, 33 (2), 13-23 (Japan); Chem. Abstr. 1988, 709. 115696t. (N164) Hlgashi, K.; Hagiwara, K. Water Sci. Techno/. 1988, 20, 55-82. (N165) Chaksac. M.; Audic, J. M.; Faup, G. M. Teah., Sci.. M e w s : Gsnie Urbain-Genie Rue1 1987, (9), 389-95 (Fr); Chem. Abstr. 1988, 108. 43635. (N166) Katayama, A.; Gomez, L., Maria, M.; Ker, K.; Hkai, M.; Shda, M.; Kubota, H. So//Sci. Plant Nutr. (Tokyo) 1987, 33.471-86 #em. Abstr. 1987. 107, 218627k. (N167) Knuutinen, J.; Virkki, L.; Mannila, P.; Mikkelson, P.; Paasivirta, J.; Herve, S. Water Res. 1988, 22, 985-90. (N168) Cruczwa, J. M.; Alford-Stevens, A. J.-Assoc. Off. Anal. Chem. 1989, 72 (9,752-9. MISCELLANEA

(01) Fujimoto, C.; Watanabe, T.; Jinno. K. J . Chromatcgr. Sci. 1989, 27, 325-8. (02) Hiroi, T.; Ouchi, K.; Okuyama, T.; Noguchi, K. Bunseki Ka&u 1988, 37, 659-64 (Japan); Chem. Abstr. 1989, 710, 38316m. (03) Nakazawa, H. Anal. Left. 1987, 20, 1751-64. (04) Lochmuiier, C. H.; McGranaghan, M. B. Anal. Chem. 1989, 87, 2449-55. ( 0 5 ) Okada. T. Anal. Chem. 1988, 60, 2116-19. (06) Imai, H.; Masujima, T.; Morita-Wada, I.; Tamai, G. Anal. Sci. 1989, 5 , 389-93. (07) Bennett, H. P. J.; James, S. Ana/. Biochem. 1989, 179, 222-8. (08) Williams, R. A.; Macrae, R.; Shepherd, M. J. J . Chromtogr. 1989, 477, 315-25. (09) Speri, P. L.; Stanczak, R. J. LC-GC 1988, 6, 431-2. (010) Hirayama, C.; Ihara. H.; Hamada, K.; Kinodrlta, S.; Yonemura, S.; Motozato, Y. J . Chfomatogf. 1988, 368, 391-4. (011) Hirayama, C.; Ihara. H. Jpn. Kokai Tokkyo Koho JP 63,246,657, 13 Oct. 1988. (012) Adachi, S.; Watanabe, T.; Kohashi, M. Agfic. 6b/. Chem. 1989, 53, 1597-602.

Supercritical Fluid Chromatography T.L.Chester* and J. D.Pinkston The Procter & Gamble Company, Miami Valley Laboratories, P.O. Box 398707, Cincinnati, Ohio 45239-8707

INTRODUCTION Supercritical fluid chromatography (SFC) grew steadily through the 19808. The number of manuscripta published r year is now approximately 10 times the rate in the 19708. #e number of practitioners has increased by an even greater factor, as has the overall interest level in the technique. No longer is SFC just a curiosit . Today it is a recognized necessity in many analytical lagoratories. Therefore, it is with pleasure that we present another signal of this growth and acceptance; the inaugural fundamental review in SFC. 394 R

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In the last two fundamental review +sues, SFC was included in Gas Chromatography. In even earher issues, some aspects of SFC progress were mentioned in both the gas chromatography and column liquid chromatography reviews. For this fundamental review we have selected articles from the 1988 and-1989 ublication years of Chemical Abstracts (except for ref 3 whicE had not yet been abstracted by CA). We have not included papers on supercritical fluid extraction (SFE)unlesa they have included an SFC analysis. Thus, papers concerned with subjects like SFE/GC and SFE/LC are not included here. In addition to the papers we cite, there are approxi@ 1990 American Chemical Society

SUPERCRITICAL FLUID CHROMATOGRAPHY Thomas L. Cheder received his B.S. in chemistry from the Fiorlda State University in 1971. Spent a year at me the Baychem Carparallon (now Mobay) in Charleston. SC. and then began graduate studies at the UniversiIy of Florida under the direction 01 J. D. Winetardner. He received the Ph.0. degree in 1976 and jolned Pmcter 8 Gamble where he is currentk Head of the Seoaralions and Optical Spectroscopy Section. Corpmate Research Division, at the Miami

beginning his graduate studies in 1980. he spent a year working with G. Spiteller at the Universihr 01 Bavreulh in West Germanv as a DAAD' F d b w He received his B . i . chemistry and math in 1979 from Ouachil Baptist UniversiIy in A*adelphia. AR. He ~urrentlva member 01 Procter LL Gamble ni

mately 40 general reviews we have omitted. The ruriosity seeker or novice needing eneral information or hackground reading on SFC may fin2 help in one or more of the honks that have recently appeared ( 1 - 4 ~ . We would dso like to point out the publication of two new journals containing a wealth nf infiirmation regarding the use of supercritical fluids and SF(:: Thr Journal I J ~Suprrcrrrrral Fluids. which began in 19RR. and The Journal of Microrolumn S r p a m r i o n s , which began in 1989.

THEORY AND FUNDAMENTAL MEASUREMENTS Significant ains were made in the past 2 years, particularly in understan$ing.SFC processes and how the mohile-phase

choire (among gas. liquid, and supercritical fluid, affects performance in rnlumn chromatovaphy. Schmitz and Klesper studied the effect of temperature and C'O, density on retention and effiriena using a silica gel stationary phase (51. Schnutz compared the elution properties o i supercritical CO. and several alkanes (fi.n. He found the eluent5 were similar when compared under cimditions ni constant frrr vulume, hut that CO, \\as quite differrnt from the alkanrs when rompnred at cnnstant densities. Klesper. Schmitz. and othen later reported their progress using -gradient" methods. which included programmed rhanges in temperature. pressure, eluent cnmpn;ilion, and velority & / 2 , . (There is a trend developing nnw to reserve the term "gradient elution- in SFC to desrribe onlv mnhile.phase composition programming.) Klesper showed the dependence # i f iolute partition rntios and r r w lution on temperature and pressure as three-dimensiiinal plnw and related the influence of solute vapor preswre and snltibility to the observed retention and selertivity 18). Other wurk involved the USQ of ronserutive multiple -gradients" to ^tune" sprcific separntinna with adjustments in resolution and analysis timr (!%/2). These programs were not limited u,just (me mnbilr phase domain but rould have been set to deliver a given mobile phase in gaseous. superrritical fluid. and liquid forms in a single chromnti8graphir procrdurr. Two other groups are working to further develop theory linking GC. SFC, and LC. Martire advanced his ^Unified Theory of Adsurption Chromatography" in which he derived a general equation to desrribe scdute retention in achroma. this theory the m&iC phase may tngraphicsptwn ( / . ? - l f i ~In he an ideal gas, a moderately nnnideal gas, a supercritiral fluid. or a liquid. Then, starting i r m 1)arry.s law, he developed

more general equations for spatial and temporal column parameters and applied them to gas, supercritical fluid, and liquid chromaroyraphy (17). lshii and co-workers studied retention in LC and SFC (18-20). They first used methanol and diethyl ether as the mobile phase (18). Later they built a system capable of using a CO, mobile phase in gas, supercritical fluid, or liquid states and studied positive and negative temperature programming (19,ZO). This led to their "Unified Fluid Chromatography" in which GC, SFC, and LC separations are performed in series in a single chromatographic analysis (21, 22). Steuer et al. studied the kinetics and thermodynamic effects of composition gradients on silica in LC and SFC and pressure wxwnunina in SFC (23). They found that 3WlMH) column GolGmes of mobile phase are required to stabilize a normalphase column after a change in the composition of a liquid mobile phase. However, with a supercritical CO, mobile phase, columns reequilihrated after only 2 column volumes of flow following a pressure program and after 10-30 volumes following a modifier change. Jinnoet al. studied and modeled the retention of polycyclic aromatic hydrocarbons (PAHs) in SFC (24,ZS). They used a CO, mobile phase and a variety of bonded-phase silica packings. Yonker and Smith described the effect of solute partial molar volume in the stationary phase on retention in SFC (2fi,27)and the effect of phase ratio and column type on retention (28)and reviewed retention processes in SFC (29). Wheeler and McNally studied the effects of several solute functional groups on retention in packed and capillary SFC (30).Bartle et al. studied the temperature dependence of the retention of several PAHs on the ODS stationary phase using a CO, mobile phase (31). They could explain the observed retention from fugacity coefficients calculated from the Peng-Robinson equation of state. Strubinger and Parcher measured the surface excess (Gihhs) adsorption isotherms of CO on silica and several bonded-phase packings (32,33).The isotierms peaked near the critical pressure and indicated the presence of multiple layers of adsorbed mobile phase. The authors concluded that at least a monolayer will normally exist under conditions typically used for SFC. The type of bonded phase had little effect. Brown et al. used SFC to predict supercritical fluid enhancement factors (34). Barker et al. used SFC to estimate solubilities of solutes in CO, (3.5).Chester et al. separated the effects of solute enthalpies of solution in the stationary and mobile phases on retention and derived conclusions regarding the performance of SFC compared to GC and LC (36). And Rerger studied the retention of fluoranthene in methylsilicone-coated capillary columns using a CO mobile phase (37). He was able to estimate the solutemobile phase enthalpy of interaction by comparing the retention under SFC and GC conditions on the same column. Schoenmakers et al. described the influence of sample size on retention in packed-column SFC. They suggested a mixed retention mechanism involving both the chemically bonded phase and residual silanol groups (38). Schoenmakers and Uunk described the effects of pressure drops in packed SFC columns on retention and efficiency for three different ODS packings using a CO, mobile phase (39). Schoenmakers later compared capillary and packed SFC columns, focusing on stationary-phase film thickness, analysis speed, and the effect of pressure drop on column efficiency (40). Roth and Ansorgova derived equations describing the pressure drop in a capillary SFC column as a function of the experimental parameters (41). Finally, Smith et al. developed a retention index scale based on a homologous series of alkyl aryl ketones and compared the results to a scale based on n-alkanes (42, 43).

STATIONARY PHASES SFC columns packed with HPLC-like packings have historically been much more retentive than the typical capillary SFC columns. This high retention has limited the scope of possible analyses for packed-column SFC since many solutes either cannot he eluted without the use of solvent modifiers or cannot he detected with them. However, much progress has been made to produce more inert and less retentive packings for SFC. It is clear that the pore structure and specific surface area of silica-based packings play a tremendous role in retention ANALYTICAL CHEMISTRY. VOL. 62, NO. 12. JUNE 15. 1990

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in SFC. En elhardt et al. discussed the differences in using supercriticaffluid mobile phases versus liquid mobile phases with typical HPLC packings (44). Nomura et al. found that the retention behavior of polar solutes (like pyridine) was affected by both the pore structure and carbon loading of their ODS silica stationary phase (45). Dean and Poole found that polar solutes interact strongly with silanol groups and su ested the use of low-surface-area packings with a chemical5 %onded stationary phase to combat the problem (46). Ashraf-Khorassani et al. described a stationary phase (Deltabond) made by cross-linkingand bond' a polysiloxane to silica (47). Solute access to uncappedzanol groups is greatly reduced compared to conventional bonded-phase silica packings. Green and Bertsch described a method for uniformly coating alass beads with immobilized silicones (48). Roeder. Ruffmi,-et al. used a polymer coating technique to immob& a chirally substituted polysiloxane either on the walls of a capillary column or on silica gel particles (49,50). They reported that homogeneous coatings of controlled thickness were easy to produce. Naturally, roblems with silanol oups can be avoided by using a nonsiica substrate for bond%g the station This was demonstrated by Khosah et al. who madx&\yi: phase packin s from alumina particles (51). Chang et a f investi ated a capillar column coated with a smectic liquid c r y s d n e stationary p L e for the separation of petroleum hydrocarbons (52). The found the resolution of their solutes to be much better with 8FC (using COPmobile phase) than when using the same stationary phase in GC. They reported this occurs because the stationary phase is more highly ordered at the lower SFC temperature. Two groups used the mass spectrometric tracer pulse chromatography method to study stationary-phase solvation promises. Yonker and Smith determined the heats of sorption of 2-propanol for transfer from a COPphase to a SE-54 stationa phase (53).They found the amount of 2-propanol sorbeTto the stationary phase changed inverse1 with the pressure applied to the mobile phase. Selim and gtrubinger found evidence of sorption of pentane mobile phase into SE30 and SE-54 stationary phases (54) and studied the effect of methanol dopin on the partition behavior of pentane into the stationary pkase (55). Takeuchi et al. examined the use of porous glass beads as a stationary phase with methanol or diethyl ether mobile phase for the separation of styrene and methylphenylsiloxane oligomers (56). These same authors also used hot NaOH solution to etch the inside surface of a fused-silica tube and then used it to separate styrene oligomers (57).

by size-exclusion chromato raphy (63). Fields et al. studied the e#& of pressure and temperature on retention with doped mobile hases (64). And Anton et al. modified a packed-column S I C instrument to pressure program a mixed mobile phase for use with a capillary column (65): Fields and Grolimund evaluated SF for use with capillary columns and an FID (66).They veriaed that the solvating power of SF, is low but found an advantage compared to CO for the elution of primary amines. Hellgeth et al. evaluated SF as a mobile phase used with packed columns (67). &mer et al. used COz doped with acetonitrile and a chiral ion-pairing agent to separate enantiomers of amino alcohols as diastereometric ion pairs (68). These authors reported that the resolution and efficiency are better than in HPLC. Ashraf-Khorassani et al. used a COz mobile phase containing methanol and tetramethylammonium hydroxide to se arate phenylthiohydantoin (PTH) derivatives of amino a c i z (69).

INSTRUMENTATION, TECHNIQUES, AND PERFORMANCE Sample Introduction

Just a few year 0,SFC (especiallywith capillary columns) was not considere a trace analysis technique. The effective sample volumes allowed were so small that solute concentrations in the injected sample solution had to be around a tenth of a percent or more, even when subnanogram masses m of the solutes could be detected. A great deal of progress l been made to understand the injection rocess and to increase the effective sample volumes without groadening the solute peaks. Bohm et al. studied split ratios from 1:3 to 1:500 and reported seeing a density-dependentsolvent effect when the split ratios were toward the low end of their range (70).Hirata et al. described a direct injection procedure for packed capillary columns (71).Buskhe et al. develo ed a rocedure to inject up to 0.5-pL volumes onto 50-pm4.l capitary columns usin a valve to vent the solvent vapor (72).Niessen et al. adaptei the phase-switching technique used in LC/MS to remove the desired solutes from a (blood) plasma matrix, desorbing the solutes with a supercritical fluid for analysis by SFC (73). Bruno described two modifications to a conventional injection valve to remove solvent from the sample solution during the injection process (74).And Hawthorne and Miller modified a four-port valve for postinjection removal of solvent from an uncoated inlet tube (75).They reported no loss of solutes larger than C alkanes. Injection techniques for capillary SFC were reviewe3 by Andersen (76)and by Lee et al. (77).

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MOB1LE PHASES

Restrlctors and Veloclty Control

There was significant progress in the continued development of mobile hases, modifiers, and programming durin the review perid. Phillips and Robey measured the strengtt and selectivity of supercritical COPrelative to liquid hexane (58). They found that the addition of methanol a t 3% had little effect on the solvent properties of C02 usmg substituted benzenes as test probes and a packed ODS column. Schmidt et al. used C02 with isopropyl alcohol as modifier to elute antibiotics from packed and capillary columns (59). The found best results in this application with an 8% dopin levef McNally et al. assessed the effects of modifiers in C02!or the elution of agricultural compounds using Vydac silica stationary phase (60). Geiser et al. described the use of water as a modifier for SFC with a CO mobile phase with seven different stationary phases (61). $his is significant because water addition (through the use of a saturator column) was shown to improve the performance of all seven stationary phases tested and because water contributes no background current to a flame ionization detector. This technique expands the sco e of packed-column SFC for analytes not containing chromop!ores or other detectable moieties other than their carbon content. Smith et al. used the surfactant Aerosol OT with water as a modifier in su ercritical ethane and propane under conditions causinge!t formation of reverse micelles (62).This greatly increases the solubility of polar solutes in the otherwise nonpolar mobile phase. Fujimoto used supercritical dichloromethane as the mobile phase to separate polystyrenes

Several re Tts were made regarding mobile-phase-velocity Flow restrictors are required to interface the control in outlets of SFC columns with lower pressure detectors. Green and Bertach compared several different restrictors in capillary SFC (78).Raynor et al. described a simple means of preparing rugged tapered capillary restrictors (79).The advantage of ru gedness makes installation easier into detectors like the FIb Column efficiency often decreases toward the end of a pressure or density program. This has several causes. First, a simple restrictor usually allows the mobile hase velocity to increase with increasing system pressure. ncreasin the mobile phase velocity above the optimum lowers the cokmn efficienc . At the same time the increase in pressure raises the mobie-phase viscosity. Also, the solutes requiring hi her elution pressures are usuall of higher molecular weight ?and larger molar volume) than dose eluting earlier. These factors combine to rapidly decrease the solute diffusion coefficients in the mobile phase which, in turn, lowers the optimum mobile-phase velocity. Unfortunately, since the velocity is increasing with pressure when it should decrease, significant efficiency losses can result. Control of the mobile-phase velocity throughout the pressure- rogrammed chromatographic problem. Naturally, control run could be used to minmii starts with understanding the roceases involved. Be er (and Toney) modeled flow throug! a restrictor and use?the restrictor temperature to independently ad'ust the mobilephase mass flow and column linear velocity 60-82). Huston and

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Bernhard modified a fluid mechanics equation describing the flow of a com ressible fluid through a converging nozzle and applied it to [FC (83). Independent control of column presme and velocity can also be accomplished by the use of make-up flows. Kumar patented a method to control column velocities in which the flow from the ump is split between the column and a mete- valve anfthen recombined in a column discharge outlet containing a restridor (84). Raynie et al. adjusted the outlet pressure of a restrictor with make-up flow and were able to maintajn constant linear velocity during pressure-programmed capillary SFC (85). The detector was operated a t column pressure ahead of the restrictor; however, the design should also work with low-pressure detectors following the restrictor. Yet another means of controlling the outlet pressure (and velocity) in acked-column SFC was demonstrated by Saito et al. (86). $hey used a high-speed switching valve to alter the outlet ressure b virtue of the fraction of time the valve was open. Lprcxiucigility of retention times with this method was better than 1%using a COz mobile phase.

troduction of liquid and solid samples into a packed-column SFC instrument (99). Saito et al. used SFE and preparative SFC to prepare tocopherol from wheat germ owder (100). McNally and Wheeler coupled SFE and SF8 to examine sulfonylurea herbicides from soil, plant materials, and culture medium (101). In later work they used both packed and capillary SFC and optimized extraction conditions with the application of radiotracers (102). Anton et al. coupled SFE with capillary SFC to rapid1 generate ualitative information (103). Xie et al. cou led SFI$ with capllary SFC by decompressing the extract &rough a restrictor into a cryogenically cooled concentrator (104). In addition, fractions were collected from a restrictor at the column outlet, and recoveries of a biologically active solute, ouabain, were measured by activity assay. Raynor et al. used SFE/capillary SFC/FT-IR to determine PAHs in coal-tar pitch (105). Liebman et al. used an artificial intelli ence approach to link Sam ling techniques involving SF#, desorption, reaction, an! concentration with on-line packedcolumn SFC and capillary GC (106).

Miscellaneous Inslrumentatlon Topics

Other SFC-Coupled Technlques

Later et al. used synchronized temperature/density proamming with capillary SFC and demonstrated the benefits separatin a series of dimethylpolysiloxane oligomers (87). &is metho! a t least partially compensates for the column efficiency losses mentioned earlier. Bruno built an SFC instrument specificall for hysicochemical measurements and used it to measure tge d&usion coefficients of toluene in C02 (88). Saito and Yamauchi used recycle SFC to preparatively separate two phthalates (89). Bornhop et al. used a splitter a t the outlet of a capillary column to operate an FID and a UV-visible detector simultaneously (90).

SFC can receive fractions from other chromatographic columns or can deliver fractions to other columns. Le and Guzowski coupled packed-column SFC and capillar y ? k to analyze gasoline (107). Campbell et al. used a columnswitching technique to perform SFC/SFC analyses of petroleum products (108). Lurie used HPLC/SFC to analyze impurities in illicit cocaine (109). And Davies et al. reviewed coupled chromatographytechniques including the use of SFC (110). SFC-coupled techniques were also used in refs

SFC Performance Issues

Su pliers of SFC- rade CO will, on request, furnish the mob& phase partialf chargea with helium, most of which goes into the unused ieadspace. The urpose is to increase the pressure in the supply cylinder a ove the normal CO va r reasure and increase the efficiency of transferring liquid C&?tfrough an educator tube and into a pump during the pump-fillin operation. Porter et al. reported retention time re roducibihy problems whenever they used helium-charged C8 as the mobile phase (91). However, Schwartz et al. (921, andlater Rosselli et al. (93),showed that good retention time precision is normal with or without helium headspace usage. All three of these studies were done with different pumping systems. This may a t least partially explain the apparent discrepancy. Richter et al. examined the injection precision of several valves used in flow-s litting, time-splitting, and direct modes in capillary SFC anfdiscovered important details that must not be ignored for best reproducibility (94). They obtained relative standard deviations of 3% and 1% using external and internal standards, respectively. Schoenmakers discussed optimization of parameters in chroma aphy in general and bcussed an o timiition problem in S C as an example (95). And Crow ana) Foley optimized the use of short (0.6-3m), 50-pm4.d. columns to separate ethoxylated alcohol oligomers (96).

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SFC-COUPLED TECHNIQUES SFEISFC

Because sample introduction requirements in SFC are more demanding than those in traditional HPLC, especially when narrow-bore ca illary SFC columns are used, better in'ection techniques n e J to be developed and made available. dn-line supercritical fluid extraction (SFE) is a powerful alternative semde introduction techniaue for SFC and has been investigaied by several roups. Engelhardt and gross combined SFE with packed-column SFC and flame ionization detection and aDD1ied it to several food and dru characterization examples 197). Hedrick and Taylor descrlqbed an SFE packed-column SFC system they used to extract diisopropy methylphoaphonatefrom aqueous samples in (and just below) the part per million range (98). Jahn and Wenclawiak used a miniextractor and a microextractor to remove solutes from adsorbates and for direct in-

i

(111-1 13).

SFC DETECTORS In this section we have cited papers focusing on SFC detector develo ment. Many additional papers involve the novel use of SFC Setectors as applied to a particular application and are cited in the Applications section. One of the most striking points in reviewing the work for this section was that there were no papers concerned with the further development of the flame ionization detector as a universal detedor in SFC. Does this indicate that the detector is mature, that all users are ha py, or that all the easy problems have already been solved.!The FID is still the most widely used detector in SFC, and its absence here should not be interpreted as a sign of unpopularity. SFC/FT-I R

Much progress has been made in the application of infrared spectrometric detection in SFC. Griffiths (114) and Taylor and Calvey (115)have reviewed the recent advances. Wieboldt (with various co-authors) develo ed and im roved a flow-cell approach for FT-IR detection 816-118). !Phis a roach is ap licable to both packed- and capillary-column S f 8 and can aciieve identification limits of 10 ng of solute delivered in a COz mobile phase. Morin et al. studied the effect of temperature and density on the background IR s ectrum of the COz mobile phase (119). Ikushima et al. useton-line FT-IR to monitor both SFE and SFC (111). Shah -?tal.used flow-cell FTIR to detect as little as 2 ng of caffeine (120). Raynor et al. eliminated solvent interference by deposit' eluting solutes onto KBr disks and performing FT-IR o f x n e (105, 121). Fuoco et al. compared six different FT-IRsampling khni ues for SFC detection with mobile-phase elimination (122). ?hey determined that conventional transmission of eluted materials collected on flat IR windows gave the best combination of sensitivity, spectral accuracy, and linearity. And Raymer et al. demonstrated SFC detection by isolatin the eluted materials in a matrix of frozen CCll (Le., SFC/gmatrix-isolation FT-IR) (123). Also see refs 113 and 124. SFWElement-Selectlve Detectlon

Just as in GC and HPLC, heteroatoms can be detected in SFC effluents by a number of element-selectiveapproaches. Fujimoto et al. evaluated an inductively coupled plasma-atomic emission spectrometer for detecting ferrocene via iron emission (125). In later work these authors reported that the ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990

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total flow of polar-modified COz from a microbore column could be introduced into the plasma source without reducing performance (126). Other means of producing atomic emission can also be coupled to SFC column outlets. Luffer et al. (127) and, in a companion paper, Galante et al. (128) used a surfacewave-sustained microwave-inducedplasma. They studied the effecta of various operating parameters, optimized the plasma for the detection of sulfur, and achieved a detection limit of 25 pg/s. Skelton et al. investigated a radio frequency plasma as an excitation source for the atomic emission detection of sulfur and chlorine (129). Spectral interferences from both COPand N,O mobile phases were small, allowing sulfur detection limits in the 50-200 pg/s range. In addition to atomic emission, there are many other ways of roducin element-selective detection signals. Morrissey ani H ill use! a hydrogen-atmosphereflame ionization detector to achieve selectivity toward metal-containing solutes (130). The detector behaved much like when used in GC, showed no ill effects from the COP mobile phase, and achieved a selectivity factor as high as 40000 for iron (in ferrocene) relative to hydrocarbon. David and Novotny examined the response of a nitrogen and hosphorus thermionic detector with capill SFC (131) an$ applied it to the determination of denvatasteroids (132). Both COzand N20mobile phases worked, and modifiers could be added up to 10 mol % without ound problems. Foreman et al. (133,134)and causing et (135) described the use of chemiluminescence Bornhop detection in SFC. Olesik et al. characterized and optimized a flame hotometric detector (FPD) for sulfur detection and achieveta limit of 8 pg/s (136). Pekay and Olesik later used SFC-FPD to separate sulfur-containing polymers, achieving better results than by SFC-FID (137).

hac%

the restrictor (153). The utility of the latter interface was demonstrated by spectra of various polar, thermally labile, and higher molecular weight compounds in both electron ionization (EI) and negative chemical ionization (NCI) modes. R e i o l d et al. used a modiied desorption/chemical ionization probe as their SFC/MS interface in which only the last 1mm of the flow restrictor was heated (154). They report outstanding results with ammonia CI of derivatized oligosaccharides, showing ammonium adduct ions for trimethylsilyl-derivatized glucose polymers in excess of 5 kDa. Cod et al. discussed the coupling of SFC to Fourier-transform M i using the dual-cell design to accommodate the as load (155). Owens et al. presented an interface probe wit[ independent control of the flow restrictor stem and tip temperatures (156). They discussed the effects of tuning, probe tip tem erature, ion source pressure, and other practical aspects of FC/MS. The drive toward higher mass range instruments was evident in the work of Pinkston et al. (157). They observed ammonium adducts of poly(dimethylsi1oxane) oligomers and derivatized oligosaccharides to the mass spectrometer’s 3-kDa mass limit. Pinkston et al. also proposed supercritical fluid injection of polysiloxane solutions for high-mass calibration and tuning (158). Trichothecene mycotoxins were used by Roach et al. as probe molecules to investigate the effects of restrictor temperature and C02mobile phase on various types of negative CI (159). Restrictor temperature affected chromatography but not ionization conditions. COz did not interfere with negative CI except when using methane/nitrous oxide for proton abstraction. The ability to rapidly switch from SFC/MS to GC/MS was stressed in the developments reported by Blum et al. (160) and by Hawthorne et al. (161).

ff

SFC/Mlscellaneous Detectors

Hill and Shumate reviewed the similarities and differences of high-, ambient-, and low- ressure detectors in SFC (138). Bornhop and Wangsgaard 8 3 9 ) and Richter et al. (140) reviewed optical detection methods and the use of GC detectors in SFC, respectively. Fields et al. found and solved problems in the ap lication of UV-absorption detection in SFC (141). Density-Bependent changes in the base line resulted from changes in the refractive index. However, cooling the cell reduced the problem. France and Voorhees used a multichannel UV detector to identify selected esticides and herbicides as they eluted from a capillary 8FC column (142). Kennedy and Wall detected a triazole fungicide metabolite by coupling acked-column SFC with an electron-capture detector (143f With their method they achieved a detection limit of 1 ng/mL. Sim et al. used photoionization detection with packed-column SFC (144). Their approach involved decompressin the mobile hase at the entrance of the ionization cell. doffman and ereibrokk (145) and Nizery et al. (146) used light scattering from the eva orated mobile phase exiting acked columns as a means of gtection. Allen et al. successhly cou led proton nuclear ma etic resonance with SFC (147). EaJerton, Morrissey, and Ell used ion-mobility spectrometry for SFC detection and showed that reactant ions were not affected by the presence of C02 (148, 149). And, although not yet reduced to ractice in SFC detection, noteworthy work was performefby Michael and Wightman in which they rformed voltammetry in flowing supercritical CO2 (150, 1 5 1 r SFC/YS

The combination of mass spectrometric detection with SFC is a powerful tool for the identification and quantitation of mixture components. It is therefore not surprising that many have worked to improve the SFC/mass spectrometry (SFC/MS) interface and to apply this method over the past 2 years. The molecular weight range of SFC is beyond that of traditional quadru ole mass spectrometers developed for GC/ MS. Thus a goo! deal of effort has been devoted to interfacing SFC and higher mass range sector and quadrupole instruments. Huang et al. first used a heated direct insertion probe, placed opposite the capillary SFC flow restridor, to counteract the cooling effect of the fluid ex ansion (152). They then developed an independently heatagle interface probe housing 398R

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APPLICATIONS As SFC instrumentation becomes more wides read, the number of SFC applications has own steadily. d a n y of the applications we cite can be adgessed by other analytical techniques. However, many others cannot be, and some advantage (like low temperatures, selectivity, detection options, and molecular weight range) exists for nearly every application cited. Natural Products and Drugs

Lipids. Many publications dealing with the characterization of thermally-labile or less volatile lipids have appeared over the past two years. Holzer et al. used GC/MS and SFC to characterize the less olar lipids from a hydrothermal vent sediment collected in t i e Guaymas Basin, Gulf of California (164). The more polar lipids were analyzed by SFC. Sakaki et al. studied lipids in fungi, especially fatty acids and their esters, using SFC (165,166). A comparison of SFC and HPLC separations was also presented (165). Packed SFC/ET-IR was used for the separation and characterizationof thermally labile sesquiterpene hydrocarbons by Morin et al. (167). Glycosphingoli ids were derivatized and separated by Kuei et al. using SFe/FID (168). Compounds with molecular weights up to 3000 were resolved based on structural differences in the carbohydrate or hydrocarbon moieties. Steroids. Raynor et al. compared capillary SFC FID to packed SFC/UV for the separation of ecdysteroi s (169). They obtained their best results with the packed-column system. This same class of steroids was also studied by Morgan et al. using acked S F C / W (170). These two groups later used packed 8FC MS with thermospray ionization to study these steroids (1 1). Steroids from physiological fluids were profiled by capillary SFC with phosphorus-thermionic detection by David and Novotny (132). A novel thio-

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SUPERCRITICAL FLUID CHROMATOGRAPHY

phosphinic ester derivative was employed. Fatty Acids and Esters. Many reports of the SFC separation of fatty acids and their esters have appeared. The effects of varying the column packing and saturating the COP mobile phase with water on the se aration of free fatty acids were investigated by Geiser et al. b1). Unmodified COz was used by both Nomura et al. (172)and Gorner et al. (173)who separated free fatty acids and their esters and unsaturated methyl esters, respectively. A preparative-scale separation of eicosapentaenoic acid was described by Kubota et al. (174). Amino Acids. The phenylthiohydantoin (€""HI derivatives of amino acids were separated by Ashraf-Khorassari et al. (69) and Berger et al. (175)using packed SFC UV. Addition of tetramethylammonium hydroxide to the Oz/methanol mobile phase was im rtant for the elution of the acidic and basic PTH-amino a c i g Derivatization with 9-fluoroenylalkyl chloroformate reagents was used in the com arison of SFC and HPLC separations of amino acids by Veugey et al. (176). Enantiomeric separations were studied in this work and in the work of Dobashi et al. (177). Dobashi and co-workers compared SFC and HPLC separations of racemic N-4-nitrobenzoylamino acid isopropyl esters. The time required for the separation was less than 5 min, and enantioselectivity was comparable to that obtained in HPLC (177). Drugs. Trace impurities in illicit cocaine were determined by usin a multidimensional HPLClSFC s stem by Lurie (109).kactions from the size-exclusion HPEC system were switched into the injection system of the SFC system. Both aminopropyl-bonded and bare-silica packed columns with a methanol, water, and amine-modified COz mobile phase were investi ated by Janicot et al. to obtain an optimum opiumalkaloifi separation (178).Smith and Sanagi (179)determined that an octadecylsilyl-bonded silica column was unsuitable for barbiturate separations, while reasonable se arations were obtained on a poly(styrene)-divinylbenzene cokmn. On-line SFE/SFC was used for the analysis of veterinary drug residues in pig kidney by Ramsey et al. (112).Detection was by tandem mass spectrometry using a movin belt interface. Kalinoski et al. used their 'high-flow-rate" S&;C/MSinterface to obtain positive CI spectra of high molecular weight, biologically active compounds such as the cyclic undecape tide c closporin A, molecular weight 1202, and two thermafy labii, ionic polyether antibiotics (180). Niessen et al. used SFC with a novel precolumn-in'ection method for the analysis of thermally labile mitomycin (73).The precolumn allowed for the isolation of the com ound of interest from other sample components and from t\e injection solvent before SFC analysis. White et al. described the SFC separations of cyclosporin, several ionophores (ionic antibiotics), and fat-soluble vitamins (181). Miscellaneous Natural Products. Balsevich et al. used packed SFC/MS with both thermospray and movingbelt E1 ionization methods to separate and identify indole alkaloids from Catharanthus roseus (182).Over 60 components were detected in thermospray SFC/MS. Polar and thermally sensitive trichothecene mycotoxins were studied by Roach et al. using negative ion CI SFC/MS (159).

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late-eluting peaks would not have been eluted in GC. Richter et al. presented a variety of a plications of SFC to food components and contaminants 886). Man of the mixtures contained components which decompose u d e r the conditions necessary for their elution in GC. Among these were triglycerides, a hops extract, celery seed oil, grapefruit oil, and carbamate pesticides extracted from parsle . Owens et al. used NH3 CI SFC/MS to characterize trimetgylsilyl-derivatized corn-syrup solids (156). Ammonium adduct ions were observed to the mass limit (m/z 3000) of the mass spectrometer. Frew et al. used SFC/MS and SFC/FID to characterize labile carotenes and xanthophylls of relative1 low volatility (187). Later, Schmitz et al. used capillary SF8/UV to characterize carotenoids (188). Their SFC results were compared to HPLC W separations. Trans and cis isomers were resolved bySF

L.

Pesticides and Herblcldes

The most active authors in this a plication area over the past 2 years have been Wheeler anfMcNally (60,101,102, 189). Most of their work has centered around thermally labile herbicides and pesticides. They used a variety of such molecules as probes in a comparison of capillary SFC, packed SFC, and HPLC (189). Packed SFC provided the fastest analyses, the lowest limits of detection, and the reatest reroducibility. Two later publications descriEe coupled gFE/SFC of sulfonylurea herbicides, their precursors, and their metabolites from a variety of matrices (101)and of diuron and linuron from soil (102).COz alone was not an effective extraction solvent for these latter two, but modifying the COz with methanol or ethanol increased the recovery by SFE to more than 95%. Niessen et al. used a novel, on-line "phase switching system" to isolate diuron from plasma before SFC (190).Detection was either by mass spectrometry using the moving-belt interface or by UV absorbance. Advantages of SFC coupled to UV multichannel detection were demonstrated by France and Voorhees when ap lied to the analysis of certain pesticides and herbicides (1427.Kalinoski and Smith applied packed-column SFC/MS using their high-flow-rate interface to the determination of or anophosphorus insecticides (191).Packed-column SFC/M8 was also used by Berry et al. in the characterization of pesticide mixtures (192). They used a thermospray interface. An example of the selectivity offered by ion mobility detection after SFC was shown by Morrissey and Hill in their determination of 2,4-dichloro henoxyacetic acid (2,4-D) (149). Fourier-transform infrarecfdetection was used in a comparison SFC for the analysis of pyrethrins of ca illary GC and cap by gieboldt et al. (193) -IR detection revealed that the most thermally labile pyrethrins degraded in the GC analysis, despite the use of short columns and thin stationary-phase films, while the insecticides remained intact during the SFC separation. Knowles et al. evaluated the SFC behavior of a variety of thermally labile carbamate, chlorinated, and organophosphate pesticides (194).

%

Surfactants

Foods

later used a comto characterize trituration as well as some positional isomer information was derived from the mass spectrometric data. Perrin and Prevot discussed the use of SFC in the analysis of fats and oils and provided examples of SFC separations of toco herols and glycerides (185). These authors concluded that EFC did not offer significant advantages over GC and HPLC in this area. However, the reviewers would like to point out that their conclusion was reached using instrumentation that would not be considered state of the art (e.g., it lacked pressure-programming ability). Miscellaneous Food Applications. The molecular weight ran e and universal detection characteristics (for or anics) of S C / F I D makes it an excellent tool for surveying unLown non lar samples. Chester et al. demonstrated this point wit; S F G F I D profiles of chewing gum extracts, red and black pepper extracts, and a honeycomb extract (36).Many of the

SFC has long been recognized as a powerful tool in this area, especially in the case of less volatile surfactants which are not easily detected in HPLC. Many nonionic ethoxylated surfactants fit in this category and are of widespread use. Thus it is not surprising that many publications dealing with the SFC of these mixtures have appeared over the past 2 years. Knowles et Matsumoto et al. (1951,Onuska and Terry (IN), al. (197),Geissler (1981,Kalinoski and Jensen (1991,and Pinkston et al. (200)all used SFC in the characterization of, or analysis for, nonionic ethoxylated surfactants. Geissler established SFC/FID response factors using single-isomer ethoxylated standards (198). Matsumoto et al. (195)and Pinkston et al. (200)used SFC/MS in their work, while Kalinoski and Jensen (199)compared SFC/FID and carbon-13 nuclear magnetic resonance spectroscopic characterizations of ethoxylated alcohol mixtures. The two techniques were complementary in nature. Polymers and Polymer Additives

The use of SFC for the characterization of low-molecular-weight polymers has become more widespread over the past lALYTICAL CHEMISTRY, VOL. 62,

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2 years. Schmitz et al. compared the separations of poly(2vinylnaphthalene) oli omers on columns coated with either poly(dimethylai1oxanef or poly(methylbiphenylsi1oxane) stationary phase (201). The molecular weight range of the former was greater, while the latter provided greater resolution of the oligomers. Gemmel et al. used gradients of CO /acetonitrile or COz/methanol to separate relatively polar ofigomers on a column acked with a polystyrene stationary phase (202). Among t e oligomers separated were poly(ethy1ene glycol), poly(capro1actone diol), epoxy repolymers, and poly(2vinylpyridine). Prepolymers of pfenol-formaldehyde resins were separated by Mori et al. using constant pressure and negative temperature programming (203). The properties of polymers are often modified or preserved by additives. A variety of light stabilizers and antioxidants, includin those in an actual poly( rop lene) extract, were separates by Ra or et al. by capilLy JFC (121,204). The eluted compounrwere then analyzed off-line by FTIR. Good spectra could be obtained at the 100-ng level. A t the other end of the spectrum, Fujimoto et al. have examined the use of su ercritical CH2C1 for size-exclusion chromatography of polygtyrene) (63). T i e range and resolution of liquid and supercritical fluid mobile phases were compared.

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Fossil Fuels

SFC was applied to the characterization of mixtures derived from fossil fuels early in its development. It is therefore not surprising that the application of SFC to fossil fuel mixtures is widespread. Examples of petroleum applications were published by Thiebaut et al. (205),Campbell et al. (IO@, Levy and Guzowski (Ion, Wright et al. (206), Schwartz ( 2 0 , Coulombe and Duquette (2081, and Lee,Fuhr, et al. (209,210). Most of these works wed packed-column SFC and were aimed at h drocarbon grou analysis or simulated distillation. The worz of Levy and 8uzowski included the capability of diverting a portion of the SFC effluent to a capillary GC for more detailed fingerprinting of Gasoline fractions (107). Wright et al. demonstrated the utility of mass spectrometric detection after separation into hydrocarbon groups (206). Schwartz showed that direct injection of undiluted, low-viscosity crude oils is possible in packed-column SFC (207). Total-aromatic-content results obtained by Lee et al. compared well with proton nuclear magnetic resonance and fluorescent indicator methods (209). Publications in the coal-derived mixtures area include those by Ra or, Barker, et al. (105, 211), Lee (212),and Wright et al. 6 6 ) . Ra or et al. used the powerful combination of on-line SFE/S$/off-line FT-IR for the characterization of polyc clic aromatic components of a coal tar pitch (105). Lee found that the concentrations of certain compound classes correlated with the rank of vitrinite coals (212). Di- and triterpenoih were found in greater concentrations in low-rank coals, while lar e, condensed-ring aromatics were more abundant in hi&-rank coals. Explosives and Propellants

The low-temperature separations available with certain supercritical mobile phases, such as CO ,have attracted the attention of those interested in the anaiysis and/or characterization of mixtures containing explosives or propellants. Douse found that capillary SFC with thermal energy analysis detection was a rug ed, sensitive method for the trace analysis of relatively nonpoh explosives (213). Detection limits were in the tens of picograms. Griest et ai. used packed-column SFC for the characterization of explosives and their manufacturin byproducts (214). Ashraf-Khorassani and Ta lor studied iouble-base ro ellant extracts (113)and propel& on-line FT-IR detection. Distabilizers (124) by 8FC!with chloromethane extraction and SFE were compared (113). SFJ3 allowed eater sample amounts to be deposited on the column and FTYR provided sensitive detection for compounds that do not respond well by FID (113). Amines and Nitrogen-Contalnlng Compounds

Certain amines form insoluble salts when mixed with COz, a popular SFC mobile phase. Thus SFC separations of these compounds have received special attention. Fields and Grolimund found that a previously postulated limit, stating 400R

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that amines with K values below 9 will react, is valid for most secondary akyf amines and secondary cyclic amines, except in ca8e9 of steric hindrance (215). The limit is not valid for tertiary amines. Aromatic amines and azaarenes were separated on analytical-scale and microbore SFC columns by Ashraf-Khorassani and Taylor (216). Their results were compared with both reversed- and normal-phase HPLC separations. The SFC separations displa ed greater chromatogra hic resolution with easier methodr development. David a n 1Sandra used trifluoroacetylation for the capillary SFC of ali hatic amines (217). Quaternary ammonium salts could only ge chromatographed with COPafter their conversion to tertiary amines. Mlscellaneous Appllcatlons

A few miscellaneous applications should also be mentioned. Steuer et al. described the SFC of ionizable organic com unds A using COz with acetonitrile and ion-pairing modifiers chiral counterion was used for the separation of enantiomeric 1,2-amino alcohols. Calvey et al. studied the peracetylated aldononitrile derivatives of monosaccharides using packedcolumn SFC and FT-IRand FID detection (218). The authors state that SFC provided greater insight into the reaction mixture than did GC. Derivatization was also featured in the work of David and Novotny (219). They used capillary SFC with nitrogen-thermionic detection to separate the quinoxalinol derivatives of a-keto acids. Simultaneous pressure and temperature programming were required to o timize the separation. Pinkston et al. used NH, CI and E1 !$FC/MS to confirm the identity of the trimethylad 1derivative of inositol tri hosphate (200). They also used SF$/MS to study the SFC beiavior of thermally labile peroxides (200). In one of the few applications of SFC to trace-level environmental analysis, Borra et al. preconcentrated nonvolatile trace organics in environmental water sam les using microcolumn HPLC (220). A second, high-resogtion separation of this mixture was performed by using capillary SFC/FID. Jahn and Wenclawiak ventured into or anometallic separations with a comparison of the SFC betaviors of three palladium @-diketonates on silica and bonded-phase packed columns (221). The best separations were obtained on silica with a methanol-modified COz mobile phase.

6).

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