High Temperature Gas-Chromatographic Separations Using Glass Capillary Columns and Carborane Stationary Phases Milos Novotny,' Ramon Segura, and Albert Zlatkis Department of Chemistry, Unioersity of Houston, Houston, Texas 77004
Poly-m-carborane siloxane polymers form homogeneous thin films on etched and silylated glass capillaries. High-efficiency capillary columns prepared with crystalline, waxy, and liquid carborane polymers were used u p to 350 OC. Areas of application a r e demonstrated for cholesterol esters, triglycerides, and complex mixtures of high-boiling hydrocarbons. Compounds of molecular weight u p to approximately 800 were chromatographed i n the gas phase with good efficiency a t high temperatures. Some instrumental problems of the direct injection procedure used a t high temperature a r e discussed.
pounds during their passage through the chromatographic system. Surface-treated glass capillary columns (1-3) and beads (4) coated with thin films of thermostable stationary phases appear t o be the most promising approach t o the analysis of high-boiling compounds. Their applications to the high-resolution analysis of steroids have been demonstrated (5, 6). Numerous stationary phases for gas-liquid chromatography have been suggested. However, very few of them can be used a t temperatures higher than 200 "C. Silicone polymers have been used almost exclusively in high-temperature work. New thermostable phases recently synthesized include poly-
THEANALYSIS of complex mixtures boiling higher than about 300 O C , such as heavy petroleum constituents and many biological compounds, is often a n extremely difficult task. Gasliquid chromatography is presently the method of greatest potential value for resolving structurally similar compounds in such mixtures. However, a number of problems arise due t o the thermal instability of stationary phases as well as the possibility of a catalytic decomposition of the separated com-
(1) M. Novotny and K. Tesarik, Chromatogruphiu, 1, 332 (1968). (2) K. Tesarik and M. Novotny, in "Gas-Chromatographie 1968," H. G. Struppe, Ed., Akademie-Verlag GmbH, Berlin, 1968, p 575. (3) M. Novotny and K. D. Bartle, Chromatograpliia, 3, 272 (1970). (4) P. Bocek and A. I. M. Keulemans, Technological University of
Eindhoven, the Netherlands, personal communication, January 1971.
Present address, Department of Chemistry, Indiana University, Bloomington, Ind. 47401.
( 5 ) M. Novotny and A. Zlatkis, J . C/iromatogr. Sci., 8, 346 (1969).
(6) M. Novotny and A. Zlatkis, Ckrornatogr. Rev., 14, 1 (1971). al 0
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Figure 1. Chromatogram of cholesterol esters Conditions: 12-m X 0.3-mm, i.d., column coated with DEXSIL-300 GC
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
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Figure 2. Chromatogram of 5 % solution of coconut oil in acetone Conditions: 28-m X 0.3-mm, i.d., column coated with crystalline carborane polymer
m-phenoxylene (3,polyamides (8), and isocyanate-based polyimides (9). Thermostability and a remarkable selectivity toward steroids are combined advantages of the polyimide substrates (9, 10). A number of semiorganic polymers based on boron compounds were developed and studied by Schroeder et al. (IZ). Among them, poly-m-carborane siloxanes (12) were shown t o be polymers of unusual temperature stability. Their use as stationary phases in gas chromatography has been suggested (13). By varying their composition, the methyl polymers can be prepared as crystalline, rubber-like, or waxy materials. Viscous liquids can be produced when a certain number of phenyl groups a r e incorporated into the siloxane chains (11). The applicability of the carborane phases for high-temperature, high-resolution gas chromatography was made in this study. Chemically-modified glass capillary columns coated with thin films of carborane phases and a direct injection (7) J. H. Beeson and R. E. Pecsar, ANAL.CHEM., 41, 1678 (1969). (8) R. G. Mathews, R. D. Schwartz, J. E. Stouffer and B. C. Pettitt, J. Chrornatogr. Sci., 8, 508 (1970). (9) R. G. Mathews, R. D. Schwartz, M. Novotny, and A. Zlatkis, ANAL.CHEM., 43, 1161 (1971). (10) M. Novotny and A. Zlatkis, J. Chromatogr., 56, 353 (1971). (11) H. A. Schroeder, Inorg. Macromol. Rel;., 1,45 (1970). (12) S. Papetti, B. B. Schaeffer, A. P. Gray, and T. L. Heying, J . Polym. Sci., Parr A - I , 4, 1623 (1966). (13) R. W. Finch, Airaiabs ResearchNores, lO(3), July 1970. 10
procedure (14) were used t o separate hydrocarbon, cholesterol ester, and triglyceride mixtures. Compounds of molecular weight up t o approximately 800 were analyzed. EXPERIMENTAL Preparation of Columns. A commercially available ap-
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paratus (Hupe Busch, Groetzingen, West Germany) was used for the preparation of glass capillaries. Their inner surfaces were chemically modified according to the procedures reported earlier (1-3). Three to five per cent solutions of carborane phases in toluene were used for coating (15). After drying overnight, the colbmns were heated a t 1 "C/min. up to 340 "C and conditioned for several hours. Helium was used as the carrier gas. Materials. DEXSIL 300 G C (Analabs, Inc., North Haven, Conn.) and experimental carborane polymers 700-8-73 and 700-8-95C (Chemical Systems, Inc., Santa Ana, Calif.) were used as stationary phases. 7001-8-73 was a crystalline polymer and 7001-8-95C a viscous liquid. Toluene was chromatoquality. The sample of Shell wax was obtained from R. J. Olson, Shell Oil Company, Houston, Texas. Gas-Chromatographic Procedures. A Varian Aerograph gas chromatograph, Model 1200, with a flame ionization detector and a Hamilton inlet system was used. The seals for connection of glass capillary columns were made from high(14) K. Grob and G. Grob, J. Chromatogr. Sci., 7, 584 (1969). (15) M. Novotny, L. Blomberg, and K. D. Bartle, ibid., 8, 390 ( 1970).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
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temperature septa (Applied Science Laboratories, Inc., State College, Pa.). Dilute solutions of the samples were injected directly (14) onto the capillary columns. Because of its limited solubility, the sample of Shell wax 700 was introduced into the system by means of a solid sampler (Hamilton Company, Inc., Whittier, Calif.).
Most silicone phases can be effectively coated o n the inner wall of etched and silanized glass capillaries ( I , 3, 6). Also, carborane phases show similar behavior and create homogenous and efficient films. However, the polarity of carborane phases appears t o be slightly different from that of the common nonpolar polysiloxanes (IO). Thus, for example, the separation effect for the pair, cholestanol-cholesterol, is more favorable with DEXSIL 300 GC than with the silicone substrates. The temperature stability of carborane phases is remarkable (10,13). When the preliminary results with trimethylsilyl steroids had indicated significant possibilities for the carborane phases, our studies were extended t o the more difficult high-boiling mixtures, such as lipids. Gas chromatography of unaltered sterol esters and triglycerides on short packed columns have been reported by many investigators (16). However, even a
carbon number separation is quite difficult with such systems and no isomers can be distinguished. Improved separations can be expected by using efficient capillary columns. The chromatograms obtained with sterol esters and triglycerides a r e shown in Figures 1 and 2. Broad peaks and a number of shoulders in the chromatogram of coconut oil sample suggest the real complexity of this mixture. However, it is also evident that the column efficiency alone can hardly resolve minor structural differences in large triglyceride molecules. The situation is even more difficult in the case of cholesterol esters which are known to have longer retention times than predicted (17). A detailed knowledge of the composition of petroleum is of importance in planning manufacturing procedures and refining operations and in controlling the efficiency of the cracking processes and large-scale operations. Although capillary columns providing efficiencies in the order of lo4t o lo6 theoretical plates have been successfully applied t o the analysis of lower hydrocarbons, high-resolution separations of mixtures higher than CZ5are uncommon. Separations of hydrocarbons of up t o about with good efficiency were made in this study. The chromatogram of the constituents of a n “essentially nonvolatile” sample from the bottom of the distillation column (Shell wax 700) on a 28-m long glass capillary column
(16) A. Kuksis, in “Methods of Biochemical Analysis,” D. Glick, Ed., Interscience, New York, N.Y., 1966, Vol. XIV, p 325.
(17) A. Kuksis, Can. J. Biochern., 42,407 (1964).
RESULTS AND DISCUSSION
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coated with a crystalline carborane is shown in Figure 3. A liquid carborane phase was used t o chromatograph lower homologs of the hydrocarbon mixtures. Figure 4 demonstrates the separation of a hydrocarbon blend dissolved in toluene. The resolution of individual homologs permits the determination of other isomers present as trace peaks among the main fractions. The quantitation of high-boiling hydrocarbons should not present any problems with the direct injection procedure. However, losses were observed with more polar molecules such as cholesterol esters at temperatures higher than 300 “C. In particular, an elution of biologically important higher esters was difficult. Despite the high stability of carborane phases, 350 “ C seems t o be a practical limit for analytical work. The silylation treatment which is necessary t o block adsorptive sites on the glass surface becomes unstable at extremely high temperatures (320-350 “C). Although capillary columns can bc programmed up to about 360 OC, extended exposure to this temperature will result in a loss of silyl groups and irreversible adsorption of more polar molecules. Since the silylation treatment is also used to enable a homogeneous spreading of carborane phases, a sudden decrease of column efficiency may be experienced. Serious problems in the high-temperature work are encountered due to the liiiiited thermostability of injection septa and seals. An appreciable adsorption of samples and a “memory effect” are often observed together with bleeding and greatly reduced durability of silicone seals. Use of the
recently described DEXSIL-impregnated asbestos (18) or glass-to-metal seals (19) may possibly improve this situation, Ghost peaks due t o bleeding at the injection temperatures up t o 300 O C with well-conditioned septa (HT-9 Type, Applied Science Laboratories, Inc., State College, Pa.) did not interfere with chromatograms and were eluted within or immediately after the solvent peak. Because of their great temperature stability, the carborane phases should be very useful in gas chromatography-mass spectrometry combination, where the column bleeding often seriously disturbs interpretation of mass spectra. Although no experiments on this subject were made in this study, Shoemake (20) recently found that bleeding of carborane phases is minimal if special treatment procedures and conditioning are used. ACKNOWLEDGMENT
The authors are grateful t o Chemical Systems, Inc., Santa Ana, Calif., for providing the samples of carborane polymers.
RECEIVED for review July 12, 1971. Accepted August 12, 1971. This work was supported in part by the Robert A. Welch Foundation (Grant No. E-363). (18) M. Beroza and M. C . Bowman, ANAL.CHEM., 43, 808 (1971). (19) J. Ganansia, C. Landault, C. Vidal-Madjar,and G. Guiochon, ibid.,p 807. (20) G. R. Shoemake, University of Houston, Texas, personal communication, July 1971.
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