Charge Exchange Mass Spectra of Trimethylsilyl Ethers of Biologically Important Compounds: An Analytical Technique Barbara Jelus and Burnaby Munson Chemistry D e p a r t m e n t University of Delaware N e w a r k Dei 1977 7
Catherine Fenselau D e p a r t m e n t of Pharmacology and E x p e r m e n t a l T h e r a p e u t m Johns Hopkins University School of M e d i o n e Baltimore Md 27205
Many compounds of biological importance are converted to their trimethylsilyl (TMS) ethers to facilitate their separation by gas chromatography ( I , 2). The electron impact spectra, as well as the methane chemical ionization spectra, of the T M S ethers of some of these compounds exhibit low intensity or non-existent molecular ions and very extensive fragmentation (3-8). In order to identify rapidly the TMS ether derivatives of biological compounds, a GC/MS compatible technique H ions and retains the major fragthat gives M or M ment ions is desired. Use of common GC carrier gases such as Nz,Ar, or He as charge exchange reagents (9) was not always satisfactory since the spectra resembled the electron impact spectra of the compounds. Figures 1 and 2 show the nitrogen charge exchange and the 70-eV electron impact spectra of the per-TMS derivative of methyl chenodeoxycholate. Both exhibit no molecular ion and much fragmentation to analytically useless low mass ions. (All work reported was obtained in experiments using direct insertion probe introduction of the TMS ether samples into a high pressure (CI) source (10) of a CEC 21-llOB mass spectrometer. Impurities present in the samples could be identified in experiments involving fractional sublimation of the sample into the mass spectrometer source. This problem will be eliminated in GC/MS experiments because the TMS derivatives will be separated prior to mass spectral analysis.) The technique of using mixed reagent gases has been discussed previously (11, 12). In this method, several per cent of a second reagent gas is added to the main reagent gas. The majority of direct electron ionization is of the main reagent gas and some of these ions react with the second reagent gas. The ionization of the additive which is present in trace amounts is produced by reactions of ions from both reagent gases. The relative amounts of ionization of the additive by the two sets of reagent ions depends on the ratios of the two reagent gases. The addition of 5-10% of many compounds to Nz drastically changes the distribution of ions from Nz+, N2+, and N4+: i-C4H10
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S Langer, S. Connel, and I . Wender, J . Org. Chem.. 23, 50 (1958) A. E. Pierce, "Silylation of Organic Compounds," Pierce Chemical Society, Rockford I l l . , 1968. J. A McCloskey, A. M . Lawson, K. Tsuboyama, P. M . Krueger, and R. N. Stillwell. J. Amer. Chem. Soc.. 90, 4182 (1968) J. H Duncan, W. J. Lennarz, and C . C. Fenselau. Biochemistry 10, 927 (1971). S . Billets, P S. Lietman, aiid C. C. Fenselau, J . Med. Chem.. 16, 30 (1973). W H. Elliott. "Bile Acids" in "Biochemical Appiications of Mass Spectrometry," G . R. Waller, Ed.. Wiley Interscience, Ne& York, N.Y , 1972, Chap. 11. 291-312. K . Okuda, M . G Horning, and E. C . Horning, J . Biochem. l T o k y o i . 71, 885 (1972) J L. Smith and W J A . Vanden Heuvel. Anal. Lett., 5, 51 (1972) N . Einolf and B. Munson. J . Mass. Spectrom. ion Phys., 9, 141 (1972) J. Michnowicz and 8 . Munson. Org. Mass Spectrom.. 4, 481 (1970) G P. Arsenault, J . Amer. Chem. Soc.. 94, 8241 (1972). D. F. Hunt and J F. Ryan I l l , Ana/. Chem.. 44, 1306 (1972).
gives mostly CdHs+; NH3 gives mostly NH4+; tetramethylsilane gives mostly (CH&Si+; and NO gives mostly NO+. Addition of small amounts (up to 15 mole 70)of a proton transfer reagent, such as isobutane or ammonia, to the charge exchange gas gave predominantly ions that correspond to M + H-TMSOH and little or no enhancement of M H . Use of tetramethylsilane, hexamethyldisiloxane, and hexafluoroacetone as additives to the "carrier gas" also gave ions corresponding to M-TMSO and little or no X ions where X is enhancement of molecular ions or M a fragment from the added reagent. A mixture of nitric oxide with nitrogen or argon or helium, in which the NO is 2-15 mole Yc of the mixture,
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Figure 2. Electron impact mass spectrum of the TMS ether derivative of methyl chenodeoxycholate with 70-eV electrons A N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 6 , M A Y 1974
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impact or N p charge exchange spectra of the same compound. The spectra obtained with He NO, Na NO, and Ar NO mixtures were very similar. The abundant M + H ions observed in some of the spectra are attributed to water impurities in the charge exchange reagents. The technique of NO/N2 charge exchange has been successful in enhancing abundances of M + ions of several other compounds whose electron impact mass spectra (as TMS derivatives) show little or no molecular ions. These compounds include the per-TMS derivatives of methyl lithocholate, methyl cholate, the methyl ester of 6-bromo2-naphthylglucuronide, guanosine, and P-glycerolphosphate. Pure NO has been used previously as a reagent gas (9, 13), but the lifetimes of conventional filaments are short. With these mixtures of nitric oxide and common GC carrier gases, no deleterious effects were noted on the samples of hot wire filaments. It is projected that the conventional carrier gases will be used in the gas chromatograph and that NO will be added in the low pressure region of the inlet lines to the mass spectrometer. It is hoped that NO (Ar NO or He + NO) mixtures will these Wp prove to be analytically useful for other types of compounds that give low intensity or non-existent molecular ions in other mass spectrometric techniques.
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Charge exchange spectrum of the per-TMS ether of methyl chenodeoxycholate with N z / N O mixture Reagent pressure 0 8 Torr (6 1% NO) and source temperature 21 0 "C Figure 3.
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proved to be the reagent system that gave the desired results of enhancement of molecular ions and retention of the major fragment ions. Figure 3 shows the nitrogen/ nitric oxide charge exchange spectrum of the trimethylsilyl ether of methyl chenodeoxycholate. In all of the N 4 N O charge exchange spectra, there is an enhancement of molecular ions, retention of the major fragment ions, and loss of the extensive fragmentation observed in the electron
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Received for review October 10, 1973. Accepted December 5 , 1973. This work was supported in part by a grant from the National Science Foundation, G P 20231. (13) D F Hunt and J F Ryan I l l , Chem C o m m u n , 620 (1972)
Liquid Coal Compositional Analysis by Mass Spectrometry J . T. Swansiger, F. E. Dickson, and H.T. Best Gulf Research & Development Company. Pittsburgh. Pa. 75230
With the increasing emphasis on the efficient use of existing energy resources which do not adversely affect the environment, liquid products derived from coal appear very promising as a possible liquid fuel source (1-3). A typical coal liquefaction process normally employs a solvent system that promotes depolymerization and hydrogenation of the coal which results in a fuel with enhanced heat content and reduced sulfur content. The coal liquids used in this work were produced by a bench-scale pilot plant employing a catalytic hydrogenation process. The feed coal is ground and slurried with a solvent derived by recycling part of the product. The feed slurry is combined with hydrogen, heated, and passed through a catalytic reactor of proprietary design. The reactor product goes to a gas-liquids separator where hydrogen is recovered for recycle. The liquid product goes to a solids separator where ash and undissolved coal are removed ( 4 ) . (1) Hydrocarbon Research, lnc., Project H-Coal Report on Process Development, Report PB 173765, Clearinghouse Federal Scientific Technique Information. Springfield, Va. (2) Spencer Chemical Division, Solvent Processing of Coal to a Deashed Product, August 1962 to February 1965, R&D Report No. 9, OCR, Washington, D.C. (3) 1973 Annual Report of the Office of Coal Research, Clean Energy from Coal-A National Priority, U.S. Government Printing Office, Washington, D.C., pp 67-75. ( 4 ) H. G. Mcllvried, S W. Chun, and D . C. Cronauer. unpublished data
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ANALYTICAL CHEMISTRY, VOL. 46, NO. 6 , M A Y 1974
Techniques for the compositional analysis of coal liquids have become of importance in order to optimize operating parameters and monitor reaction products. Other workers have applied mass spectrometry to the study of coal (5, 6), coal extracts ( 7 ) , coal tars ( B ) , and oils derived from coal ( 9 ) , primarily using existing group-type methods developed for petroleum analysis (10, 11) and low ionizing voltage techniques (12) to obtain a carbon number distribution. Sharkey, Shultz, and Friedel have indicated (13) that two type analysis methods for analyzing aromatics in petroleum (14, 15) are not directly applicable to coal hy( 5 ) H . W . Holden and J . C. Robb. Fuei. 39, 39 (1960). (6) A. G. Sharkey. Jr., J L. Shultz. and R. A Friedel. "Advances in Coal Spectrometry, Mass Spectrometry," Washington, U.S. Department of the Interior. Bureau of Mines, 1963. (7) T Kessler. R. Raymond, and A. G. Sharkey. Jr., Fuel. 48, 179 (1969). (8) J L. Shultz. R. A. Friedel, and A. G. Sharkey, Jr , "Mass Spectrometric Analyses of Coal-Tar Distillates and Residues," Washington. U.S Department of the Interior. Bureau of Mines, 1967 (9) A G. Sharkey. Jr., J . L. Shultz, and R. A. Friedel, Fuel. 41, 359 ( 1962). (10) R . J . Clerc, A. Hood. and M . J . O'Neal. A n a / Chem , 27, 868 (1955). (11) G. F. Crableand N . D . Coggeshall.Ana/. Chem.. 30, 311 (1958) (12) H E. Lumpkin, Ana/ Chem 30. 321 (1958). (13) A. G. Sharkey, Jr , J . L Shultz, and R A. Friedel. "Analytical Methods in Mass Spectrometry." Washington, U.S. Department of the Interior, Bureau of Mines, 1967.
