Chemical Ionization Mass Spectrometric Determination of Organic Compounds in Solution at the Part per Million Level Sir: The general analytical utility of Chemical Ionization Mass Spectrometry (CIMS) has been demonstrated by Field ( 1 , 2 ) , Munson (3, 4 ) , and Fales (5, 6). CIMS gives simpler spectra and has somewhat higher sensitivity (7, 8 ) than conventional mass spectrometry. In normal practice, the CI reagent gas flows continuously into the mass spectrometer source through a special gas inlet system (9).Volatile samples may be injected directly into the reagent gas stream while solids are usually introduced via a conventional solids probe ( 7 ) . We wish to report that excellent CI spectra can be obtained by direct injection of dilute solutions into a heated batch inlet system. The solvent serves as the reagent gas while the solute is the analyte. In the examples shown here, water is used as the solvent. The principal reagent ions are (10-12) HsO+(HzO), where n = 0-5. The mass spectropeter used was a DuPont 21-490B modified in house for CIMS by closing up the source and providing differential pumping with a Varian-NRC VHS-4 1200 l./sec diffusion pump (13). Sample introduction is possible either by the direct insertion probe or via injection into the DuPont instrument's heated inlet system. High voltage discharges are minimized by maintaining the source housing pressure a t less than 2 X Torr. Any discharges that do occur are grounded out through the source housing. Optimal spectra were obtained by maximizing the water cluster a t mle 73 a t the collector using the various tuning parameters of the instrument. Dilute aqueous samples of known composition were prepared and their CI spectra examined. Adequate pressure for chemicai ionization was obtained with the injection of 15 to 30 microliter aliquots of solution into the 1-liter inlet l./sec system connected to the source through a 1.9 X leak. The pressure of H20 in the source was approximately 0.25 Torr. The inlet system and source were maintained a t 120-140 "c. The water CI spectra of many compounds have been observed. Typical spectra for aniline, valeronitrile, acetone, and diethyl ether are shown in Figure 1; water peaks have been subtracted. Each spectrum was obtained under identical conditions using a 100 parts per million aqueous solution. No fragmentation was observed for any of the compounds studied. The lowest mass peak seen is a large M 1, representing the protonated sample. Additional peaks are seen for water clusters added to the protonated parent ion. Thus, peaks occur a t M 1, and then a t regular 18 mass unit intervals thereafter. Valeronitrile (Figure 1) demonstrates a common feature of water chemical ionization spectra. The base peak is most often found a t M 19. Acetone and diethyl ether show the same general profile, but have much larger M 1 peaks. Usually, the M 1 peak is a t least ten percent of the base peak. The aniline spectrum seems to exhibit an inverse relationship between peak height and water cluster size. I t is of interest to note that aniline seems to prefer the addition of a proton, while acetone, diethyl ether, and valeronitrile prefer the addition of H30'. Other variations in peak intensities have also been observed which strongly suggest that structural and conformational information can be obtained from the relative intensities of (M l), (M 19), (M 37), etc. The detection limit with the existing apparatus is one
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"/E Figure 1. Water CI spectra obtained by injecting 25 pl of 100-ppm solutions of (A) aniline, (B) valeronitrile, (C) diethyl ether, a n d (D) ace-
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part per million (in aqueous solution). The peak heights for M 1 and M 19 appear to be fairly linear with concentration. Using the one-liter inlet volume and 25 pl of solution, usable CI spectra can be obtained for approximately 12-20 minutes following sample injection. This long time is desirable for exploratory studies but is much longer than required to obtain an analysis of the solution, and thus does not give maximum sensitivity in terms of ng of sample needed to produce a CI spectrum. Better sensitivity was obtained using an inlet volume of approximately 10 cm3 and a smaller leak which gave good spectra with 10 pl of 5 ppm solution, corresponding to the introduction of about 50 ng of compound. From this work it is apparent that Chemical Ionization Mass Spectrometry has great potential for the rapid, simple, and direct determination of organic materials in dilute solutions.
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LITERATURE CITED (1) F. H. Field, M. S. B. Munson, and D. A. Becker in "Ion Molecule Reactions in the Gas Phase," P. Ausioos, Ed., American Chemical Society, Washinqton, D.C., 1966, pp 167-192. (2) F. H. Field, Accounts Chem. Res., 1, 42 (1968). (3) M. S. B. Munson and F. H. Field, J. Amer. Chem. SOC., 8 8 , 2621 (1966). l d ) M S B Munson Anal Chem.. 43 (13). 28A (1971) (5) H. M. Fales, G. 'W. A. Milne, 'and T . Axenrod, Anal. Chem., 42, 1432 (1970). 161 G. W. A. Milne. H. M. Fales, and T. Axenrod, Anal. Chem., 43, 1815 (1971). (7) D. Beggs, M. Vestal, H. M. Faies, and G. W. A. Milne, Rev. Sci. insfrum., 42, 1578 (1971). (8) H. M. Fales. Y . Nagai, G. W. A. Milne, H. G. Brewer, Jr., T. J. Bronzert, and J. J. Pisano, Anal. Biochem., 43, 288 (1971). (9) J. H. Futrell and L. H. Wojcik, Rev. Sci. lnstrum., 42, 244 (1971). (10) D. Beggs and F. H. Field, J, Amer. Chem. Soc., 93, 1567 (1971). 111) D. F. Hunt, C. N. McEwen, and R . A. Upham, Anal. Chem., 44, 1292 (1972). (12) J. Yinon, presented at the Twenty-Second Annual Conference on Mass Spectrometry and Allied Topics, Philadelphia, Pa., May 19-24, 1974. (13) D. Martinson, P. Price, R. Upham, H. S. Swofford, Jr., and S. E. Buttrill, Jr., manuscript in preparation. ~
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A N A L Y T I C A L C H E M I S T R Y , VOL. 47, N O . 1, J A N U A R Y 1975
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Philip Price D. P. Martinsen R. A. Upham H. S. Swofford, Jr. S. E. Buttrill, Jr.
