Anal. Chem. 1982, 5 4 , 2375-2376
LITERATURE CITED (1) Arpino, P. J.; Krien, P. Proceedings of the 26th Annual Conference on (2) (3) (4) (5) (6) (7) (8)
(9) (10) (11) (12) (13) (14) (15) (16) (17) (18)
Mass Spectrometry aind Allied Topics, St. Louis, MO, 1978; pp 426-428. Arpino, P. J.; Guiochon. G.; Krien, P.; Devant, G. J. Chromatogr. 1979, 185,529-547. Arpino, P. J.; Krien, P.; Vajta, S.; Devant, G. J. Chromatogr. 1981, 203, 117-130. Mauchamp, B.; Krien, F’. J. Chromatogr. 1982, 236, 17-24. Henion, J. D.; Wachs, 1’. Anal. Chem. 1981, 53, 1963-1965. Evans, N.; Williamson, J. E. Blomed. Mass Spectrom. 1981, 8 , 316-321. Schaefer, K. H.; Levssn, K. J. Chromatogr. 1981, 206, 245-252. Melera, A. Adv. Mass Spectrom. 1980, 88,1597-1615. Kenyon, C. N.; Meleria, A.; Erni, F. J . Anal. Toxlcol. 1981, 6 , 216-230. Dedieu, M.; Juin, C.; Arpino, P. J.; Bounine. J. P.; Guiochon, G. J . Chromatogr. 1982, 25 I , 203-213. Arpino, P. J.; Guiochon, G. J. Chromatogr. 1982, 251, 153-164. Blakley, C. R.; Carmody, J. J.; Vestal, M. J. A m . Chem. SOC.1980, 102, 5931-5933. Blackley, C. R.; Carmody, J. J.; Vestal, M. ,417al. Chem. 1980, 52, 1636-1641. Irlbarne, J. V.; Thom$on, B. A. J . Chnm. Phys. 1976, 64, 2287-2294. Tsuchiya, M.; Taira, T. Int. J. Mass Spectrom. Ion Phys. 1980, 34, 351-359. Blackley, C. R.; Vestal, IM. L. presented at the 30th Annual Conference on Mass Sepectrometry and Allied Topics, Honolulu, HI, 1982; abstract paper MPA 11. Hunt, D. F.; Crow, F. W. Anal. Chem. 1978, 50, 1781-1784. Hunt, D. F.; McEwen, C. N.; Harvey, T. M. Anal. Chem. 1975, 47, 1730-1734.
2375
Woodward, R. B. Pure Appl. Chem. 1973, 33, 145-177. Schulten, H. R. Int. J. Mass Spectrom. Ion Phys. 1979, 32, 97-283. McFarlane, R. D.; Torgerson, D. F. Science, 1976, 191, 920-922. Barber, M.; Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. N. Biomed. Mass Spectrom. 1981, 8 , 492-495. (23) Graham, S. W.; Dowd, P.; Hercules, D. M. Anal. Chem. 1982, 54, 649-654. (24) Rinehart, K. L., Jr.; Cook, J. C., Jr.; Maurer, K. H.; Rapp, U. J. Antibiot. 1974, 27, 1-13. (25) Arpino, P. J.; Guiochon, G. Anal. Chem. 1979, 5 1 , 682A-701A. (19) (20) (21) (22)
M. Dedieu* C. Juin Nermag Corp. 49 quai du Halage 92500 Rueil-Malmaison, France
P. J. Arpino G. Guiochon Ecole Polytechnique Laboratoire de Chimie Analytique Physique 91128 Palaiseau, France
RECEIVED for review May 20,1982. Accepted August 16,1982. Financial support for this work was received from the French “Ministgre de l’lndustrie et de 1’Equipement” under contract NO. 79-2-35-059.
Capillary Gas Chromatography/Mass Spectrometry with a Microwave Discharge Interface for Determination of Radioactive-Carbon-Containing Compounds Sir: Minor compounds in complex organic matrices represent an analytical challenge to organic mass spectroscopists, both in their detection and in structure elucidation. In biochemistry and pharmacology it is often possible to employ radiolabeled precursors when characterizing unknown metabolites. Gas chromatography with detection of radioactivity has long utilized nuclear decay t o permit very specific compound localization in clomplex chromatograms (1, 2). The design of combustion chiambers and radiochemical detectors significantly degrades the chromatographic resolution attainable with capillary columns (3-5). The amount of 14C normally employed in metabolism experiments suggests that mass spectrometric detection of 14C should compete with radiochemical detection--i.e., 1000 dpm equals approximately 100 pg of 14C. The limitation of mass detection is interference from 13C,2H, l8O, etc., which contribute to a significant and variable M + 2 background for most organic molecules. Quantitative degradation of organic molecules to di- or tristomic products simplifies heavy nuclide detection. We have exploited a low-pressure microwave-induced plasma interface to convert all carbon-containing compounds in a capillary GC effluent to CO and C 0 2 which can be measured in a conventional mass spectrometer without loss of chromatographic resolution and with a detection limit of 300-500 dpm. EXPERIMENTAL SECTION A Varian 1400 gas chromatograph equipped with a capillary splitter inlet set at 400:l (Supelco) was used with a HewlettPackard 50 m X 0.3 mm 11.d.SE 54 coated fused silica capillary column. The end of the column was extended into a 1/4 in. Swagelok “T”where its 1 mL/min helium flow was mixed with 0.1 mL/min of oxygen (UHP Matheson) and then passed through a 1/4 in. o.d., 4 mm id., 10 cm long quartz tube into a microwave
discharge cavity. Connections to the quartz tube were made with Vespel SP22 (40% graphite) ferrules and Swagelok fittings. The polyimide coating on that part of the capillary column which extended into the make-up “T” was burned off prior to assembly to minimize background. The microwave cavity was of a novel design (details are available upon request) to perform under a relatively high-pressure oxygen-rich environment and was powered by a 2450-MHz Raytheon Microtherm microwave power supply. The quartz tube was coupled to a 1 m section of 0.030 in. i.d. stainless steel capillary tubing which ran concentrically through the direct introduction probe into the mass spectrometer ion source. The interface was heated to approximately 230 OC up to the discharge cavity and operated at ambient temperature from that point on. A mixture of 514 ppm butane in helium was obtained from Matheson. The [l-14C]palmiticacid (50 mCi/mmol, New England Nuclear) was methylated with diazomethane in ether and compared for purity with unlabeled methyl palmitate and palmitic acid using conventional GC/MS techniques. A Finnigan Model 1015 quadrupole mass spectrometer with Extranuclear SpectrEl electronics and differential pumping was used to obtain mass spectra data recorded with a Ribermag SADR data system or Finnigan PROMIM.
RESULTS AND DISCUSSION Because of the low operating pressures in the discharge tube, ca. 1 torr, no make-up gas was needed to ensure high chromatographic resolution. Only 0.1 mL/min of O2 was needed to combust completely any analyte introduced. This complete combustion is demonstrated in Figure 1where approximately 1 mL/min of butane/He mixture was introduced via the make-up gas line and its spectrum analyzed both without and with the discharge. No ions representing intact butane were observed after the discharge was turned on; the summed intensities of major product ions approximately equal
This article not subject to U.S. Copyright. Publlshed 1982 by the American Chemical Society
2376
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
BUTANE -DC
r
BUTANE +DC
''C-Msthyl Palmitate
lMpCi "300 dpm
1 Flgure 1. Mass spectrum of a 514 ppm mixture of butane In helium. The m l z 32 peak from the oxyaen reaction gas was not recorded. In the left panel, the discharge is not turned on; in the right panel, the
discharge is working.
IJI
+
HEXANES DC
1
h
m/z 43
61 K
i ; Selected ion recording from radiolabeled methyl palmitate in n-heptane with detectlon of 14C02and '2C02. Units on the x axis are mlnutes. The ratio of m l z 46/44 is three times that observed for an equivalent quantity of unlabeled methyl palmitate. Figure 3.
258
E1B
E98
118
33s
1%
178
IF@
lie
LIB
,e
Flgure 2. Mass chromatograms from a mixture of hexane isomers separated on a SE 54 coated fused sillca column and degraded in the mlcrowave discharge. Ten scans on the x axis correspond to 3.7 s. The relative intensitles of the signals observed at m l z 43, 44, and 57 are 61 000, 9 100000, and 0, respectively.
the expected intensities from the combustion products of butane. The chromatographic resolution obtainable with the interface is seen in Figure 2 where a sample of isomeric hexanes was injected and separated at room temperature. The peak width at half-height was approximately 1.8 s at a retention time of 2 min. The small signal a t m / z 43 is spillover from the intense m/z 44 due to incomplete mass resolution of the quadrupole analyzer. The potential of this technique as a radioactivity monitor was assessed with a sample of methyl [lJ4C]palmitate which was chromatographed isothermally at 215 OC. Figure 3 shows the detection of I4CO2at m/z 46 along with the natural 12C02 a t m / z 44. The time from the injection (not shown) was 4.2 min and the half-width was 4 s. After purity, derivatization yield, and split were accounted for, the methyl palmitate peak was calculated to contain 150 pCi which is 330 dpm. The reaction GC/MS interface is not limited to oxygen as the reactant gas nor to carbon as the analyzable element. We have seen complete reaction of carbon either with nitrogen to form HCN and (CN)2 or with Hz to form CH,. Other preliminary studies have shown detection of nitrogen, sulfur, chlorine, and bromine in the oxygen-scavenged discharge. In the future, we will report on the chemistry of this technique using a unit with heated and minimal postdischarge reactor volume. The use of a destructive interface has the advantage of generality; no information is needed about the position or the
chemistry of the element or nuclide under investigation. In this way, it may be useful in metabolic studies where all the products of a labeled precursor are sought or in s w e y analyses where an element- or nuclide-specific chromatographic detector is desired. Its construction permits retention of the high resolution of open tubular gas chromatographic columns.
ACKNOWLEDGMENT We thank Robert L. Bowman, NIH, for his interest and his design and construction of a microwave cavity used in this research, and Wayne Sieck, NBS, for many helpful discussions. LITERATURE CITED (1) Matucha, M.; Smolkova, E. J. Chromatogr. 1976, 127, 163-201. (2) Campbell, I. M. Anal. Chem. 1979, 51, 1012A-1021A. (3) Weber, H.; Holler, M.; Breuer, H. J. Chromatogr. 1982, 235, 523-526. (4) Ernst, L. A.; Emmons, G. T.; Naworal, J. D.; Campbell, I. M. Anal. Chem. 1981, 53, 1959-1961. (5) Gross, D.; Gutekunst, H.; Blaser, A.; Hambock, H. J. Chromatogr. 1980, 198, 369-396.
'
Professor of Pharmacology, George Washington University, Washington, DC, on sabbatical leave 1981-1982.
Sanford P. Markey* Laboratory of Clinical Science National Institute of Mental Health Building 10, Room 3N262 Bethesda, Maryland 20205
Fred P. Abramson' Clinical Pharmacology Branch National Cancer Institute Bethesda, Maryland 20205
RECEIVED for review May 14, 1982. Accepted July 26, 1982.
