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sources mentioned above and required greater effort in isolating the experiment from them. In spite of this difficulty, there was a 25% improvement in signal to noise (measured in the same manner as above) using the intracavity method with the 500-mm radius of curvature mirror vs. the same method with the 200-mm radius mirror. This result represents a 2O:l detection advantage for the intracavity method over the external PBD method. Registry No. Carbon, 7440-44-0. LITERATURE CITED (1) Boccara, A. C.; Fournier, D.; Badoz, J. Appl. Phys. Lett. i980, 36, 130. (2) Jackson, W. B.; Amer, N. M.; Boccara, A. C.; Fournier, D. Appl. Opt. 1981, 20, 1333. (3) Aamodt, L. C.; Murphy, J. C. J . Appl. fhys. 1983, 54, 581. (4) Mandeiis, A. J . Appl. Phys. i983, 54, 3404. (5) Low, M. J. D.; Morterra, C.; Severdia, A. G. J . Mol. Catal. 1983. 2 0 , 311. (6) Pichon, C.; Leiiboux, M.; Fournier, D.; Boccara, A. C. Appl. fhys. Lett. 1979, 35, 435. (7) Low, M. J. D.; Lacrolx, M.; Morterra, C. Appl. Spectrosc. 1982, 36, 582.
(8) Low, M. J. D.; Lacrolx, M.; Morterra, C. Spectrosc. Lett. i982, 75, 57. (9) Fournier, D.;Boccara, A. C.; Badoz, J. Appl. Opt. i982, 21, 74. (10) Johnson, C.; Brundage, R. T.; Giynn, T. J.; Yen, W. M. J . fhys., Colloq. (Orsay, Fr.) 1983, 44, 248. (11) Coddens, M. E.; Poole, C. F. Anal. Chem. i983, 55, 2429. (12) Chen, T. I.; Morris, M. D. Anal. Chem. 1984, 56, 19.
Tsutomu Masujima Asutosh N. Sharda Lindsay B. Lloyd Joel M. Harris Edward M. Eyring* Department of Chemistry University of Utah Salt Lake City, Utah 84112
RECEIVED for review June 11, 1984. Accepted July 31, 1984. Financial support of this work by a grant from the Department of Energy (Office of Basic Energy Sciences) is gratefully acknowledged.
Solution introduction Mass Spectrometry: A Simple Alternative to Desorption Techniques Sir: A considerable number of techniques have recently been developed for obtaining mass spectra from nonvolatile and thermally labile samples. These new techniques generally require specialized source facilities such as field desorption ( I ) , secondary ion mass spectrometry (2), fast atom bombardment (3), liquid ionization (4),electrohydrodynamic ionization (5), and laser desorption (6). Plasma desorption mass spectrometry (7),which is one of the more successful of the new techniques, requires unique mass spectrometry facilities. Direct introduction of liquid chromatographic effluents into mass spectrometer ion sources requires the presence of both a liquid chromatograph and an interface system in the mass spectrometry laboratory. The thermospray (8)interface and other methods of direct liquid introduction (9) have allowed determination of mass spectra of compounds that would not normally vaporize under conditions of direct probe introduction. It was the objective of this research to develop a procedure for obtaining mass spectral data on nonvolatile compounds with minimal modifications to a commercial mass spectrometer. The experiments with direct interfaces between liquid chromatography and a mass spectrometer have clearly shown that overcoming a compound’s lattice energy by introducing the compound in solution makes it possible to obtain mass spectral information from compounds that would be destroyed during attempts to evaporate them directly from a solid matrix. Many LC/MS interfaces that have been described have typically used nebulizers with holes a few micrometers in diameter for solution introduction. This report presents a method for using the same principles for sample introduction without the need for modifying the instrument or utilizing a liquid chromatograph td present the sample to the mass spectrometer. EXPERIMENTAL SECTION The mass spectrometer used in these experiments was a Finnigan 4510 GC/mass spectrometer equipped with a temperature controlled direct insertion probe. The instrument was operated in ita normal positive or negative chemical ionization mode with 0.4 torr of isobutane, as the reagent gas. Both source temperature 0003-2700/84/0356-2977$01.50/0
and direct insertion probe temperature were treated as experimental variables and are reported with the spectra. Primary ionization was accomplished with an 89-eV beam of electrons. Solution Injection System. Figure 1 illustrates the tubes that we have used for direct injection of solutions of nonvolatile compounds. A 2 mm 0.d. Pyrex capillary was fashioned to replace the quartz or ceramic probe tip that we usually use with this mass spectrometer, The conventional probe tip has a diameter of 2 mm and a length of 11 mm. After an approximately 12 mm segment of capillary tubing sealed at one end was cut, the open end of the tube was fire polished in a micro oxygen-natural gas flame. The fire polishing was continued until the outlet diameter was between 250 and 220 pm. This was checked periodically by cooling the capillary and inserting a 220-pm fused silica needle from a conventional on-column capillary GC injection syringe. With a small amount of practice, probe tips like that in Figure 1 can be made in any quantity desired. Solytions were prepared by dissolving substrates in HPLC water from a Milli-Q system to obtain a concentration of approximately 1 mM/L. Approximately 5 p L portions of these solutions were used to fill the capillaries. The capillaries were introduced into the mass spectrometer as probe tips in the conventional way except that the pump-down time in the direct probe interlock was limited to 2 s.
RESULTS AND DISCUSSION Figure 2 illustrates a solution introduction mass spectrum obtained by inserting a capillary containing 5 KL of a 2 mM solution of erythromycin A in water into an isobutane CI plasma. The evaporation profile for the solution is illustrated in Figure 3. The background positive and negative CI spectra of water are given in Figure 4. The protonated molecule of erythromycin is prominent in Figure 2 at m / z 734. Fragment ions at m / z 658,876, and 716 correspond to loss of C2H403, C2H202,and water, respectively,from the protonated molecule. The fact that the protonated molecule is not the base peak in this chemical ionization mass spectrum suggests that the ionizing conditions in the water-isobutane plasma generated during the injection process are more energetic than in a simple isobutane chemical ionization plasma (10). The smooth evaporation profile which is illustrated in Figure 3 was characteristic of only approximately half of our 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
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