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(31) Ohashi, M.; Barron, R. P.; Benson, W. R. J. Am. Chem. SOC. 1081, 103, 3943. (32) Bowie, J. H.; Lewis, 0. E.: Cooks, R. G. J. Am. Chem. SOC.B 1067, 621. (33) Gilland, J. C., Jr.; Lewis, J. S. Org. Mass Spectrom. 1074, 9 , 1148. (34) Unger, S. E.; Day, R. J.; Cooks, R. G. Int. J. Mass Spectrorn. Ion Phys. 1081, 39, 231. (35) Venkataraman, K. “The Chemistry of Synthetic Dyes”; Academic Press: New York, 1952; Vol. 1, p 20. (36) Ryan, T. M.; Day, R. J.; Cooks, R. G. Anal. Chem. 1080, 52, 2054. (37) Roilgen, F. W., private communication, 1981.
(38) Barofsky, D. F.; Giessmann, U. Int. J. Mass Spectrom. Ion Phys. 1083, 4 6 , 359. (39) Unger, S. E.;Day, R. J.; Ryan, T. M.; Cooks, R. G. Presented at 28th Annual Conference on Mass Spectrometry and Allied Topics, New York, May 25-30, 1980.
RECEIVED for review June 9, 1983. Accepted August 31, 1983. The support Of the Science Foundation (CHE 8011425) is gratefully acknowledged.
Mass Spectrometry with Direct Supercritical Fluid Injection Richard D. Smith* and Harold R. Udseth Chemical Methods and Kinetics Section, Pacific Northwest Laboratory (Operated by Battelle Memorial Institute), Richland, Washington 99352
Dlrect fluid lnjectlon mass spectrometry utilizes supercritical fluids for solvatlon and transfer of materials to a mass spectrometer chemical Ionization (CI)source. Available data suggest that any material soluble In a supercritlcai fluld Is transferred efficlentiy to the ionlzatlon reglon. Mass spectra are presented for mycotoxins of the trlchothecene group obtained by use of supercritical carbon dloxide with isobutane as the CI reagent gas. Direct fluid lnlection MS/MS is also illustrated for major ions in the isobutane chemlcal lonlzation of T-2 toxin. The effect of pressure and temperature upon solublilty in supercrlticai fiulds Is described and illustrated for diacetoxyscirpenoi. A potentlal method Is also demonstrated for “on-line fractlonation” durlng MS analysis using pressure to control supercrltical fluid solubility. Mass spectra are also presented for polar compounds, using supercrltical ammonla, and the extension to complex mixtures Is described. The fundamental bask and experimental requlrements of the direct fluid lnjectlon process are discussed.
New methods for mass spectrometric analysis of nonvolatile or thermally labile compounds utilize a range of ‘‘soft” ionization techniques which result in ion formation at or near a surface. These techniques include field desorption ( I ) , laser desorption (LD) (2),secondary ion mass spectrometry (SIMS) (3, 4), fast atom bombardment (FAB) (5),californium-252 plasma desorption (6),“in-beam” chemical ionization (7-I2), ion desorption from droplets (13,14),particulate impact (15), electrohydrodynamic ionization (I6),and a variety of rapid heating techniques (17). These techniques, however, all suffer from difficulties which include: often substantial matrix effects; frequently complex mass spectra and, in some cases, ions produced with moderate abundances at nearly every mlz value (particularly with FAB,SIMS, and LD); response which varies greatly with compound polarity; quantitation which is difficult; spectra which are currently difficult to predict and interpret; and sensitivity to the precise chemical and physical nature of the surfaces involved. Conventional electron impact (EI) or chemical ionization (CI) methods offer some advantages in comparison to desorption ionization methods. The internal energy deposited in the ionization processes can be readily controlled by varying electron energy for E1 or the reagent gas for CI. The extensive experience with E1 and CI methods assists interpretation and prediction of spectra. Additionally, matrix effects are avoided,
quantitation is straightforward and ionization efficiences and detection limits are relatively uniform. Previously, the two most universal methods for transfer of nominally nonvolatile neutral molecules to the gas phase have been the direct liquid introduction (DLI) method, developed for HPLC/MS interfacing (18),and rapid heating techniques (17). These methods also have distinct limitations. The DLI method requires a “desolvation chamber” for evaporation of liquid droplets in a region adjacent to the CI source where the temperature must be carefully controlled and where optimum temperatures may be different for different compounds. Rapid heating techniques, on the other hand, are sensitive to both the sample substrate and heating rate and present difficulties in spectrum acquisition during the brief heating period. A new alternative approach involves the use of a supercritical fluid or “dense gas” for efficient transfer of material to the gas phase in a CI source. The method of direct fluid injection mass spectrometry (DFI-MS) is being explored in our laboratory for the purpose of interfacing capillary column supercritical fluid chromatography (SFC) with mass spectrometry (19). The direct interfacing of SFC was discussed more than a decade ago (20) and Randall and Wahrhaftig (21)have described an approach using supersonic molecular beams. Unfortunately, this approach appeared to suffer from instrumental complexity and extensive solvent cluster formation. While SFC technology continues to progress rapidly, the columns available to date are best suited for relatively nonpolar compounds. The DFI-MS (and DFI-MS/MS) methods, however, are readily applicable to a wide range of compounds using a variety of fluids. The DFI method allows mass spectra to be obtained for essentially any compound soluble in a supercritical fluid and, hence, allows a rapid qualitative evaluation for fluid phase solubility. This report discusses the basic aspects of DFI-MS and presents examples of the range of possible application.
EXPERIMENTAL SECTION Most of the instrumentation used for DFI-MS is similar to that previously described for SFC-MS (19). Figure 1gives a schematic illustration of the DFI-MS instrumentation. All fluids are either distilled or cleaned with various filters and adsorbent materials prior to use. The required high-pressure pulse-free liquid flow is generated with a Varian 8500 syringe pump (8OOO psi maximum pressure). The liquid flow is filtered and cleaned as necessary (and depending upon the fluid) before the temperature is elevated above the critical temperature. A final cleanup stage consists of elevation above the critical temperature prior to the injector which results in removal of suspended particulate matter or precipitation
0003-2700/83/0355-2266$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983
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PRESSURE MONITOR AUXILIARY GAS
ANALYZER W COLLISION CHAMBER
I
0.2 pL INJECTOR
I
LIQUID BATH AND CIRCULATOR
0.4 pn
FILTERS
Flgure 1. Schematic illustration of the instrumentatlon for direct fluid Injection mass spectrometer.
of material soluble in the liquid but insoluble in the supercritical fluid. This has been found to allow trouble-free operation of the DFI orifice for several days, and up to several weeks in some instances, depending upon the fluid. The 0.06 or 0.2 pL Valco injector valve can be maintained at the fluid temperature or, more typically, operated at ambient temperature. In the latter case the elevation to supercritical temperatures occurs rapidly to prevent loss of material due to possible limited solubility in the subcritical liquid. The injector is mounted at the end of the DFI probe which may be conveniently removed from the vacuum chamber using a standard 0.5 in. diameter direct insertion interlock. The length of the 0.1 mm i.d. platinum-iridium tubing, used for transfer between the injector and DFI orifice, is approximately 35 cm providing a total volume of approximately 3 pL. DFI probes have been constructed by using both electrical heating and fluid circulation to maintain isothermal conditions up to the DFI injector. Use of the electrically heated probe is generally restricted to temperatures >lo0 OC where heating of the DFI orifice by the ion source block is not a problem. Fluid circulation has been used for probe operation to -200 OC with a temperature gradient of
