Anal. Chem. 1992, 64, 233-238 (14) White. R. T.. Jr.; Douthlt, G. E. J . Assoc. Off. Anal. Chem. 1985, 66, 766-769. (15) Pratt, K. W.; Kingston, H. M.; MacCrehan, W. A,; Koch, W. F. Anal. Chem. 1988, 60, 2024-2027. (16) Nadkarni, R. A. Anal. Chem. 1984, 56, 2233-2237. (17) Fassett. J. D.; Kingston, H. M. Anal. Chem. 1985, 57, 2474-2478. (18) Kingston, H. M.; Jassie, L. B. In htroduction to Microwave sample ReperaHon: Theory 8 d Practice; Kingston, H. M., Jassie, Lois B.,
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Eds.; American Chemical Society: Washington, DC, 1988; Chapters
11 and 6. (19) Matusiewicr, H.; Sturgeon. R. E.; Berman, S. S. J . Ana/. At. Spect”.1989, 4 , 323-327.
RECEIVED
for review June 10, 1991. Accepted October 11,
1991.
Supercritical Fluid/Supersonic Jet Spectroscopy with a Sheath-Flow Nozzle Chung Hang Sin,’Matthew R. Linford, and Steven R. Goates*
Department of Chemistry, Brigham Young University, Provo, Utah 84602 INTRODUCTION Complex and high molecular weight samples require highresolution methods for detailed analysis. One method with considerable discriminating power is supersonic jet spectroscopy (SJS). Highly resolved “fingerprint” spectra are obtained when sample molecules, entrained in a carrier fluid, are “cooled” in a free expansion of the fluid into a vacuum (1-3). Additionally, coupling SJS to chromatography allows for very high analytical resolving power (1). Conventional SJS with a gas-phase carrier is limited by the volatility and thermal stability of the sample. However, supercritical fluids can be employed to introduce nonvolatile or thermally labile species into the jet, either by direct supersonic expansion of the supercritical fluid (SF) carrier ( 4 , 5 )or by injection of the SF carrier into a secondary gas expansion (6-8). Employing SF carriers and especially linking supercritical fluid chromatography (SFC) to SJS poses several challenges, most of which are related to conflicting flow restriction requirements (9). Capillary SFC requires high restriction to maintain pressure in the column and low flow rates to achieve efficient separations. SJS benefits from much higher flow rates and abrupt restriction. In addition, because pressure and flow rate conditions are set by the chromatography part of the experiment, nozzle temperature is the only parameter which can be adjusted to control the formation of “van der Waals” clusters downstream from the nozzle. Such clusters, which spoil spectral selectivity, are prone to form with the polyatomics employed as SF eluents or carriers (4). Finally, decompression of the SF carrier in the throat of the nozzle can cause it to condense if the nozzle temperature is not high enough ( 1 0 , I I ) and can cause nonvolatile analyte to precipitate, especially with an extended restrictor (4, 12, 13). Stiller and Johnston (14)demonstrated how the sheath-flow approach could be used to solve flow mismatch problems in gas chromatography/SJS. The principle of sheath-flow operation is the channeling of the column effluent through an orifice by a flowing sheath of makeup gas (see Figure 1). Other advantages of such an arrangement are a reduction of effective dead volume in the nozzle and, with a monatomic makeup gas, greater supersonic cooling. Improved sensitivity is also obtained when the channeling results in hydrodynamic-like focusing of the analyte (14,15). We hoped for similar advantages in SF/SJS experiments. We had also predicted (4)that the use of a monatomic makeup gas would inhibit van der Waals cluster formation, allowing the nozzle to be operated at lower temperatures. *Towhom correspondence should be addressed. * Current address: Department of Chemistry, Northern Illinois University, DeKalb, IL 60115.
Unfortunately, secondary-flow methods proved to be more difficult to apply to SFC/SJS than to GC/SJS. Imasaka et al. (7)achieved some success, but only at the expense of large sample flow rates, high detection limits,and chromatographic band broadening. About the same time as Stiller and Johnston were conducting their sheath-flow experimentswith gases (14),we tried and failed in similar SF experiments with high molecular weight samples. Anderson and Johnston (16)were also unable to transport nonvolatile molecules into the jet through a sheath-flow nozzle, but they made an enlightening study of the performance of the nozzle with naphthalene in SF and liquid carriers. Their work along with our studies of direct SF expansions (4,IO)has led to a sheath-flow nozzle design which delivers all the hoped-for benefits except for sample focusing and has allowed SJS to be coupled to capillary SFC (8). In this paper investigations into the parameters that govern the performance of the nozzle are reported.
EXPERIMENTAL SECTION Apparatus. The overall experimental setup was similar to that described more completely in ref 9. Sample line pressures up to 400 atm could be obtained with a modified ISCO syringe pump or up to 150 atm, with a Perkin-Elmer syringe pump. The vacuum chamber, pumping system, and x-y-z positioning apparatus have been described previously (4). Background pressure in the chamber was monitored with a calibrated Varian ion gauge; it ranged from 1 X lo4 to 8 X lo4 Torr, depending on the pressure of the makeup gas flowing through the nozzle. A Lambda Physik excimer-pumped dye laser (EMG 102, FL2002) was the excitation source. Laser-induced fluorescencewas passed through a long-pass wavelength filter into an EM1 9789 photomultiplier tube, and the signal was amplified by a Tektronix AM502 amplifier, processed by a PAR No. 162 boxcar with No. 165 gated integrator,and either digitized directly by a MetraByte Chrom-1 computer interface or digitized later from strip chart records by a Silk Scientific UnPlotIt device. Nozzle. Figure 1shows a diagram of the sheath-flow nozzle, which includes a few modifications to the design proposed in ref 17. It is composed of Scientific Glass Engineering fittings and a tapered outlet cap assembly, made by the BYU Research Machine Shop. Because the cone could not be machined to a small enough opening, a laser-drilled 25-pm pinhole was used as the orifice. Pinholes in 3/8-in.stainless steel disks were purchased from Lennox Laser or Optimation,and ’/,& disks were cut from these stock disks. Graphitized vespel gaskets (SGE) and Teflon tape were used to seal the three parts together as depicted. Sample was carried through a fused-silica capillary column (50 pm inner diameter) into the sheath cone where it wm swept out of the nozzle by a makeup gas. The column ran through a larger, more rigid capillary in order to keep it centered in the gas flow; it extended into the cone onequarter to onehalf the length of the cone. Fluid pressure was maintained along the length of the column by a primary flow restrictor at the end of the capillary column (back-pressure of the sheath gas gives secondary restriction; see
0003-2700/92/0364-0233$03.00/0 0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992 ,
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