Anal. Chem. 1991, 63, 251-255
25 1
Probe Interface for Supercritical Fluid Chromatography/Fourier Transform Mass Spectrometry Eldon R. Baumeister, C. David West, C a r l F.Ijames, and Charles L. Wilkins*
Department of Chemistry, University of California-Riverside, Riverside, California 92521
A versatlie probe-mounted Interface for supercritical fluld chromatography/Fourier transform mass spectrometry has been designed, constructed, and evaluated. The interface Is fully contained wRMn a 40 In. long stainless steel tube and provides for accurate transfer line temperature control and separate control of a nonlnductiveiy wound probe tip heater. The tip can be contlnuously held at temperatures above 600 OC. Separate current and thermocouple vacuum feedthroughs allow for convenlent tlp removal and SFC pressure/flow restrlctor replacement. The tlp heater utlllzes a copper disk to protect the fraglle restrlctor tlp and to serve as an anode for the electron beam source. The probe is easily inserted through a standard probe lock wRhout dlsruptlon of the hlghvacuum system. Functional characterlstlcs of the new SFC/FTMS probe Interface are demonstrated by analyses of cholesterol and a polyaromatlc hydrocarbon mixture. Cholesterol shows mlnlmal dehydration, as evidenced by the appearance of Its molecular Ion as the base peak In Its mass spectrum. Detection limits of approximately 100 pg are demonstrated for aromatic hydrocarbons.
INTRODUCTION Because of its much-reduced mobile-phase flow rate, capillary supercritical fluid chromatography has made it feasible to directly interface SFC with mass spectrometers. The wide acceptance of supercritical fluid chromatography/mass spectrometry (SFC/MS) as a potentially important tool for analysis of nonvolatile and thermally unstable compounds is evidenced by numerous recent publications and review articles (1-6). The surge of interest in SFC and SFC/MS can be explained by two factors. First, the diffusion rates and viscosities of supercritical fluids allow for fast and efficient separations of nonvolatile or thermally unstable compounds. Such separations are generally not possible by gas chromatography/mass spectrometry (GC/MS). The physical properties of supercritical fluids allow the use of liquid-phase detectors and, after decompression through an appropriate restrictor, many gas-phase detectors. Second, SFC/MS typically has several potential advantages over LC/MS such as simpler direct interfacing, greater speeds of analysis, greater sensitivity, better chromatographic resolution, and most important, the ease of mobile-phase removal. Although supercritical fluids work well with most GC detectors if compatible mobile phases are used, the greater information provided by mass spectrometers makes them attractive as SFC detectors. Following the pioneering work by Smith and co-workers (7, 8), a great many SFC/MS interfaces have been constructed. Most are described in the review articles cited, but recent papers of particular pertinence to the present study are those by Kalinoski and Hargiss (9),Sheeley and Reinhold (3),and Huang and co-workers (IO). Fourier transform mass spectrometers have the necessary scanning speed, high mass range, and high resolution to fully exploit SFC. FTMS (Fourier transform mass spectrometry)
does require that relatively low analyzer pressures (lower than lo-' Torr) must be maintained for optimal results. The feasibility of interfacing SFC with Fourier transform mass spectrometers was first demonstrated by Lee et al. in 1987 (11). Their results showed that the high gas load from an SFC interface could be tolerated and that several modes of ionization of eluants were possible. However, as these workers observed, several limitations existed in their interface. In particular, the restrictor could not be heated, making analysis of high-mass analytes problematic. A second demonstration of SFC/FTMS, employing an improved heated interface was reported by Laude and co-workers later that year (12).In that report, several SFC separations of relatively simple mixtures were described and the utility of FTMS as a detector for SFC was further demonstrated. Because neither of these two approaches to the SFC/FTMS interface utilized a completely removable probe-mounted arrangement, use, modification, and servicing of these interfaces was inconvenient. The initial work in SFC/FTMS prompted us to design a probe interface system' which is convenient to use, has good temperature control, and is contained entirely within a standard FTMS probe. Good temperature control is a prerequisite if thermally labile materials are to be analyzed. Ideally, such an interface would incorporate an easily exchanged integral restrictor and could be operated for extended periods with the tip temperature above 600 "C. For maximum sensitivity, it should also have a tip position which is easily adjustable, relative to the FTMS cell, during operation. Ideally, the interface should be useable for GC/MS without any significant alterations. Here, the design, construction, and testing of such a versatile probe-mounted SFC/FTMS interface is described. Use of this interface provides a convenient means of accomplishing both GC/FTMS and SFC/FTMS experiments without the need for modification of the mass spectrometer. Because the requisite analyzer pressures for FTMS are much lower than those of most mass spectrometers, with a correspondingly greater mismatch between the SFC requirements and the analyzer, FTMS provides a stringent test of this design. From the present results, it appears that the interface should be equally suitable for other types of mass spectrometers. EXPERIMENTAL SECTION Supercritical Fluid Chromatography System. The supercritical fluid chromatography/Fourier transform mass spectrometry system is diagrammed in Figure 1. The first component consists of the GC oven, SFC pump, and injector. The second element includes the heated transfer lines and probe interface, and the final component is the Fourier transform mass spectrometer. SFC separations were conducted by using a Varian Vista 6000 gas chromatograph oven (Palo Alto, CA) fitted with a Brownlee Labs Microgradient System syringe pump (Santa Clara, CA). A Valco Model C14W 60-nL injection valve (Houston, TX) is used along with a Lee Scientific 8 m X 50 pm SB Phenyl-5 bonded (0.25 pm thick) column (Salt Lake City, UT). All samples were commercially available from Aldrich Chemical Co., Milwaukee, WI. Spectral grade benzene was used to dissolve the samples. Pure COz, obtained from Scott Speciality Gases (San Bernardino, CA), was used as mobile phase. SFC/FTMS runs 0 1991 American Chemical Society
252
ANALYTICAL CHEMISTRY, VOL. 63, NO. 3, FEBRUARY 1, 1991 A
B
Figwe 1. Block diagram of SFCIFTMS system: (A) COPcylinder; (B) high-pressure syringe pump: (C) 0.45-pm filter; (D) injection valve; (E) gas chromatograph; (F) transfer line; (G) probe interface: (H) Fourier
transform mass spectrometer. See text for details. -G Figure 3. Tip heater: (A) SGE zero dead-volume union, modified as described in text; (B) noninductively wound heater wire; (C) ceramic
adhesive; (D) 1.5 mm 0.d. copper cylinder; (E)hole for capillary restrictor, 0.380 in. i.d.; (F) probe tip disk: (G) thermocouple wires: (H) heater wires. See text for details.
F
Figure 2. Probe interface: (A) fused-silica capillary restrictor; (8) 'Il6-in. stainless steel tube: (C) 5/16-in. stainless steel tube: (D) 1/8-in.
copper tubing, wrapped with heating tape and glass tape insulation: (E) vacuum feedthroughs; (F) heater wires: (G) thermocouple wires. were pressure-programmed with initial linear flow velocities of 4 cm/s at 1800 psi. During analyses, the injection transfer line region was maintained at a constant temperature to match both the column and postcolumn transfer line temperatures. The temperature of the postcolumn heated transfer line was carefully controlled. This heated transfer line contains the last meter of the capillary SFC column and delivers eluant directly to the removable probe interface. Injection and postcolumn heated transfer lines are constructed of thin-wall 1/16-in.stainless steel tubing wrapped with Clayborn Labs F-28 resistive heating tape (Fort Collins, CO) and covered with insulation and Fiberglas tape. Multiple thermocouples are placed along the tubes to verify that uniform heating occurs along their entire length. Probe Interface Design. The removable probe interface is made up of two components, the body of the probe and the heated tip. Figure 2 is a schematic diagram of the probe body. The outer part consists of an 0.873 in. 0.d. X 40 in. No. 304 stainless steel tube. Contained within the outer stainless steel tube is a noninductively wrapped and well-insulated, resistively heated (Clayborn Labs F-28 resistive heating tape) '/,gin. stainless steel tube housing an integral Guthrie-type restrictor (White Associates, Pittsburgh, PA). As shown in the diagram of the tip heater winding in Figure 3, noninductive winding is accomplished by bending the wire double and wrapping the doubled wire around the piece to be heated. When inserted, this heated region is located within the magnetic field of the Fourier transform/mass spectrometer and is temperature-controlled by one output of a dual-output 1.8-mA 40-V dc power supply (Harrison Laboratories, Berkeley Heights, NY) Two type K thermocouples and two electrical extensions run the length of the tube and connect feedthroughs at the probe end washer to appropriate plugs at the atmospheric side of the probe. On the high-vacuum side of the probe interface is a welded probe end washer. This washer was machined so that four 0.154 in. 0.d. Del-Weld feedthroughs (Insulator Seal Inc., Hayward, CA) and a chromatography fitting could be welded into it. Thus, the probe end washer contains a 3/ 16-in.X 0.4-mm glass-lined miniunion (MVSU/004, Scientific Glass Engineering, Inc. (SGE), Austin, TX)and the four vacuum feedthroughs. Two copper feedthroughs provide current to a tip heater while the other two are made of K-type thermocouple
materials used to monitor the tip heater temperature. The SGE fitting is located off-center in the end washer because the center line of the probe does not match the center line of the cell. This geometry allows for optimizing tip position relative to the electron ionization filament's electron beam by simply turning the probe either clockwise or counterclockwise. The chromatography column is connected to the integral restrictor with a Valco 1/32-in.zero dead-volume union. On each side of the union the column and restrictor are supported by the */16-in.stainlesssteel transfer lines. The column to restrictor union region is separately resistively heated, insulated, and monitored by thermocouples. All resistive heating tapes not within the superconducting FTMS magnetic field or its fringe field are controlled by standard variable transformers. Tip Heater Design. The probe tip heater (Figure 3) is constructed on an SGE zero dead-volume miniunion nut. The top half of the miniunion nut has been turned round while the lower half maintains its hexagonal pattern to permit tightening with a wrench. A copper extension protrudes beyond the miniunion nut. All but the final millimeter of a 7 mm long piece of 1/4-in. cylindrical copper stock was turned down to 1.5 mm and then pressed and silver-soldered into the end of the SGE miniunion nut, with the 1/4-in.disk furthest from the miniunion nut. The 1 mm thick X 1/4 inch diameter tip disk thus formed serves both to protect the fragile restrictortip and to provide a grounded anode for the electron beam. The copper tip extension is drilled to accept the 0.375 mm 0.d. fused-silica Guthrie restrictor. The copper extension provides the best thermal contact with the fused silica. The miniunion nut with its copper extension is coated with a very thin insulating layer of finely ground Cotronics No. 940 ceramic adhesive (Brooklyn, NY). For temperature monitoring, a 32 gauge K-type thermocouple is secured 1 mm from the end of the tip heater and the tip heater is wrapped noninductioely with 4 in. of a single strand of bare Clayborn Labs F-28 resistive heater wire. Both the thermocouple and heater wire are potted in place with several thin layers of the ceramic adhesive. Thermocouple and electrical connections between the tip heater and the probe end washer were made by spot-welding the wires to the feedthrough pins in the probe end washer. After adding a thin coating of silver solder to the two copper feedthrough pins, this mode of attachment became quite convenient. The tip heater is controlled by the second output of the 1.8-mA 40-V dual dc power supply. This heated tip can be conveniently heated to over 600 "C. The restrictor used is a White Associates 1m X 0.375 mm 0.d. X 50 pm i.d. uncoated, deactivated fused-silica integral type with a nominal flow of 0.2 mL/min at STP of COPas a gas a t 830 psi head pressure. The restrictor is vacuum-sealed in the SGE miniunion fitting with a 0.4-mm graphitized Vespel ferrule. Fourier Transform Mass Spectrometer. A modified Nicolet FTMS-1000 mass spectrometer equipped with a Nicolet FTMS-2000 4.76-cm cubic 80% transmissive stainless steel dual differentially pumped cell was operated at a magnetic field strength of 2.9 T. Source and analyzer cell chambers were pumped with 700 and 300 L/s Alcatel diffusion pumps, respectively.
ANIALYTICAL CHEMISTRY, VOL. 63, NO. 3, FEBRUARY 1, 1991
Ty ical background pressure in the absence of C02 flow is 1 x 10 Torr. The source/analyzer pressure ratio for the experiments described was always greater than 100 with supercritical COz as mobile phase, even at SFC pressures as high as 6000 psi. The electron gun is located on the analyzer side of the vacuum chamber with the beam aligned to pass through the 2-mm conductance limit into the source cell. A standard Teflon-sealed probe lock on the source side flange is used for insertion of the SFC/FTMS interface probe into the high-vacuum chamber of the mass spectrometer. Standard Nicolet GC/FTMS software was utilized with a Nicolet 1280 computer to acquire and process the SFC/ FTMS data. SFC/FTMS Instrumental and Software Parameters. The instrumental parameters used in this work were selected to emphasize sensitivity rather than mass resolution. Consequently, the FTMS parameters were chosen to maximize the number of ions in the cell, up to the space charge limit. Typical parameters are a 600-kHz bandwidth (with low pass filters set to 615 kHz), 16K data points acquired and augmented by 16K zeroes prior to a 32K Fourier transform, a 30-ms 13-V electron beam time, 200 sweeps coadded per data file, a trap voltage of 1.76 V, and the maximum emission current beneath the space-charge effect limit. COz+ ions were ejected following the beam event. Use of these ionization conditions results in what is probably a combination of both COz charge exchange and direct electron ionization. A total experiment time of about 50 ms/spectrum was required, resulting in a total data acquisition time of 10.5 s for 200 coadded transients, allowing acquisition of six to eight data files per chromatographic peak. Chromatography was conducted to minimize analysis time rather than to achieve high chromatographic resolution. A very sharp pressure ramp was utilized. An initial isobaric period of 10 min at 1800 psi was followed by a pressure ramp to 5000 psi over the next 5 min, followed by a second isobaric period of 10 min at 5OOO psi. At 1800 psi, the linear flow velocity was 4 cm/s. The SFC column and transfer lines were carefully maintained at 150 "C, with the mass spectrometer vacuum enclosure, source cell, and analyzer cell held at 125 "C. The optimal tip temperature for the samples analyzed in the present study was found to be 300 "C.
J
RESULTS AND DISCUSSION SFC/FTMS Probe Interface Evaluation. Initial testing of the probe interface involved evaluation of the temperature control of the various heated zones and pressure/flow characteristics of the integral restrictor and heater tip. The injection and postcolumn transfer line regions are easily controlled by variable transformers over a 50-250 "C range. Because ample insulation is used, the transfer lines are thermally stable, although it is also possible to change their temperatures with rapid reequilibration. The probe interface and tip heater are conveniently controlled by the dual dc power source. Similarly, the integral restrictor heater region is also easily temperature-controlled over a 50-250 "C temperature range but requires more time for equilibration because of the insulation and its containment inside the stainless steel probe tube. The noninductively wrapped tip heater can be heated from room temperature to over 600 "C and equilibrates rapidly. The probe's integral restrictor and tip heaters require dc power sources, since it is important that they not interfere with magnetic field homogeneity near the cell. The ceramic coating used on the tip heater serves as a good electrical insulator for potting the thermocouple and heater wire to the all-metal nut assembly, without impeding rapid heat transfer. Unfortunately, when the ceramic tip is exposed to the atmosphere, it rapidly absorbs atmospheric moisture. This is only a very minor problem, as the moisture is easily removed once the probe is reinserted by application of tip heater current. In practice, the probe interface can be inserted through the probe lock and electrical and chromatographic connections made in under 10 min. Furthermore, the complete system can be made fully operational within 30 min. Thus, the SFC/FTMS probe interface can be installed and removed rapidly, allowing complete sample inlet flexibility
253
Table I. Dual-Cell Pressure Differentials in the FTMS during SFC Probe Interface Delivery of COzo source/
column head pressure, psi 1500 1600 2000 2500 3000 3500 4000 4500 5000 5500 6000
source ion gauge, Torr 5.2 X 5.6 X 7.2 X 9.3 X 1.1 x 1.3 x 1.5 x 1.7 x 2.0 x 2.3 x 2.8 x
10" 10" 10" 10" 10-5 10-5 10-5 10-5 10-5 10-5
10-5
analyzer ion gauge, Torr 3.8 X 4.1 X 5.3 X 6.9 X 8.5 x 1.0 x 1.2 x 1.4 x 1.5 x 1.9 x 2.6 x
10"
10-8
10-7 10-7 10-7 10-7 10-7
10-7
analyzer pressure ratio 137 137 136 134 129 130 125 121 133 121 108
a Measured at the ion gauges (calibrated for nitrogen), which are mounted outside the fringing field of the magnet. The true pressures are probably somewhat higher at the cell. The vacuum chamber was maintained at 125 "C, transfer lines, at 150 "C, and the restrictor tiD, at 300 "C.
with the mass spectrometer. An additional advantage of this design is that it obviously could be converted easily and quickly into a GC/MS interface by simply replacing the fused-silica integral restrictor with a deactivated, uncoated piece of fused silica to match the desired GC column. The tip heater would then be set to match the other transfer line temperatures. Earlier prototype tip designs developed problems resulting from charging of the tips. In those cases, the effect of charging was to seriously limit transient duration and stability, due to electrons reflected back into the cell. In order to provide an anode for the electron beam, the copper-disk extension was devised. This disk is in electrical contact with the body of the probe, which is grounded to the vacuum chamber. Use of this design prevents charging of the tip heater. The tip disk may also prove to be a convenient electrode for possible extended-cell suspended-ion trapping experiments (13). As mentioned earlier, location of the off-center SGE fitting in the probe end washer allows for easy rotation of the restrictor in the xy plane until it is in direct line with the E1 beam, thus maximizing ionization of the eluants. This distance from the front trap plate of the source cell is also controllable, because the probe can be inserted any desired distance in the z direction (along the magnetic field axis). COPflow through the probe interface restrictor was measured with the probe tip l in. from the source trap plate. With the flow of C 0 2 off, the system can be maintained a t a Torr. With the column and background pressure of 1X transfer lines maintained a t 150 "C and the tip heater a t 300 "C, the SFC pressure was programmed from 1500 to 6000 psi while the source and analyzer pressures were monitored. A summary of SFC pressure versus source and analyzer pressures is recorded in Table I. Of note is the fact that the source/ analyzer pressure ratio always remains over 100, even with an inlet pressure of 6000 psi. Higher ratios are observed for SFC pressures below 5500 psi. Another very important fact is that with the high flow rate restrictor (0.2 mL/min as a gas a t STP a t 830 psi head pressure) used for this work, the analyzer pressure remains acceptable, in the low lO-'-Torr range. This appears to be the lowest analyzer pressure for an SFC/MS system yet reported. The present probe interface reduces total source and analyzer pressures by more than 1 order of magnitude compared to the previously described interface (12). Analysis of Cholesterol. Historically, cholesterolhas been used as a sensitive test for thermal activity of GC/MS in-
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 3, FEBRUARY 1, 1991
Table 11. SFC/FTMS Data for the Polyaromatic Hydrocarbon Mixture Separation elution order
compd
mol w t
1 2
fluorene phenanthrene pyrene
166 178 202
perylene
252
3 4
1'""'l""'""l"""'l""'""I"""'"I'"""" 1 0 0 10.5 11.0 11.5
9 5
G.C.
12.0
RUN T I M E I N M I N U T E S
I,--=-++
12.5
quantity injected, ng 2 8 6 10
13.0
Figure 4. SFC/FTMS total ion reconstructed chromatogram of 100 ng of cholesterol injected.
G
C
RUN T I M E I N M I N U T E S
Flgwe 6. SFC/FTMS total ion reconstructed chromatogram of the PAH mixture.
Figure 5. SFC/FTMS spectrum of 100 ng of cholesterol injected.
terfaces and jet separators (4). Because cholesterol dehydrates readily, typical cholesterol mass spectra obtained by GC/MS or with heated probes have the dehydration product (m/z 368) as the base peak and show only low-abundance molecular ion ( m / z 386). Thus, the appearance of the cholesterol mass spectrum can serve as an indication of the ability of the interface to handle thermally labile compounds. Figure 4 is the SFC/FTMS reconstructed total ion chromatogram obtained when 100 ng of cholesterol is injected, and its SFC/FTMS spectrum is shown in Figure 5. Judging by the signal to noise ratio of about 20:1, a lower detection limit of 5-10 ng under the current, not fully optimized, SFC/FTMS conditions should be possible. The base peak in the mass spectrum is the molecular ion ( m / z 386) with the dehydration product ion ( m / z 368) having a relative abundance of only 26%. Aside from the high relative abundance of the molecular ion, the spectrum has much in common with conventional E1 spectra. A somewhat unusual feature of this cholesterol mass spectrum is the abundance of m / z 301 ions. Under the relatively soft ionization conditions used, fragmentation of the 20-22 carbon bond to give an 84 Da isohexyl neutral fragment and the m / z 301 ion may account for this. The present data for cholesterol indicate that the probe interface delivers the analyte to the MS detector without significant thermal decomposition or dehydration. This is good evidence that the probe interface has adequate temperature control and the tip heater provides its high temperature only a t the very end of the restrictor. Analysis of Polyaromatic Hydrocarbons. In order to demonstrate the combined SFC/FTMS system's ability to separate and analyze mixtures, a simple four-component mixture of intermediate molecular weight polyaromatic hydrocarbons (PAH) was analyzed. Table I1 lists the compo-
100
150
MASS
IN
A
200
M
L
250
100
Figure 7. SFC/FTMS spectrum of 2 ng of fluorene injected.
nents in the mixture, along with the quantity of each injected, their molecular weights, and elution order at 5500 psi. Figure 6 is the SFC/FTMS reconstructed total ion chromatogram for this separation. A representative spectrum from the first mixture component, 2 ng of fluorene, is shown in Figure 7. These PAHs are often used for sensitivity determinations, because they form abundant molecular ions and are subject to minimal fragmentation. It appears from this spectrum that sensitivity is good, with detection limits at the subnanogram level. To further examine this premise, 500 pg of pyrene was injected. The reconstructed chromatogram is shown in Figure 8 and the mass spectrum in Figure 9. From this spectrum a detection limit of about 100 pg is estimated. It is expected that these detection limits will improve when optimal SFC/FTMS conditions are determined. However, present sensitivity is already similar to that obtained for PAH analysis with other E1 SFC/MS systems (e.g. full-scan spectra obtained at the "low nanogram level" with a Hewlett Packard 5988 SFC/MS system with single-ion-monitoring detection limits of ca. 25 pg estimated) (14) and approaches the sensitivity obtained with GC/MPI/FTMS (MPI = multiphoton ioniza-
Anal. Chem. 1991. 63, 255-261
255
limits for resolution, both chromatographic and mass spectral, are yet to be determined. Finally, we hope to examine the use of suspended-ion trapping procedures, recently developed by Laude and co-workers (13),to see if what we view as the major limitation of the technique, namely its limited mass spectral dynamic range, can be alleviated.
ACKNOWLEDGMENT We thank Mr. Eugene Ethridge and Mr. Frank Forgit of the University of California-Riverside, Department of Chemistry machine shop, for their contributions to both the design and construction of the probe interface. LITERATURE CITED (1) Chester, T. L.; Pinkston, J. D. Anal. Chem. 1990, 62, 394R-402R. 12) Smith, R. D.; Wright, B. W. I n Microbore Column Chromatography; Yang, F. J., Ed.; Chromatographic Science 45; Marcel Dekker: New York, 1989; pp 307-368. (3) Sheeley, D. M.; Reinhold, V. N. J . Chromatogr. 1989, 474, 83-96. (4) Owens, G. D.; Burkes, L. J.; Pinkston, J. D.; Keough, T.; Simms. J. R.; Lacey, M. P. In Supercrltlcal Nuid Extraction and Chromatography; Charpentier, B. A., Sevenants, M. R., Eds.; ACS Symposium Series 366; American Chemical Society: Washington, DC, 1988; pp 191-207. (5) Smith, R. D.; Kalinoski, H. T.; Udseth, H. R. Mess Spectrom. Rev. 1987, 6 , 445-496. (6) Wright, B. W.; Kalinoski, H. T.; Udseth. H. R.; Smith, R. D. HRCB CC, J . High Resolot. Chromatogr Chromatogr Commun 1986, 9 , 145-153. (7) Smith, R. D.; Fjeidsted, J. C.; Lee, M. L. J . Chromatogr. 1982, 247, 231-243. (8) Smith, R. D.; Udseth, H. R.; Kalinoski, H. T. Anal. Chem. 1984, 56, 2971-2973. (9) Kalinoski. H. T.; Hargiss, L. 0. J . Chromatogr. 1989, 474, 69-82. (10) Huang, E. C.; Jackson, 8 . J.; Markides, K. E.;Lee, M. L. Anal. Chem. 1988, 60, 2715-2719. (11) Lee, E. D.; Henion, J. D.; Cody, R. B.; Kinsinger, J. A. Anal. Chem. 1987, 5 9 , 1309-1312. (12) Laude, D. A.; Pentoney, S. L.; Griffiths, P. R.; Wilkins, C. L. Anal. Chem. 1987, 59, 2283-2288. (13) Hogan, J. D.; Laude, D. A. Anal. Chem. 1990, 62, 530-535. (14) Hawthorne, S. B.; Miller, D. J. J . Chromatogr. 1989, 468, 115-125. (15) Sack, T. M.; McCrery, D. A,; Gross, M. L. Anal. Chem. 1985, 57, 1290-1295.
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RECEIVED for review July 23,1990. Accepted October 29,1990. Partial support of this research by the National Science Foundation (Grant CHE-11685) is gratefully acknowledged.
Techniques for Postcolumn Derivatization in Gas ChromatographyIMass Spectrometry Woodfin V. Ligon, Jr.* and Hans Grade
General Electric Company, Corporate Research and Development, Schenectady, New York 12301
The connection of the output of a conventional split-type ca-
INTRODUCTION
ceed at or near the ambient GC oven temperature and at atmospheric pressure. I n addition the reactions must be complete in less than 1 8. Bromlnatlon, deuterium exchange, and a variety of acylation reactions have been demonstrated.
that can be filled with various catalysts to accomplish conversions such as hydrogenations and oxidations. Teeter et al. (7) have also described postcolumn hydrogenation. Chaffee et al. (8) have described a catalyst-filled microreactor for
0003-2700/91/0363-0255$02.50/00 1991 American Chemical Society
