Anal. Chem. 1999, 71, 1610-1616
High-Speed, Vacuum-Outlet GC Using Atmospheric-Pressure Air as Carrier Gas Heather Smith,†,‡ Edward T. Zellers,†,§ and Richard Sacks*,†
Department of Chemistry and Department of Environmental and Industrial Health, University of Michigan, Ann Arbor, Michigan 48109
A model is developed to explore operating conditions and performance tradeoffs for high-speed GC using atmospheric-pressure air as carrier gas and a vacuum pump to draw the carrier gas and injected samples through the system. The model is based on the rate theory for open tubular columns and conventional equations for gas flow in capillary tubes. The model predicts the effects of column outlet pressure, column length, column diameter, and detector dead time on the number of theoretical plates generated in a 30-s analysis spanning a retention factor range from 0 to 5. The outlet (detector) pressure range considered is 1-100 kPa (0.01-1.0 atm). A 0.1-mmi.d. column is found to generate more plates than either larger or smaller diameter columns because of the constraint of using atmospheric pressure at the column inlet. About 25 000 plates are generated with a 2.5-m-long column for outlet pressures less than ∼20 kPa. The model is validated with a high-speed GC instrument using a cryofocusing inlet system and a photoionization detector. The number of theoretical plates measured for o-xylene agrees very well with the model predictions for the lowest pressure case. System performance degrades at higher outlet pressures and with smaller diameter columns because of increased dead time of the detector. Results are considered in the context of designing portable GC instruments for ambient VOC analysis. Capillary gas chromatography (GC) is the most widely used method for the analysis of mixtures of volatile and semivolatile organic compounds. It is unsurpassed in providing high selectivity, sensitivity, accuracy, and precision over a very wide dynamic concentration range. It is also very slow, typically requiring minutes to tens of minutes per sample. Recognition of this limitation has led to a number of advancements aimed at increasing sample throughput. Several recently introduced technologies hold great promise for the development of practical and robust high-speed GC instruments.1 Key features now available include fast electronics and data processing,2 enhanced inlet †
Department of Chemistry. Present address: Department of Chemistry, Eastern Michigan University, Ypsilanti, MI 48197. § Department of Environmental and Industrial Health. (1) Sacks, R.; Smith H.; Nowak, M. Anal. Chem. 1998, 70, A29. (2) Klemp, M.; Peters, A.; Sacks, R. J. Environ. Sci. Technol. 1994, 28, 369A. ‡
1610 Analytical Chemistry, Vol. 71, No. 8, April 15, 1999
systems,3-6 microbore column technology,7 high-speed temperature programming,8 columns with tunable selectivity,9-12 the capability for rapid, on-column sample cleanup,1 and the use of vacuum outlet techniques for improving both resolution and detector response time.13-16 Computer models for high-speed GC separations also have been developed which aid in the design of separation strategies for specific application.17,18 Application of high-speed, capillary GC to the direct measurement of VOCs in the environment has a number of obvious advantages. Several portable high-speed GC instruments are now available, some of which use enhanced inlet systems, micromachined injection and sampling valves,19 and high-speed temperature programming.1,20 These instruments may also use microbore (0.1-mm-i.d.) capillary columns to improve resolution for fast analysis. However, these instruments are still relatively large, in part, owing to the need for on-board gas supplies. The elimination of on-board gas supplies would reduce instrument size and weight considerably but would require the use of air as the carrier gas. There are several drawbacks to this approach. In particular, with atmospheric-pressure detection, the air would have to be compressed, and binary diffusion coefficients in air are unfavorable for high-efficiency column operation when the relatively high flow rates needed for high-speed separations are used. In addition, some stationary phases are degraded from exposure to oxygen, and air is a poor performer in some detectors. To avoid on-board compressed gases, a detector that requires no additional gas supplies must be used. Detectors of this type (3) van Es, A.; Janssen, J.; Bally, R.; Cramers, C.; Rijks, J. HRC & CC, J. High Resolut. Chromatogr. Chromatogr. Commun 1987, 10, 273. (4) Liu, Z.; Phillips, J. J. Microcolumn Sep. 1989, 1, 249. (5) Klemp, M.: Akard, M.; Sacks, R. Anal. Chem. 1993, 65, 2516. (6) Sacks, R.; Klemp, M.; Akard, M. Field Anal. Chem. Technol. 1997, 1, 97. (7) Van Es, A. High-Speed Narrow Bore Capillary Gas Chromatography; Huthig Buch Verlag: Heidelberg, 1992. (8) MacDonald, J. J.; Wheeler, D. Int. Lab. 1998, 28, 6. (9) Sacks, R.; Akard, M. J. Environ. Sci. Technol. 1994, 28, 428A (10) Akard, M.; Sacks, R. Anal. Chem. 1994, 66, 3036. (11) Akard, M.; Sacks, R. Anal. Chem. 1995, 67, 2733. (12) Akard, M.; Sacks, R. Anal. Chem. 1996, 68, 1474. (13) Cramers, C.; Scherpenzeel, G.; Leclercq, P. J. Chromatogr. 1981, 203, 207. (14) Puig, L.; Sacks, R. J. Chromatogr. Sci. 1991, 29, 158. (15) Leclercq, P.; Cramers, C. A. HRC & CC, J. High Resolut. Chromatogr. Chromatogr. Commun 1987, 10, 269. (16) Hail, M. E.; Yost, R. A. Anal. Chem. 1989, 61, 2402. (17) Smith, H.; Sacks, R. Anal. Chem. 1997, 69, 145. (18) Smith, H.; Sacks, R. High Resolut. Chromatogr., submitted. (19) Lee, G.; Ray, C.; Siemers, R.; Moore, R. Am. Lab. 1989, 21 (2), 110. (20) Ehrmann, E. U.; Dharmasena, H. P.; Carney, K.; Overton, E. B. J. Chromatogr. Sci. 1996, 34, 533. 10.1021/ac981153w CCC: $18.00
© 1999 American Chemical Society Published on Web 03/06/1999
Figure 1. Schematic of the instrument used for vacuum outlet GC with atmospheric-pressure air as carrier gas. The portion shown in the box is a cryofocusing inlet system. T, capillary trap tube; R1-R4, fused-silica capillary restrictors; V1-V4, valves; C, separation column; PID, photoionization detector; VP1 and VP2, vacuum pumps; CG, carrier gas (tank air) inlets; P1, column head-pressure meter; P2, detector pressure meter.
usually are based on closed-cell designs, and the cell volume may be sufficient to cause significant extracolumn band broadening. The alternative of operation under reduced column outlet pressure would help offset this problem. The use of vacuum outlet GC provides a means for improving column efficiency and detector performance.13-16 By connecting the detector to a vacuum pump, carrier gas density along the entire column length is reduced. Since gas-phase binary diffusion coefficients scale inversely with gas density, larger diffusion coefficient values are obtained at reduced operating pressure. This shifts the optimal carrier gas velocity (velocity giving the minimum height equivalent to a theoretical plate) to higher values, which can significantly shorten analysis time. In addition, operation of the detector at subambient pressure increases local carrier gas velocity in the detector, which reduces the effects of dead volume. Although portable GC instruments employing vacuum outlet operation are under development, the rational design of such instruments has not been reported previously. This report describes the use of a model to design a high-speed, vacuumoutlet GC system using atmospheric pressure air as carrier gas. Results of experiments aimed at validating the model also are described. EXPERIMENTAL SECTION Apparatus. The laboratory instrumentation used for these studies is not suitable for portable instrumentation but does allow for the validation of the model and for the evaluation of atmosphericpressure air the carrier gas for high-speed GC. Figure 1 shows a schematic layout of the high-speed GC instrument adapted for use with atmospheric-pressure air as carrier gas. The instrument employs a high-speed cryofocusing inlet system (Cryointegrator model L, Chromatofast Inc., Ann Arbor, MI) (shown in box) and a photoionization detector (PID) (model PI 52-02A, HNU Systems, Newton, MA). This inlet system was used in these studies because it produces very narrow injection plugs.5 This feature is useful for the evaluation of column performance and detector dead-time effects. However, the inlet system was not designed for atmosphericpressure operation, and the pneumatic restrictors used in the system produce pressure drops that necessitated the use of a
compressed-air tank to supply carrier gas at above 1-atm pressure. The gas delivery pressure was adjusted to give a column head pressure of 1.0 atm. The inlet system uses a capillary, metal cold-trap tube (T in Figure 1) to collect and focus the sample vapor introduced at point S. An on-board vacuum pump VP1 pulls sample into the trap tube from S. The sample pressure was 1 atm (gas bag samples) for the work reported here. The trap tube is cooled to ∼-100 °C by a continuous flow of cold nitrogen gas. After sample collection and focusing, the flow direction in the trap tube is reversed, and the tube is heated rapidly by the current pulse from a capacitive discharge power supply. This injects a nearly symmetric vapor plug ∼10 ms in width (σ value) into the capillary separation column C. Components labeled R1-R4 are fused-silica capillary tubes used to control gas flows through the system. Components labeled V1V3 are gas-switching valves. Dry, clean tank air is supplied continuously to carrier gas inlets CG. The trap tube is connected to VP1 through sampling valve V2. When V2 is open, air containing organic vapors from S is pulled through T from right to left. After sample collection is complete, purge valve V1 is opened, and carrier gas flushes R2 and R3, thus preventing contamination of subsequent samples. The sample is focused as a narrow condensedphase plug near the right-hand end of the trap tube. Next, V2 is closed, and the trap tube is pressurized from CG through R1. This reverses the flow direction in the trap tube in preparation for sample injection. Pressure transducer P1 (Omega model PX302-050AV) between the trap tube and the column is used to measure the column head pressure, which is read on a digital meter (Omega model DP280P10). A bleed line (R4) connected to the compressed air supply is used to prevent diffusion of sample into the gauge line. The PID pressure P2 is monitored with an Omega PX243-15BG5V transducer and an Omega model DP25-E meter. The PID is connected to pump VP2 through valve V3. A twostage mechanical pump (Cenco, model HYVAC 14, Central Scientific Co., Chicago, IL), which can reduce the detector pressure to less than 1 kPa, was used. Valve V3 is closed during sample collection and opened prior to injection. The detector pressure and thus the column outlet pressure are controlled by adjustment of the manual needle valve V4, which bleeds ambient air into the vacuum line. The PID uses a 10.2-eV lamp and has a cell volume of
