Cryogenically Cooled Microloop System for Sampling and Injection in

used to focus sample plugs from a split injector, but it also has been used for gas ..... Therefore, the volume per unit time that can be sampled acro...
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Anal. Chem. 1996, 68, 701-707

Cryogenically Cooled Microloop System for Sampling and Injection in Fast GC Anthony J. Borgerding and Charles W. Wilkerson, Jr.*

Chemical Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

A new inlet system for fast gas chromatography has been built in which analyte compounds in the gas phase are cryogenically trapped onto a sample loop, isolated, revaporized, and injected. Sample loops are constructed of 20 cm lengths of 50 and 100 µm i.d. fused-silica tubing. Since the volume of these loops is small, analytes could be injected with initial bandwidths of less than 10 ms. Loops can be coated to increase the effectiveness with which compounds are trapped. Sample loops with thicker, less polar films trap hydrocarbons at relatively higher temperatures than those with more polar, thinner films. Use of 50 µm i.d. sample loops results in narrower injection bandwidths compared to 100 µm i.d. loops, but this does not necessarily translate into better chromatographic resolution. The wider bore tubes allow for more efficient sampling and shorter retention times for similar column head pressures. The injection system has been used to demonstrate sampling and fast GC analysis of a 10 ppbV mixture of volatile organic compounds. Gas chromatography (GC) is a very important means of analysis for volatile and semivolatile organic compounds. The instrumentation is reliable and relatively inexpensive, and GC data are very reproducible. A drawback to the technique is that analyses usually take at least 10 min and can be as long as 1 h, depending on the nature and complexity of the sample. However, beginning with Desty1 over 30 years ago, researchers have developed GC instrumentation and techniques that have reduced analysis times to a few seconds, and in some cases less than 1 s. These advances have the potential to increase the productivity of GC and to expand its uses dramatically. This so-called high-speed or fast gas chromatography (fast GC) is accomplished using high carrier gas flow rates and short columns; in effect sacrificing chromatographic resolution in exchange for speed. Some of the resolution that is lost can be regained by reducing the inner diameter of the column.1-5 Fast GC experiments are almost always performed isothermally. Under these conditions, the peak widths of eluting compounds are highly dependent on the initial injection impulse. Therefore, minimization of this band duration is critical to achieve (1) Desty, D. H. In Advances in Chromatography; Giddings, J. C., Keller, R. A., Eds.; Marcel Dekker: New York, 1965; Vol. 1, pp 199-228. (2) Schutjes, C. P. M.; Vermeer, E. A.; Scherpenzeel, G. J.; Bally, R. W.; Cramers, C. A. J. Chromatogr. 1984, 289, 157-162. (3) Schutjes, C. P. M.; Vermeer, J. A.; Rijks, J. A.; Cramers, C. A. J. Chromatogr. 1982, 253, 1-16. (4) van Es, A.; Janssen, J.; Cramers, C.; Rijks, J. HRC CC, J. High Resolut. Chromatogr. Chromatogr. Commun. 1988, 11, 852-857. (5) Wollnik, H.; Becker, R.; Gotz, H.; Kraft, A.; Jung, H.; Chen, C.-C.; Van Ysacker, P. G.; Janssen, H.-G.; Snijders, H. M. J.; Leclercq, P. A.; Cramers, C. A. Int. J. Mass Spectrom. Ion Processes 1994, 130, L7-L11. 0003-2700/96/0368-0701$12.00/0

© 1996 American Chemical Society

adequate separation of multiple components in a short time. Numerous techniques have been developed for minimizing injection band duration, including the use of fluid logic gates,6-8 valves,9-11 other mechanical devices that sample a small fraction of a gas stream,12-15 or simply using split injection with high split ratios and flow rates.5 Unfortunately, all of these approaches result in only a small portion of the original sample being injected onto the column, making them inadequate for the analysis of trace samples. A more efficient means of minimizing injection band duration has been developed by Sacks and co-workers.13,16-25 In their technique, compounds in the gas phase are cryofocused onto metal tubes, followed by ballistic heating of the tube to ∼200 °C in a few milliseconds. The trapped compounds are rapidly vaporized and injected into the chromatographic column with a band duration on the order of 10 ms. Initially, this interface was used to focus sample plugs from a split injector, but it also has been used for gas samples taken from loops19,20 and bags.24 The latter applications allow sampling of larger volumes, resulting in higher analyte signals for a given concentration. This paper describes a new method for sampling and injection in fast GC. Compounds from the gas phase are sampled through a small-volume loop where they are cryofocused. Sample and carrier flow are stopped as the loop is isolated and warmed to revaporize the trapped analytes. The gas-phase compounds are then injected into the flow stream. Heating of the injection loop is mild, occurring over 20-60 s to an ultimate temperature that (6) Gaspar, G.; Arpino, P.; Guiochon, G. J. Chromatogr. Sci. 1977, 15, 256261. (7) Annino, R.; Leone, J. J. Chromatogr. Sci. 1982, 20, 19-26. (8) Schutjes, C. P. M.; Cramers, C. A.; Vidal-Madjar, C.; Guichon, G. J. Chromatogr. 1983, 279, 269-277. (9) Myers, M. N.; Giddings, J. C. Anal. Chem. 1965, 37, 1453-1457. (10) Jonker, R. J.; Poppe, H.; Huber, J. F. K. Anal. Chem. 1982, 54, 2447-2456. (11) van Es, A.; Janssen, J.; Bally, R.; Cramers, C.; Rijks, J. HRC CC, J. High Resolut. Chromatogr. Chromatogr. Commun. 1987, 10, 273-279. (12) Tijssen, R.; van den Hoed, N.; van Kreveld, M. E. Anal. Chem. 1987, 59, 1007-1015. (13) Peters, A.; Klemp, M.; Puig, L.; Rankin, C.; Sacks, R. Analyst 1991, 116, 1313-1320. (14) Arnold, N. S.; McClennen, W. H.; Meuzelaar, H. L. C. Anal. Chem. 1991, 63, 299-304. (15) Peters, A.; Sacks, R. J. Chromatogr. Sci. 1991, 29, 403-409. (16) Ewels, B. A.; Sacks, R. D. Anal. Chem. 1985, 57, 2774-2779. (17) Lanning, L. A.; Sacks, R. D.; Mouradian, R. F.; Levine, S. P.; Foulke, J. A. Anal. Chem. 1988, 60, 1994-1996. (18) Mouradian, R. F.; Levine, S. P.; Sacks, R. D. J. Chromatogr. Sci. 1990, 28, 643-648. (19) Mouradian, R.; Levine, S.; Sacks, R.; Spence, M. Am. Ind. Hyg. Assoc. J. 1990, 51, 90-96. (20) Rankin, C.; Sacks, R. J. High Resolut. Chromatogr. 1990, 13, 674-678. (21) Puig, L.; Sacks, R. D. J. Chromatogr. Sci. 1991, 29, 158-164. (22) Klemp, M.; Sacks, R. J. Chromatogr. Sci. 1991, 29, 243-247. (23) Peters, A.; Sacks, R. J. Chromatogr. Sci. 1992, 30, 187-191. (24) Akard, M.; Sacks, R. D. J. Chromatogr. Sci. 1994, 32, 499-505. (25) Klemp, M.; Sacks, R. J. High Resolut. Chromatogr. 1991, 14, 235-240.

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Figure 1. Schematic diagram of the cryogenically cooled microloop injection fast GC system: (A) heated valve enclosure; (B) column oven.

is only 10-30 °C higher than that of the column. The duration of the injection pulse is dominated by the flow rate of the carrier gas and the volume of the trapped sample after revaporization, as opposed to the time required to revaporize the analyte in the case of the cryoinjector developed by Sacks. For example, sample filling a 20 cm × 50 µm i.d. loop (0.4 µL) should be flushed by a 2.3 mL/min flow stream in 10 ms. Experimental injection band duration measurements presented here confirm this prediction. While small-volume injection valves have been used previously for fast GC,9-11 they were not useful for measuring sub-ppmV concentrations of analytes, since the overall sample volume was limited to the size of the injection plug. In our injector, analytes from sample volumes up to several milliliters are trapped onto the small-volume loop, improving the achievable limit of detection. EXPERIMENTAL SECTION Sampling and Injection System. A schematic of the cryogenically cooled microloop injection system is shown in Figure 1. It is contained inside of a 4 in. × 4 in. × 3.5 in. resistively heated valve enclosure (Supelco, Inc., Bellefonte, PA). Sample and carrier gas flows were controlled using a gas sampling valve with 1/16 in. fittings (Model C6WP, Valco Instruments, Houston, TX). Sample loops were constructed of 20 cm lengths of fusedsilica tubing (coated or uncoated) with internal diameters (i.d.) of either 50 or 100 µm. Bare fused-silica capillary tubing and DB-5 columns with 0.05 and 0.1 µm film thickness were purchased from J&W Scientific (Folsom, CA). SB Phenyl-5 and SB Octyl-50 columns with 0.25 µm film thickness were purchased from Dionex (Sunnyvale, CA). Dead volume in the capillary tube connections was minimized using PEEK sleeves and ferrules (Upchurch 702

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Scientific, Oak Harbor, WA). The sample loops were cryogenically cooled using a liquid nitrogen cold trap system (SGE, Austin, TX). This system delivers liquid nitrogen to a stainless steel tee, through which the fused-silica sample loops were passed. Temperature was measured using a Fluke Model 52 digital thermometer with a J-type wire thermocouple inserted into the tee along with the sample loop. The specific procedure for sampling and injecting compounds, starting under conditions where the microloop is warm, is as follows: The six-port valve is turned halfway, stopping both the sample and the carrier gas flow, and the liquid nitrogen is switched on to cool the loop to the trapping temperature (∼25 s to reach -20 °C, 45 s to reach -90 °C). When the trapping temperature has been reached, the valve is opened to the load position and gases are sampled through the loop (valve position is as indicated by the dotted lines in Figure 1). Samples at ambient pressure were pulled for a specified time through the sample loop by a vacuum pump (Model 8910, Welch, Skokie, IL). Alternatively, tank samples at positive pressure flow through the sample loop without the aid of a pump. After the sampling time is complete, the valve is again turned halfway to isolate the sample loop (stopping sample and carrier flow), and the loop is warmed to the injection temperature in the valve oven (∼30 s from -90 to +40 °C). When the injection temperature is reached, the valve is opened to the inject position, making the sample loop part of the carrier stream into the column (valve position as indicated by solid lines in Figure 1). Gas Chromatography. A Hewlett-Packard Model 5890 GC was used in all experiments. The carrier gas plumbing and flow control were modified to accommodate the new injection system; these modifications are illustrated in Figure 1. Valves that controlled the septum purge and split ratios for the split/splitless injection system were removed. Carrier gas was fed directly into the six-port valve of the microloop injection system from a fiveport valve that was used to control the column head pressure (and thus the flow rate). A pressure gauge and a vent controlled by a needle valve were built into the system immediately above where the carrier gas enters the six-port valve. A 4 m, 100 µm i.d. DB-5 column with 0.1 µm film thickness (J&W) was used for all separations. The carrier gas was hydrogen, and the flow rate was measured using a digital flowmeter (R & D Separations, Rancho Cordova, CA). A flame ionization detector was used, and signal was recorded on a HP3395 integrator (Hewlett Packard). Materials. Reagent grade dichloromethane, toluene, benzene, and cyclopentane were obtained from EM Science (Gibbstown, NJ). Chloroform, n-hexane, heptane, and octane were obtained from J.T. Baker (Phillipsburg, NJ), and carbon tetrachloride and tetrachloroethylene were obtained from Matheson (Norwood, OH). All of these chemicals were reported by the supplier as 99% or higher purity and were used without further purification. The 10 ppbV gaseous mixture of volatile organics was purchased from Scott Specialty Gases (Plumbsteadville, PA). Chlorodifluoromethane for injection band duration measurements was sampled directly from pressurized containers of Dust-Off Plus (Falcon Safety Products, Branchburg, NJ). Measurement of Injection Band Duration. The injection band duration of the microloop system was measured by replacing the GC column with a transfer line consisting of a 1 m length of uncoated 100 µm i.d. fused silica. A gas-tight syringe connected to the sampling line via a Luer-lock fitting was used to fill the

sample loop with chlorodifluoromethane at a concentration of ∼100 mg/L. The response time of the integrator was not fast enough to accurately measure this peak, so the transfer line was instead fed into the pulsed (500 Hz) electron impact ion source of a reflectron time-of-flight mass spectrometer (R. M. Jordan, Grass Valley, CA). The signal from the microchannel plate detector of the instrument was passed through a Stanford Research Model SR 440 amplifier (Sunnyvale, CA) prior to digitization by a Sonix STR-864A waveform recorder (Springfield, VA). Data acquisition, storage, and manipulation were controlled by an IBM PC server using sofware written in our laboratory. Gas Sample Preparation. Gas samples were prepared in 1 L Tedlar bags (Scott Specialty Gases) in a manner similar to that of Akard and Sacks.24 Mixtures of analytes were injected through a rubber septum into the bag using a 10 µL syringe. Zero air was added through a 3/8 in. stainless steel valve fitting to fill the bag. At least 3 h was allowed for equilibration. The bags were attached to the injection system via the 3/8 in. fitting for sampling. RESULTS AND DISCUSSION Injection Band Duration. The injection band duration in this system is dependent on many factors associated with the trapping, revaporization, and injection steps. Compounds are trapped along a certain distance of the sample loop according to, among other factors, the sampling flow rate and time and the trap temperature. During the time required for revaporization, compounds diffuse longitudinally. Thus, band durations also depend on the isolation time and the diffusivity of the analytes. During the injection step, band duration is affected by longitudinal diffusion and resistance to mass transfer in the mobile and stationary (in the case of coated loops) phases. Discussion of all these factors is beyond the scope of this paper. To demonstrate the minimum performance capabilities of this system, samples of chlorodifluoromethane were taken at ambient temperatures. Thus, the analyte is spread out across the sample loop, rather than being focused at the front. A 1 m uncoated fusedsilica tube was substituted for the column in these measurements. Under these conditions, the measured peak duration should simply be the time required to flush the volume of sample from the loop (injection band duration), plus broadening caused by longitudinal diffusion and resistance to mass transfer in the gas phase. The contributions due to broadening were calculated according to standard chromatographic theory.26 Figure 2 shows a series of injections in which the volume of the microloop and the flow rate of the carrier gas were varied. Figure 2a shows the peak measured for a 50 µm × 20 cm (0.4 µL) sample loop with an average linear velocity, corrected for temperature and pressure, of 647 cm/s. While this value is greater than that for optimal separation, high linear velocities are often used for fast GC experiments, particularly when the output pressure of the column is subambient.27,28 The calculated broadening of the peak in the transfer line due to longitudinal diffusion of the analyte is 1.5 ms, and that caused by resistance to mass transfer in the gas phase (k ) 0) is 3.6 ms. Under these highflow conditions, the injection band duration (time required to flush the loop) is 7.9 ms. The sum of these predicted contributions is in excellent agreement with the measured total baseline band (26) Lee, M.; Yang, F.; Bartle, K. Open Tubular Gas Chromatography; Wiley: New York, 1984. (27) Hail, M.; Yost, R. Anal. Chem. 1989, 61, 2402-2410. (28) Giddings, J. Anal. Chem. 1962, 34, 314-319.

Figure 2. Peaks of unretained chlorodifluoromethane, indicating injection band duration, for different sample loops at different flow rates: (a) 50 µm i.d. loop at 4.1 mL/min; (b) 50 µm i.d. loop at 2.3 mL/min; (c) 100 µm i.d. loop at 4.1 mL/min; (d) 50 µm i.d. loop with 0.25 µm film at 4.1 mL/min.

duration of 13 ms. Figure 2b shows the data obtained for the same sample loop with a corrected average linear velocity of 389 cm/s. Under these conditions, broadening from longitudinal diffusion is 3.3 ms and resistance to mass transfer accounts for 4.6 ms. The predicted time required to flush the sample loop at this flow rate is 13.1 ms. The measured baseline duration is 30 ms, which is ∼7 ms longer than the sum of the predicted values. However, as can be seen in Figure 2b, peak tailing accounts for this discrepancy. Likely causes of tailing are suboptimal coupling of the column either to the injector or to the ion source of the mass spectrometer, fluctuations in carrier gas flow during valve switching, or analyte interactions with sites on the transfer line. Peak tailing also causes disagreement with predicted values when larger sample loops are used. Figure 2c shows the results from an experiment performed with a 100 µm × 20 cm tube (1.6 µL) at a flow rate of 770 cm/s. In this example, longitudinal diffusion and resistance to mass transfer account for 1.2 and 3.3 ms, respectively, of the baseline band duration, and the predicted time for flushing the loop is 26.6 ms. As can be seen by the shape Analytical Chemistry, Vol. 68, No. 4, February 15, 1996

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Figure 3. Chromatograms generated after cryotrapping sample for 5 s at varying temperatures onto different 20 cm × 50 µm sample loops: (a) uncoated; (b) coated with 0.05 µm 5% phenyl material; (c) coated with 0.25 µm 5% phenyl material; (d) coated with 0.25 µm 50% octyl material. List of components: (1) cyclopentane, (2) hexane, (3) benzene, (4) heptane, (5) toluene, and (6) octane.

of this peak, tailing is responsible for the ∼10 ms duration at the baseline of the peak unaccounted for by summing the predicted values. Regardless of the tailing, and despite the very high flow rate of this experiment, the 4-fold increase in volume of this sample loop causes a longer injection band duration compared to loops with an inner diameter of 50 µm. However, using a 100 µm i.d. sample loop increases the amount of sample that can be taken for a fixed sample time and pressure compared to 50 µm loops. In addition, chromatographic resolution is not diminished under most conditions using the larger internal diameter loop and, in fact, is improved in some circumstances. These characteristics will be discussed below. While the data in Figure 2a-c were obtained with bare fusedsilica tubes, Figure 2d shows the injection band duration for a 50 µm i.d. sample loop coated with a 0.25 µm 5% phenyl film. Aside from the coating on the loop, conditions were the same as the 704 Analytical Chemistry, Vol. 68, No. 4, February 15, 1996

experiment whose results were shown in Figure 2a. The coating causes additional broadening due to the resistance to mass transfer in the stationary phase in the sample loop. The calculated value of this broadening is 2.1 ms. Comparing the result shown in Figure 2d to that shown in Figure 2a, it is apparent that use of a coating on the sample loop causes a slight additional broadening of the injection band duration. Effect of Sample Loop Coating on Trapping Efficiency. In this sampling and injection system, cryogenically trapped compounds are heated during the revaporization step to only 10-30 °C higher than the oven temperature. For example, the separations shown in Figure 3 were performed at a reinjection temperature of 60 °C while the separation was done at 30 °C. In contrast, systems using ballistic heating to vaporize cryofocused compounds usually heat the trapping tube to at least 200 °C. Gradual heating and lower temperature reinjection may be advantageous in that

Figure 4. Signal for hexane versus trapping temperature for 5 s samples taken using 50 µm i.d. sample loops with various coatings.

thermal degradation of analytes during this step is unlikely to occur whereas it has been noted in ballistically heated systems.25 The primary advantage of mild revaporization, however, is that coatings to enhance the trapping efficiency of the sample loop may be used without their being immediately degraded. Sample loops could routinely be used for 50 injections in this system with no noticeable change in their performance. The advantage of using coated loops is shown in Figure 3 for a series of separations of cyclopentane, n-hexane, benzene, n-heptane, toluene, and n-octane at a concentration of ∼10 ppmV. The compound mixture was sampled for 5 s at a series of temperatures to demonstrate the trapping characteristics of 50 µm sample loops with various coatings. Figure 3a shows the results for an uncoated silica sample loop. In this experiment, none of the analytes accumulates significantly until the temperature drops below -20 °C. At -40 °C, octane appears to be completely retained; i.e., its relative peak area does not increase with further decrease in temperature. Relative signals for toluene, heptane, benzene, and hexane are maximized at -60 °C, while cyclopentane is not completely trapped until the temperature reaches at least -80 °C. The trapping efficiency is improved when the sample loop is coated, even when the coating is very thin. Figure 3b shows results using a 5% phenyl coating with a film thickness of 0.05 µm. The data show that the higher boiling components (heptane, toluene, octane) begin accumulating at 0 °C. Similar signals for most of the compounds in the mixture are attained at trapping temperatures 20 °C warmer than required when an uncoated sample loop is used. This is more clearly seen in Figure 4, where the hexane signal from experiments using different sample loops is plotted versus temperature. The data show that signal for this compound begins to increase at -40 °C when trapped onto a bare silica loop, while similar behavior occurs at -20 °C for a loop coated with 0.05 µm 5% phenyl film. When the film thickness is increased, compounds are trapped at even higher temperatures. The data in Figure 3c were obtained using a 5% phenyl coating with a film thickness of 0.25 µm. Note that signal for heptane, toluene, and octane begins increasing at a trapping temperature of just +20 °C and that similar signals for these compounds occur at temperatures an additional 20 °C warmer than was shown for the sample loop with the thinner film coating. The more volatile

Figure 5. Fast separation of benzene, heptane, tetrachloroethylene, and octane using different sample loops. The signal attenuation for the top chromatogram is a factor of 2 higher than that for the lower chromatogram. See text for other conditions.

components (cyclopentane, hexane, benzene) do not exhibit this effect. However, when the coating on the sample loop is made less polar, these three compounds begin to accumulate at relatively higher temperatures. Figure 3d illustrates the results for a sample loop coated with a 0.25 µm 50% octyl film. Hexane and cyclopentane begin to show enhanced signal at 0 °C, and their signal maximizes at -40 °C, an additional 20 °C warmer than for the 5% phenyl-coated sample loop of the same thickness. Figure 4 graphically illustrates this result for hexane. In addition to enhanced trapping at higher temperatures, the maximum hexane signal generated using the 50% octyl film is higher than that generated using any of the other sample loops. These data confirm that, as expected, using thick films with a chemistry similar to the analyte of interest yield enhanced signals. Influence of Sample Loop Internal Diameter on System Performance. As discussed earlier, since changing the inner diameter of the loop from 50 to 100 µm increases its volume by a factor of 4, the injection band duration increases accordingly if the analytes are not cryofocused. However, this does not necessarily mean that chromatographic resolution decreases. Figure 5 shows separations of a ∼20 ppmV mixture of benzene, n-heptane, tetrachloroethylene (50 ppmV), and octane using as the sample loop 20 cm lengths of bare silica tubing having inner diameters of 50 (bottom) and 100 µm (top). To demonstrate minimum capabilities, samples were not cryofocused, and the entire volume of the loop contained sample. In each case, the oven temperature was 50 °C and the average linear flow rate, calculated from the elution time of methane (in a separate experiment, data not shown), was 247 cm/s. Despite the longer injection band duration, the resolution of the separation using the 100 µm i.d. sample loop is actually slightly better than in the experiment using the smaller volume loop. For example, k values for benzene and octane are 1.15 and 4.26, respectively, using the 100 µm loop, compared to 1.03 and 3.78 using the 50 µm loop. The variable condition in these two separations is the pressure required to attain the same flow rate; 30 psi was required for the Analytical Chemistry, Vol. 68, No. 4, February 15, 1996

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Figure 6. log-log plot of signal for dichloromethane, chloroform, and carbon tetrachloride versus sampling time using 50 and 100 µm i.d. tubes coated with 5% phenyl material. See text for further experimental details.

system using the 100 µm loop, while 46 psi was required for the system when a 50 µm sample loop was used. The additional pressure requirement is caused by the extra resistance to flow of the more narrow loop. At higher pressures, analyte diffusivity decreases, which causes the resistance to mass transfer to increase. This term is the dominant cause of band broadening for fast separations when high flow rates are utilized.28 The effect of pressure on chromatographic resolution is especially dramatic given the difference in predicted injection band durations (80 versus 20 ms) for the two sample loops. It should also be noted that the attenuation for the top chromatogram in Figure 5 was set a factor of 2 higher than for the lower chromatogram, indicating that the signal intensity was much higher for the 100 µm loop. This is an expected result given the larger volume of this loop. The use of the wider bore tube is also advantageous in the sampling step, since higher sample flow rates can be attained at a given pressure. Compared to a 50 µm i.d. sample loop, the gas throughput of a 100 µm loop is over 1 order of magnitude greater across a similar pressure drop. The calculated throughput of the loops described here, for a compound of mass 100 at atmospheric pressure being sampled by a pump at 1 Torr, is 2.3 × 1019 (50 µm) and 3.7 × 1020 molecules/s (100 µm). Therefore, the volume per unit time that can be sampled across the 100 µm loop increases by this factor, as illustrated in Figure 6. A 5 ppmV mixture of dichloromethane, chloroform, and carbon tetrachloride was trapped onto different sample loops at -90 °C for sampling durations that varied from 10 to 120 seconds. After isolation of the sample loop and heating it to 40 °C, the revaporized analytes were injected onto the column. Figure 6a shows the data, plotted on logarithmic scales, comparing sample loops with inner diameters of 50 µm and 100 µm coated respectively with 0.05 µm and 0.1 µm 5% phenyl films. The average signal from each of the compounds, based on 3 analyses taken for each sampling duration, is plotted. These results show an average increase in signal by a factor of 15 for chloroform and carbon tetrachloride. This is in excellent agreement with the calculated factor of 16 increase in sample throughput occurring in the 100 µm i.d. loop for each sampling duration studied. For dichloromethane the average signal increase is a factor of 9, which 706 Analytical Chemistry, Vol. 68, No. 4, February 15, 1996

Figure 7. Analysis of 10 ppbV mixture using (top) a 100 and ( bottom) a 50 µm i.d. sample loop, each coated with 5% phenyl films. See text for other conditions. List of components: (1) vinyl chloride, (2) trichlorofluoromethane, (3) dichloromethane, (4) chloroform, (5) 1,1,1-trichloroethane, (6) carbon tetrachloride, (7) 1,2-dichloroethane, (8) trichloroethylene, (9) tetrachloroethylene, and (10) 1,2-dibromoethane.

is most likely a result of incomplete sampling of this volatile component. Trace Analysis. The ability of this system to concentrate gasphase analytes makes it useful for the analysis of dilute samples. Figure 7 presents data for two analyses of a 10 ppbV mixture of 10 volatile organic compounds. The initial column head pressure was 30 psi for both separations. The top chromatogram shows the results using a 100 µm i.d. sample loop, and the lower chromatogram was generated using a 50 µm i.d. sample loop. Samples were taken from a pressurized tank. As mentioned earlier, the sampling rate using the 100 µm i.d. loop can be much faster than that using the 50 µm i.d. loop. Using the 100 µm i.d. loop, sampling was done at a pressure of 10 psi, corresponding to 6.2 mL/min, for 60 s. The average linear velocity across the sample loop, corrected for temperature and pressure, was 860 cm/ s. Using the 50 µm i.d. loop, sampling was done at 20 psi, corresponding to 1.65 mL/min (766 cm/s), for 180 s. Both samples were trapped at -80 °C and injected at 60 °C. The oven temperature was held at 30 °C. The increase in the baseline signal at ∼17 s is due to a change in the sample flow rate. The retention time of the less volatile compounds was reduced by increasing the column head pressure to 70 psi. Pressure programming of this type has previously been shown to be useful in fast GC.25 The results for both experiments show adequate signal-to-noise ratios for quantitation of all the analytes in the mixture. The resolution is slightly better under these conditions using the 50 µm i.d. loop, as indicated by the partial separation of chloroform and 1,1,1-trichloroethane, compared to complete coelution in the experiment using the 100 µm i.d. loop. In contrast to the example shown in Figure 5, the column pressure is the same for each separation. The difference in resolution in this case is caused by differences in flow rate and injection band duration. The signal intensity for all of the components in the mixture is higher when the 100 µm loop is used, despite a sampling time that was only a third as long as that for the experiment using the 50 µm loop. In

addition to the lower volumetric sampling rate for the 50 µm loop, the lower signals may also indicate incomplete retention of the analytes on the sample loop due to the length of the sampling time. CONCLUSIONS The results presented here indicate that the cryogenically cooled microloop injector is a useful alternative to other injection systems used in fast GC. Future experiments will be pursued to compare this system with a ballistically heated cryotrapping injector for analysis of actual environmental samples. In addition, valve automation and an improved cryotrapping and heating system for the microloop are being developed to minimize the

time between sampling and reinjection. Finally, we will continue experiments to further characterize the advantages gained using coated sampling loops. ACKNOWLEDGMENT This work was supported by the Department of Energy Office of Environmental Management through the DOE Methods Compendium Program. Received for review August 21, 1995. Accepted December 1, 1995.X AC950853N X

Abstract published in Advance ACS Abstracts, January 15, 1996.

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