High-speed GC Analysis of VOCs: Sample Collection and Inlet Systems 1 .
Part 1 of a Two-Part Article h e chemical analysis of organic compounds in the environment is an extremely important and complex procedure. For volatile organic compounds (VOCs), gas chromatoeranbv lGCl is the most widely used 1 method because of its ' excellent selectivity, high sensitivity, and wide dynamic concentration range. Very complex mixtures can be separated and analyzed using fusedsilica capillary separation columns; however, analysis times for this method may be era1 minutes or longer. This represents a severe limitation for rhany environmental auulications i n which large numdirs of samples must be analyzed (e.g., soil gas and groundwater analysis for hazardous waste, chemical spill site characterization, stack gas monitoring, and workplace air monitoring). Recent developments in several laboratories in the United States and Europe (1-5) have shown that the speed of GC can be dramatically increased by the use of special inlet systems with relatively short capillary columns operated at unusually high carrier gas flow rates. With these techniques, simple mixtures of VOCs can be separated and analyzed in a few seconds. This represents a n increase i n s a m p l e throughput of 1 to 2 orders of magnitude and will significantly affect a variety of environmentally based applications. This report describes cryofocusing sample collection and introduction devices for the high-speed GC
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and purgeable VOCs from water samples may take several minutes. After s a m p l e collection, the metal capillary tube is rapidly heated by a capacitive discharge power supply to revaporize t h e condensed sample and introduce ic to the separation column as a vapor plug 5-10 ms in width. This narrow sample plug is necessary to take full advantage of the separating power of modern fused-silica capillary GC columns. ~~
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Instrumentation for high-speed GC Currently available commercial GC instrumentation is inadequate for analysis times of several seconds analysis of VOCs ( 6 8 ) . These de- or less. Inlet systems, detectors, and vices use bare metal capillary or ad- electronic signal processing are the sorbent-lined fused-silica tubes to major contributors to extra column collect VOCs from dilute vapor band broadening. High-speed GC samples. The tube is cooled to a suf- requires the use of special inlet sysficiently low temperature to quanti- tems capable of delivering a very tatively collect all VOCs of interest. narrnw sample vapor plug to the Permanent gases, including nitro- separation column. In addition, relgen and oxygen, generally are not atively short capillary columns are collected. Collecting samples of am- required. These points are illustrated in bient air, workplace air, and stack gas may take several seconds; the Figure 1 and discussed in the accollection of automobile exhaust companying box on basic theory. Inlet systems. The GC system shown in Figure 2 was developed for the high-speed analysis of VOCs MARK KLEMP in environmental samples. It features a cryofocusing sample collecANITA PETERS tion and inlet system, direct atmosChromotofast, Inc. pheric pressure sampling, highAnn Arbor, MI 48104 speed electronic signal processing, and a relatively short capillary sepRICHARD SACKS aration column. The system operUniversity of Michigan ates in two modes, sample collecAnn Arbor, MI 481 04 tion and analysis.
00t3-936x19410927-369A$04.50/00 1994 American Chemical Society
Environ. Sci. Technol., Vol. 28, No. 8. 1994 369 A
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Basics for high-speed GC
be negligible. In the region of high average
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the number of peaks, if perfectly
370 A Environ. Sci. Technol., Vol. 28, No. 8, 1994
equal column length; blue
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smaller boiling point range, a longer column may be pr ferred. This assumes that temperature programming not practical on the time scale of these analyses
In Figure 2, gas flow directions for the sample collection mode ace shown by broken lines with arrowk. pneumatic components are showr by red lines, and electrical compo'nents are shown by black lines. Comportents labeled R are deactivated fused-silica capillary tubes, which are used as pneumatic restrictors for pressure and gas flow control. Components labeled V are poeumatidally operated micro gas valves. The 8-m-long. 0.25-mm-i.d. separation column is indicated by C and the carrier gas source by G. The sample collection and inlet device T, shown in detail in inset A of Figw e 2 , consists of a gas-cooled and electrically heated metal tube (red line). For the work described here, a 15-cm length of 0.30'mm-i.d. Cu (30%)/Ni (70%) tubing was used. The quartz cooling chamber (yellbw) is typically cooled to -100 "C by a flow of cold.nitrogen gas, and a .vacuum pump P pulls the sample through the tube from the source indicated by I. The sample source may be at atmospheric pressurg. After sample colle%tion is complete, valve V, is opened,, and a purge flow of pure cafrier gas remaves all traces of the sample from the inlet system, thus,eliminatine contamination and preparing ths system for collection of the nex sample. Note that valves V, and V, in Figure 1, whiGh are used to control the purge flow and the gas flow direction through the trap tube, respectively, are not in the sample flow path. Therefore, the sample never comes in contact with valve surfaces. This reduces the risk of sample alteration and contamination. After sample collection and crynfocusing, the gas flow direction through the tube is reversed by closing valve V,, and the tube is rapidly heated by a current surge from a capacitive'discharge power supply S. The trap tube temperature is ,monitored by thermocouple TC. The current surge and the trap tube temper, ature are shown in inset B of Figure 2. Peak trap temperature is typically in the range of 100-200 %. Heating is very rapid, and the sample vapor plug is delivered to the separation column in a very concentrated form, The metal trap tube is coni structed of relatively thin-walled material: the tube cools in ahout 5 s and is then ready for collection and cryofocusing of the next sample. The red portion of inset A is d heated transfer line. Note that the
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sample is condensed on the steep thermal gradient at the upper end of the metal trap tube. Before sample injection onto the column, the flow direction through the trap tube is reversed and the sample is injected from the same end of the trap tube. T h u s , t h e s a m p l e plug travels through only a very short segment of the hot metal tube, a n d this nearly eliminates thermal degradation of even sensitive compounds. Adsorbedt-coated trap tubes. The use of hare metal trap tubes is impractical for compounds that are more volatile t h a n p e n t a n e , whereas adsorbent-lined fusedsilica trap tubes can collect and cryofocus low as well as high mnlechlar weight compounds at mnre modest temperatures. Porous-polymer- and Al,O,-lined trap tubes have been used successfully as the adsorbent material for these applications.
Figure 3a shows plots of the fraction trapped versus trap temperature for pentane obtained by using (A) a 0.30-mm-i.d. bare metal trap tube and (B) a 0.32-mm4.d. fusedsilica trap tube lined with a porous polymer adsorbent coating. This adsorbent material bas a very high affinity for VOCs and can efficiently collect volatile compounds at much higher temperatures than the bare metal tube. At a temperature of ahout -120 OC, even ethane and ethylene can he trapped and held for at least several minutes. In addition, adsorbent-coated trap tubes can collect and cryofocus less volatile compounds at much more modest temperatures. Figure 3h shows the design of a cryofocusing sample collection and inlet system that uses a porouspolymer-coated trap tube (yellow). The trap tube is surrounded by a metal tube (green) that allows rapid
Environ. Sci. Technol., Vol. 28. No. 8, 1994 371 A
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Adsorbent coated trap tube (a) Comparison of bare metal tube (A) and porou polymer lined tube (6) for mpentane 1
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FIGURE 4
Chromatograms of vapor samples using the high-saeed GC instrument with a bare metal cold trap tube (a)Chromatogram of BTEX comDounds from a bag Sample
I) Chromatogram of purgeableVOCs from a soil sample
%ainednear an auto reoair facili
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372 A Environ. Sci. Technol., VoI. 28, No. 8, 1994
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heating by the use of a current surge through the metal tube. Black dots indicate connection points to the power supply. Because the fusedsilica tube that contains the porous polymer lining is very thin walled, the sample can be rapidly heated and injected as a narrow plug into the separation column. The trap tube is connected to the other system components by heated transfer lines. These heated regions are shown in red. Higher molecular weight compounds, very polar VOCs, and water vapor can be very difficult to desorb rapidly from these adsorbent-coated trap tubes. A dual trap system consisting of a bare metal trap tube upstream from the adsorbent-coated trap tube has been developed to prevent these components from reaching the adsorbent-coated trap tube. These series-coupled trap tubes can be operated at the same or different sample collection temperatures. It is also possible to heat the tubes to different temperatures and at different times for sample injection to the separation column. Detectors. The flame ionization detector [FID), photoionization detector (PID), and electron capture detector [ECD) are most frequently used for the analysis of VOCs in environmental samples. The PID and ECD are attractive because they are selective and thus can mask interfering peaks. This reduces the resolution requirements of the GC system. However, they are closed-cell detectors, and the cell volume of commercial devices may be too large for high-speed GC without the use of special techniques. The FID is most frequently used
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High-speed chromatograms of natural gas samples using the high-speed GC instrument with a porous-polymer lined fused silica trap tube A
(a) -130 'C trapping temperature
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(b) -70 'C trapping temperature
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(e) -30 'C trapping tempera
for the high-speed analysis of environmental samples because it shows good sensitivity for most VOCs with the exception of formaldehyde. However, water vapor produced in the FID can be a problem for reverse-flow sample collection because the column is backflushed (reverse gas flow direction) during the sample collection interval. If this water vapor reaches the cold trap, ice formation will rapidly clog the capillary trap tube. The backflush carrier gas source BG, shown in Figure 2. provides a forward flow of carrier gas through the final segment of the separation column and the FID during sample collection and column backflush. This completely eliminates water vapor problems from the FID. For some FID designs and for short sample collection times, this gas source can be eliminated. Other components of the highspeed GC system include a highspeed electrometerlamplifier (E in Figure 2). a 12-bit signal digitizer, and a computer used for both data collection and instrument control. All operations are automated, and the instrument will continuously cycle for high-speed repetitive VOC analysis. System performance Qualitative features. Adjustment of the capillary restrictors (R in Figure 2) controls the sample gas flow
FIGURE 6
Sample recovery versus reinjection temperature for a normal flow direc!ion metal trap tube configuration (a) and for the reverse flow direction metal trap tube configuration (b)a fb)
I 50 100 150 200 250
300 350 Reinjection temperature ('C)
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Plols labeled A, 6, and C are tor 211~omtoluene.2-bmmotoluene.and 2-chbmtoluene,respacllvely. Reprintedwith permlsslon from AnaWmi Chemistry. 1993, 652518
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Sampling time (5) i l
rate-usually in the range of 0.110.0 cm'/min. Typical sample size is about 1.0 cm'. Figure 4 shows chromatogams obtained with the highspeed GC system by using the bare metal trap tube and an 8.0-m-long, 0.25-mm-i.d. nonpolar (DB-1) fused-silica capillary column. Chromatogram a shows the high-speed separation, by use of a Tedlar gassampling bag, of BTEX (benzene, toluene, ethylbenzene, xylene) compounds. Note that the separation is complete in 20 s. Components A-F are benzene, toluene, noctane, ethyl benzene, xylene, and n-nonane, respectively. Concentrations were typically 160,ppm (v01.l vol.) in H,. Also note that the peaks are narrow and show very little tailing. The high-speed screening of BTEX compounds in soil and water samples should aid in site characterization of leaking fuel storage tanks. Chromatograms b and c show the high-speed separation of purgeable organic compounds from two different soil samples. Flow rates for chromatograms b and care different from those used for chromatogram a. Chromatogram b comes from a soil sample obtained near an automobile engine repair facility, and chromatogram c comes from roadside soil in a heavily traveled urban area. Peaks A and B identify benzene and toluene, respectively. Purging was accomplished by heating the samples in a hot water bath. The atmospheric pressure sampling capability of the high-speed GC instrument was used to collect and
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cryofocus the head-space gas above the soil samples. Although these samples are too complex for complete separation on the time scale of these studies, many components are adequate'ly separated for reliable quantification. In addition, these chromatograms are very useful in providing an overall VOC profile of the contaminated soil samples. Figure 5 shows,chromatograms of natural gas samples that were cryofocused using a. porous-polymer!ined fused-silica trap. The inside diameter of the, trap tube was 0.32 mm. The separation was performed using a 4.0-m-long, 0.32-mm4.d. AI,O, porous layer open fubular (PLOT) column to avoid the need for subambient column oven operation for these low molecular weight compounds. The oven temperatpre was 1 2 0 "C. Chromatograms a, h, and c were obtained by using trapping temperatures of -130, -70, and -30 f, respectively. Peaks A, B, C, D, and E are methane, ethane, propane, iso-butane, and n-butane; respectively. Note that in all cases the separation is complete ih about 6 s. At -130 "C, none of the methane has bled off the trap and it elutes as a well-defined peak. When the trap temperature is increased to -70 "C, methane completely bleeds off the trap. However, ethane and the other components are stili quantitatively trapped. The shift in retention time for ethane in chfomatogram b relative to chromatogram a.may be the result of migration in the trap tube prior to heating. At -30 "C neither
374 A Environ. SCI. Technol.. Vol. 28, No. 8. 1994
methane nor ethane is trapped significantly, but all other components are quantitatively trapped. Although methane can be quantitatively trapped for short periods of time at -130 OC, if the sampling interval is greater than about 10 s, significant loss of methane is observed. An important feature of the reverse-flow sampling configuration shown in Figure 2 is the dramatic reduction in thermal decomposition 6f sensitive compoun& relative to other metal cold trap designs in which the condensed sample must pass through the entire trap tube length during the injection process. Figure 6 shows peak area recovery plots for a three-component mixture containing (A) 2-fluorotoluene, (B) 2-bromotoluene, and (C) 2-chlorotoluene. For these plots, the ratio of the peak area for each component to the combined area of all three peaks was determined, and values were plbtted versus the injection temperature. Figure 6a shows results for a metal cold trap inlet system that does not use the reverse-flow sampling configuration. The plots in Figure 6h were obtained using the reverse-flow configuration shown in Figure 2 . Plot B in Figure 6a shows that at low injection temperatures the highest boiling point component, 2-bromotoluene, is not completely injected from the cold trap; at higher injection temperatures, significant decomposition is observed. Although less decomposition is observed for the 2-chloro-
toluene, significant loss in peak area does occur for injection temperatures greater than about 300 "C. With the use of the reverse-flow sampling configuration, complete recovery and no thermal decomposition are observed for each of the test compounds over the entire range of injection temperatures. Sample size and sampling duration. The sample collection system described here uses software to select sample size and sampling duration. Figure 7a shows plots of peak area versus collection time obtained by using a metal trap tube for a bag sample containing (A) n-pentane and (B) n-hexane for sample collection times in the range of 10-500 s. Sample flow rate was about 0.1 cm'tmin, and sample concentration was typically 100 ppm [vol.lvol.). The trap tube temperature was -100 OC during sample collection and was heated to 150 OC for sample injection: the oven temperature was 500 "C. The plots have linear regression correlation coefficients of 0.99998and 0.999998,respectively. Figure 7b shows similar plots for several compounds with low molecular weights obtained by using a porous polymer (PLOT) trap. The sample contained (A) ethylene, (B) ethane, (C) propane, ID) n-butane, and (E) n-pentane. Concentrations were typically 100 ppm (vol.lvo1.l. The dilution gas was air. The trap tube temperature was -85 "C during sample collection and 150 OC during sample injection. The components were separated on a 4.0-mlong, 0.32-mm-i.d. Al,O, PLOT column at an oven temperature of 110 "C. Linear regression correlation coefficients were i n t h e 0.999954.999998 range. Note that the intercepts of the plots in Figure 7 have nonzero values because of the response times of the flow switching valves. The very high linear correlations observed in Figure 7 confirm that sample collection is quantitative and breakthrough is negligible for at least 500 s. Thus, both the bare metal trap and the PLOT trap can be used for long-term cryofocusing or cryointegration of a dilute sample vapor stream. This should be useful for purge-and-trap and automobile exhaust monitoring applications as well as for other cases in which sample concentration must be integrated over a prescribed time interval. Quantitative analysis characteristics. Figure 8 shows analytical curves (log-log) for benzene ob-
tained by using commercially available gas standards. Concentrations ranged from 1.0 to 100 ppm (v01.l vol.) in air. Sample was collected in a hare metal trap at a temperature of -100 "C and was injected at 150 "C. Sample collection times were (A) 5 s, (B) 30 s, (C) 60 s, and ID)120 s. Table 1 summarizes statistical data for the log-log plots in Figure 8 . Linear regression correlation coefficients are in the 0.9954.998 range. Log-log slope values are all near the ideal value of 1.00. Note that the detection limit for a signal-to-noise ratio of 3.0 is
