Anal. Chem. 1998, 70, 4836-4840
Development and Evaluation of a Thermoelectric Cold Trap for the Gas Chromatographic Analysis of Atmospheric Compounds Michael Holdren,* Scott Danhof, Michael Grassi, Joseph Stets, Bill Keigley, and Victor Woodruff
Battelle Memorial Institute, 505 King Avenue, Columbus, Ohio 43201-2693 Anna Scrugli
Consorzio Ricerche Associate, C.P. 234, Trav. 2°Strada Est, I-09032 Assemini, Cagliari, Italy
A prototype of a thermoelectric cold trap system has been designed, built, and evaluated. The unit is used to preconcentrate trace volatile organic compounds prior to analysis. The thermoelectric cold trap unit provides an alternative to the normally used liquid nitrogen-cooled traps as a means to preconcentrate sampled components. The thermoelectric system is especially applicable in field monitoring programs, where logistical burdens are often encountered when supplying large amounts of liquid cryogens. Likewise, one does not have to be concerned with the expense and downtime associated with the use of liquid cryogens. The thermoelectric cold trap assembly can be easily interfaced with any chromatographic system using a two-position six-port valve. The operating principle is based upon the normal sample adsorption/ desorption cycle and GC analysis. The cold trap system includes a custom-designed cold trap module (commercially available thermoelectric module components have been used), an adsorbent tube heater inserted into the cold trap module, and an electronics package to control heating/cooling and timing operations. The cold trap is approximately 15 × 15 × 15 cm. The unit can achieve a cold-side temperature as low as -45 °C under steady-state conditions, and the trap can be heated to 300 °C indefinitely without damaging the thermoelectric modules. The unit also achieves very good transient performances (less than 20 s to heat from -30 to 300 °C and ∼5 min to return to the initial -30 °C set point). Many of the volatile organic compounds (VOCs) found in ambient air are present at very low parts per billion (ppb) and parts per trillion (ppt) levels (e.g., ethane, benzene, toluene). To identify and quantify these species, researchers must employ collection techniques that preconcentrate sufficient amounts of these materials for analytical detection. The use of cryogenic trapping to concentrate VOCs prior to analysis has been established as a proven technique for VOC monitoring.1-5 This method involves collecting the sample on an (1) Rasmussen, R. A.; Harsch, D. E.; Sweany, P. H. J. Air Waste Manage. Assoc. 1977, 27, 579. (2) Farwell, S. O.; Gluck, S. J.; Bamesberger, W. L.; Schuttle, T. M.; Adams, D. F. Anal. Chem. 1979, 51, 609.
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inert material (e.g., glass beads) at subambient temperatures. The temperature of the trap is below the condensation temperatures for trace VOCs but above the condensation temperature for major constituents of ambient air (e.g., nitrogen, oxygen). After collection, the trap is rapidly heated, the VOCs are desorbed, and typically the compounds are analyzed using a gas chromatographic (GC) system. An alternative trapping method involves the use of adsorbent materials to collect VOCs at ambient temperatures. Popular adsorbents include Tenax-TA, silica gel, carbon molecular sieves, and activated charcoal. Desorption again is accomplished by elevating the trap temperature prior to GC analysis. Deficiencies reported with monoadsorbent traps are their inability to completely collect and/or desorb samples possessing a wide range of molecular weights and the presence of artifact materials.6,7 By using selected adsorbents in a multibed trap configuration, collection devices can be tuned to capture a wide range of compounds with minimal artifact interferences.8-10 Following sample collection and desorption, chromatographic peak resolution is further enhanced by refocusing the desorbed material onto the head of the analytical column. To accomplish this task, cryogenic refocusing devices are required. Commercial units employing cryogens are available and connect directly to the inlet of the analytical column.11-13 Novel approaches that make use of closed-cycle coolers and/or thermoelectric devices have also been developed.14,15 Alternatively, some operators make use (3) Westberg, H.; Lonneman, W.; Holdren, M. In Identification and Analysis of Organic Pollutants in Air; Keith, L. H., Ed.; Butterworth Publishers: Boston, 1984. (4) McClenny, W. A.; Pleil, J. D.; Holdren, M. W.; Smith, R. N. Anal. Chem. 1984, 56, 2947. (5) Holdren, M.; Rust, S.; Smith, R.; Koetz, J. EPA-600/4-85-002, a final report on Contract 68-02-3487 from Battelle to U.S. Environmental Protection Agency, Research Triangle Park, NC, 1985. (6) Brown, R. H.; Purnell, C. J. J. Chromatogr. 1979, 178, 79. (7) Pellizzari, E.; Demian, B.; Krost, K. Anal. Chem. 1984, 56, 793. (8) Pollack, A., J.; Holdren, M. W.; McClenny, W. A. J. Air Waste Manage. Assoc. 1991, 41, 1213. (9) McClenny, W. A.; Oliver, K. D.; Daughtrey, H. E. J. Air Waste Manage. Assoc. 1995, 45, 792. (10) Kelly, T. J.; Callahan, P. J.; Pleil, J. D.; Evans, G. F. Environ. Sci. Technol. 1993, 27, 1146. (11) Entech Instruments Inc. (www.entechinst.com). (12) Tekmar-Dohrmann Co. (www.tekmar.com). (13) Graseby-Nutech Co. (www.graseby.com). (14) Bertman, S. G.; Buhr, M. P.; Roberts, J. M. Anal. Chem. 1993, 65, 2944. 10.1021/ac9804184 CCC: $15.00
© 1998 American Chemical Society Published on Web 10/20/1998
of gas chromatographs that are equipped with subambient oven accessories and cool the entire column. Typically, liquid cryogens such as nitrogen or carbon dioxide are used to cool the sample collection trap and to refocus desorbed material onto the head of the column. However, large amounts of cryogen are consumed during normal operations and result in added expenses and inconveniences of changing out cylinders. Likewise, logistical burdens are often encountered in moving the cryogens from the point of delivery to their final destination. Safety is also a major concern when handling operations are carried out. Several commercial gas chromatographic systems have been recently designed to automatically preconcentrate volatile organic compounds and analyze the enriched samples.11-13 All units, except one, make use of a liquid cryogen to facilitate operations.15 The nonliquid cryogenic GC system utilizes a thermoelectric means to cool the collection trap. However, this device is an integral part of the GC system and cannot be transferred to a different GC. Thermoelectric modules are small solid-state heat pumps, ranging in size from that of a fingernail to over 5 cm square. They move heat from one area to another, thus creating a temperature differential. The thermoelectric module is made up of an array of semiconductor couples connected electrically in series and thermally in parallel. With a dc potential applied, heat will be absorbed at one side of the module, thus cooling it, while heat is rejected at the other side, where the temperature rises. This phenomenon is known as the Peltier effect. With careful attention paid to the thermal isolation of cold-side surfaces, minimization of thermal losses, and promotion of heat transfer away from the hot side of the thermoelectric module, we designed a stand-alone cold trap assembly. Our interest was to investigate thermoelectric technology and develop a stand-alone cold trap unit that could be easily connected to existing GC systems and used for sample preconcentration. In this paper, we describe such a unit and present some evaluation results. THERMOELECTRIC COLD TRAP SYSTEM DESIGN The thermoelectric cold trap system includes three key components: (1) a cold trap module, (2) an adsorbent tube heater that is inserted into the cold trap module, and (3) an electronics package to control operations. Design goals for our work, based on standard VOC field measurement procedures, were as follows: (1) achieve a target cold set point temperature of -30 ( 5 °C; (2) achieve a target hot-set point temperature of 300 ( 5 °C; (c) heat the adsorbent tube from -30 to 300 °C in less than 20 s; (d) cool the adsorbent tube from 300 to -30 °C in less than 10 min; (e) minimize the size of the cold trap module (i.e., ∼15 × 15 × 15 cm) so that it could easily interface to a two-position sixport valve on any gas chromatographic system. These goals were established based on the needs for VOC monitoring at field sites. Once these were attained, the system was evaluated by chemical testing. The purpose of the chemical tests was to show that the prototype cold trap system could be employed to preconcentrate low-ppb levels of very volatile organic compounds onto the cold adsorbent tube trap and then to rapidly (15) Perkin-Elmer Corp. (www.perkin-elmer.com).
Figure 1. Key components of the thermoelectric cold trap system.
desorb the contents of the trap onto the analytical column for analysis. The target compounds, which include ethane, ethylene, and acetylene, are very difficult to preconcentrate because of their high volatility. In the following sections, we discuss the operational aspects of the cold trap, the adsorbent tube/heater, and the controller. Cold Trap. The thermoelectric cold trap assembly contains the following key components: a finned heat exchanger, thermoelectric modules, a cold block, and insulation material. The prototype system is equipped with a finned U-channel hotside heat exchanger instead of a finned single-sided flat heat exchanger. The U-channel heat-exchanger configuration is intended to maximize the heat-exchanger size (to improve efficiency) without increasing the overall cold trap package size. With the addition of a cooling fan, the overall dimensions of the assembly are 15.2 × 15.2 × 17.8 cm (6 × 6 × 7 in.). The assembly contains two thermoelectric modules that are connected in parallel. These units were purchased from Melcor Corp. The two thermoelectric modules are necessary for extended operation, for periods of time greater than 10 min at 300 °C. With the dual thermoelectric modules and the U-channel exchanger, sufficient heat is removed from the cold block assembly during the trap heating period so that the thermoelectric modules never exceed the manufacturer’s recommended maximum temperature (∼80 °C). The minimum steady-state operating temperature of the cold trap is ∼-45 °C. The time required to heat the trap to 300 °C is ∼20 s. The time required to cool the trap from 300 to -30 °C is ∼5 min. The design of the cold block was carried out with the aid of modeling using a computerized two-dimensional finite element analysis (FEA). One of the more important results from the FEA model showed that if the initial design of a flat cold-side block 2.5 × 5 × 7.6 cm (1 × 2 × 3 in.) was used, nearly all the heattransfer losses would occur from the cold body (trap) to the hot side of the heat exchanger. This information led to a trap design with a “stand-off” cold block to increase insulation between the cold and hot regions of the thermoelectric module. The final cold block design resulted in an aluminum “T” shape that had a very small volume and thermal mass. Figure 1 shows the “U” channel heat exchanger, the “T”-shaped cold block, and the adsorbent trap/heater. Analytical Chemistry, Vol. 70, No. 22, November 15, 1998
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Figure 2. GC chromatograms at a nominal challenge concentration of 100 ppb of C (loop on the left versus trap on the right).
Adsorbent Trap/Heater. A commercially available fusedsilica-lined stainless steel tube 2.0 mm i.d. by 25 cm long is used as the preconcentration trap. The center external surface of the tube is cleaned and wrapped with insulation tape. Three thermocouples are placed on top of the insulation, and another layer of insulation tape is added. A Nichrome ribbon is wrapped around the tube to provide a 3-in. heated zone. Several layers of insulating tape are applied to the ribbon to ensure a snug seal when the tube is inserted into the cold trap block. The heater’s resistance is ∼0.8 Ω. The applied voltage to the heater wire is ∼8 V ac. Controller. A control module is used with the system to carry out three functions. First, the controller supplies power to the trap and heater and controls the cold and hot temperature set points using individual temperature controllers. Second, the controller is equipped with timing circuitry to set the periods for the cold and hot cycles. Third, the unit is equipped with relays and contact closure circuitry that interface with the timing circuitry to operate the gas chromatographic valve and to start the GC and data acquisition systems. A typical GC operational run occurs as follows. At 0 min, the trap is at its cold set-point temperature (e.g., -30 °C) and the GC valve is switched to its load position. The GC valve remains in the load position for the set loading time. During this time period, sample air is drawn through the trap at a known flow rate using a pump and mass flow controller that are located downstream of the trap. At the end of the trap loading time, a number of activities occur simultaneously. The valve is switched to the inject position and the trap is heated to its hot set point (e.g., 300 °C). At the same time, the controller sends a momentary contact closure signal to start the GC and the data acquisition system. The trap heats to 300 °C within 20 s. At the end of the heating cycle, the 4838 Analytical Chemistry, Vol. 70, No. 22, November 15, 1998
trap heater is turned off (e.g., heating time of 2 min), and the trap returns to its cold set point of -30 °C (∼5 min later). The cycle time is normally determined by the amount of time needed to chromatographically resolve the compounds of interest. For a complex mixture like the 41 compounds listed in U.S. EPA TO-14 methodology, a GC run time of 30 min is needed. The total cycle time would be the GC run time plus the valve load time. CHEMICAL TESTING Chemical testing was carried out to evaluate the performance of the thermoelectric cold trap system as a preconcentration device. A target set of seven volatile organic compounds was chosen that included the common C2-C4 hydrocarbons. These compounds were targeted because they are very volatile and therefore difficult to capture with a preconcentration cold trap system. These compounds are also ubiquitous in urban atmospheres at concentrations of a few ppb. Furthermore, these compounds are among the most volatile of the ozone precursor compounds that are currently being measured in the United States by State agencies at Photochemical Assessment Monitoring Stations (PAMS). METHODOLOGY The gas chromatographic system that was assembled for chemical testing consisted of a Varian 3600 GC equipped with a flame ionization detector (FID). The detector output signal was connected to a personal computer running ChromPerfect GC software. Separation was accomplished using an aluminum oxide PLOT capillary column (Chrompack), 50 m long by 0.32 mm i.d.
Figure 3. GC chromatogram of a 41-component mixture at 10 ppb each (U.S. EPA TO-14 methodology). Table 1. Collection/Recovery Efficiencies for Target Compoundsa
a
compound
amt recovered (%)
ethane ethylene propane propylene isobutane n-butane acetylene
98 ( 3 102 ( 3 101 ( 1 105 ( 2 101 ( 1 100 ( 1 95 ( 6
Average of three runs.
Analytical separation for the compounds of interest was obtained using a temperature program of -30 to 120 °C with an initial hold of 2 min and a ramp of 15 °C/min. Zero grade nitrogen with a flow rate of 4 cm3/min constituted the carrier gas. The FID gases were ultrazero air (300 cm3/min) and ultrazero hydrogen (30 cm3/ min). The adsorbent trap was a fused-silica-lined stainless steel tube (25 cm by 0.2 cm i.d.) that was packed sequentially with two adsorbents. The first adsorbent was ∼0.02 g (2 cm long) of Carbopack B (60/80 mesh), and the second material was ∼0.02 g (1 cm long) of Carbosieve S-III (60-80 mesh). This type of adsorbent trap has also been used to perform U.S. EPA methods 624 and 8240, which address the collection and analysis of VOCs from water by purge and trap techniques. The adsorbent trap material has also been used by the Perkin-Elmer Corp. in their
auto-GC system, which is used to analyze for the ozone precursor compounds discussed earlier. Sample flow through the trap was controlled using the following: (1) a Sierra Instruments model 810 mass flow controller/readout unit and (2) a Thomas dual-diaphragm pump. The sample flow rate was controlled at 20 cm3/min with a collection time (load time) of 5 min, thereby resulting in a sample volume of 100 cm3. A flow calibration device traceable to the National Institute of Standards Technology (NIST) was used to measure actual flow, which was 104 cm3 ((3%). The trap was installed so that, during sample collection (-30 °C), air was directed initially through the Carbopack B material and then through the Carbosieve S-III. During thermal desorption (250 °C), the trapped VOCs were back-flushed off the trap. The trap was connected to a two-position VICI six-port valve, which in turn was interfaced to the GC. We prepared a calibration cylinder containing the seven volatile organic compounds, each at a nominal concentration of 1 ppm ((5%), in high-purity nitrogen. A dynamic dilution system was then used to dilute the calibration standard using ultrazero air as the diluent. TEST RESULTS Multipoint calibration curves were generated using a calibrated 1.18-cm3 sample loop as well as with the preconcentration cold trap. Very good linearity was obtained for all seven compounds as indicated by correlation coefficients of 0.99 or better. The instrument noise level corresponded to 600 ( 200 area units. Analytical Chemistry, Vol. 70, No. 22, November 15, 1998
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Using a signal-to-noise ratio of 3 as the minimum detection level (MDL), the MDL was 100 ( 30 ppb of C for all seven compounds when sampling with a 1.18-cm3 loop. The calibration curve with the loop samples ranged from 100 to 1000 ppb of C. The cold trap calibration levels ranged from 20 to 100 ppb of C. Using a MDL of 3 times the noise level (1800 area units), a MDL of 1 ( 0.3 ppb of C was observed when sampling with the cold trap and preconcentrating 100 cm3 of air. Comparison chromatograms at a nominal challenge concentration of 100 ppb of C are shown in Figure 2. The chromatogram on the left was obtained with a 1.18-cm3 sample loop. The chromatogram on the right was obtained with a preconcentrated volume of 104 cm3. Collection/recovery efficiencies were determined using experimentally measured sample loop (1.18 cm3) and cold trap (104 cm3) volumes and the corresponding signal responses from the challenge concentrations. These results are shown in Table 1 (percent recovered and percent standard deviation). The loop area response is that amount measured when the sample loop is challenged with the undiluted cylinder mixture of the seven targeted compounds (nominally 1 ppm (10%). The trap area response results when the cold trap is challenged with three diluted levels of the cylinder mixture. The percent recovered is determined by the measured trap/loop ratio divided by the dilution factor and enrichment factor product. As the data indicate, for the three preconcentration runs, the amounts recovered with the cold trap were essentially 100% (100 ( 3%). More recently, the above methodology has been applied to the analyses of ambient air samples collected at field locations with stainless steel canisters (for whole air sampling) and with adsorbent tubes (Carbotrap 200 tubes, Supelco, Inc.). Results show excellent analytical precision from repetitive analyses ((5%) and similar performance from the analyses of duplicate field samples. These results will be reported elsewhere. The two-component bed material used in this work has been replaced with other adsorbents for the analyses of more complex target species. Figure 3 shows a representative chromatogram of a 41-component mixture (U.S. EPA TO-14 methodology, 10 ppb each) using a single-bed adsorbent (0.03 g of Carboxen 569), held at -30 °C for sample preconcentration (100 cm3) and heated to 300 °C for sample desorption. The analysis of atmospheric levels of peroxyacetyl nitrate (PAN) and its homologues has also been accomplished using the cold trap system. Figure 4 shows a typical chromatogram of ambient air using a single-bed adsorbent held at -30 °C for sample preconcentration (40 cm3) and heated to 60 °C for sample desorption.
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Figure 4. Typical chromatogram of peroxyacetyl nitrate (PAN) found in the atmosphere.
SUMMARY A thermoelectric cold trap system has been designed and has been shown to successfully preconcentrate air samples containing volatile organic compounds. The thermoelectric cold trap system included a cold trap module, an adsorbent tube heater that is inserted into the cold trap module, and an electronics package to control operations. The system met the following criteria: (1) a target cold set-point temperature of -30 ( 5 °C; (2) a target hot set-point temperature of 300 ( 5 °C; (3) transition from -30 to 300 °C in less than 20 s; (4) transition from 300 to -30 °C in less than 10 min; (5) small size of the cold trap module (i.e., ∼15 × 15 × 15 cm). The prototype system was also shown to be easily interfaced to any gas chromatographic system that is equipped with a twoposition six-port sampling valve and electronic actuator. Chemical testing was carried out with a target set of seven volatile organic compounds (e.g., ethane, ethylene, acetylene). These compounds were targeted because they are among the most difficult to capture with a cold trap. These compounds are also ubiquitous in the atmosphere and are key compounds on the U.S. EPA’s list of ozone precursors. Experimental results showed excellent recoveries (100 ( 3%) when the unit was challenged with low-ppb levels of the seven target compounds. Received for review April 16, 1998. Accepted August 25, 1998. AC9804184
