Anal. Chem. 1993, 65, 2944-2946
2944
Automated Cryogenlc Trapplng Technlque for Caplllary GC Analysis of Atmospherlc Trace Compounds Requiring No Expendable Cryogens: Appllcatlon to the Measurement of Organic Nltrates Steven B. Bertman,' Martin P. Buhr, and James M. Roberts Cooperative Institute for Research in Environmental Sciences (CIRES), University of ColoradolNOAA, Boulder, Colorado 80309-0216, and NOAA Aeronomy Laboratory, 325 Broadway, Boulder, Colorado 80303
INTRODUCTION An increasing number of chromatographic methods developed for the measurement of atmospheric trace species rely on the enhanced performance, reliability, and sensitivity afforded by the use of fused-silica capillary columns (for example, refs 1-6). In turn, the use of capillary columns often requires special considerations for sample acquisition and injection.' In order to realize the advantages of optimum separation, peak shape, and sensitivity, the sample must be introduced onto the analytical column in a narrow band. This is easily accomplished for liquid samples, which can be introduced through a heated injection port on the chromatograph. On the other hand, direct injection of gas-phase samples, where a relatively large volume of air must be introduced, is complicated by the low flow rates of capillary columns relative to traditional packed columns. In our work on trace atmospheric organic nitrates, we have found capillary colulpnsto be most useful for multicomponent analysis,which has lead us to investigate several means of sample acquisition. Trapping of gas-phase samples is commonly used with capillary columns in atmospheric studies to concentrate the sample of interest in a small zone prior to introduction into the chromatograph. Acquisition of trapped samples is most often accomplished by collection of a volume of air on a solid sorbent such as activated charcoals or Tenax9 or by condensation out of the gas phase at low temperature using a liquid cryogen.10-14 The solid sorbent methods have severalpotential problems. Reactions that destroy or alter the sample can occur on the surfaces of the absorbent, and this can lead to interfering artifacts. Since absorption of the sample is ideally an equilibrium process,there exists some degree of uncertainty with regard to the efficacy of sample transfer, thereby decreasing the precision of analyses.16 Desorption of compounds from solid materials often requires heating, and the (1) Atlas, E. Nature 1988, 331, 426-8. (2) Roberta, J. M.; Fajer, R. W.; Springston, S. R. Anal. Chem. 1989, 61, 771-2. (3) Flocke, F.; Volz-Thomas, A.; Kley, D. Atmos. Enuiron. 1991,25A, 1951-60. (4) Goldan, P. D.; Kuster, W. C.; Fehsenfeld, F. C.; Montzka, S. A. Geophys. Res. Lett. 1993,20, 1039-1042. (5) Mineshos, G.; Roumelis, N.; Glavas, S. J. Chromatogr. 1991,541, 99-108. (6) Martin, R. S.; Westberg, H.; Allwine, E.; Ashman, L.; Farmer, J. C.; Lamb, B. J. Atmos. Chem. 1991, 13, 1-32. (7) Tijssen, R.; van den Hoed, N.; van Kreveld, M. E. Anal. Chem. 1987,59, 1007-15. (8)Atlas, E.; Schauffler, S. Enuiron. Sci. Technol. 1991,2661-7. (9) Roberta, J. M.; Fehsenfeld, F. C.; Albritton, D. L.; Sievers, R. E. Zdentificationand Analysis of OrganicPollutantsin the Air;Butterworth Publishers: Worburn, MA, 1984; pp 371-87. (10)Kalman, D.; Dills, R.; Perera, C.; DeWalle, F. Anal. Chem. 1980, 52,1993-4. (11) McElroy, F. F.; Thompson, V. L.; Holland, D. M.; Lonneman, W. A,; Seila, R. L. J. Air Pollut. Control Assoc. 1986,36, 710-4. (12) Grwnberg, J. P.; Zimmerman, P. R. J. Geophys. Res. 1984, 89,
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(13) Mouradian, R. F.; Levine, S. P.; Sacks, R. D. J. Chromatogr.Sci. 1990,28,643-8. (14) van Es, A.; Janssen, C. C.; Rijks, J. J . High Resolut. Chromatogr. 1988,11, 852-7. (15) Heavner, D. L.;Ogden,M.W.: Nels0n.P. R.Enuiron.Sci. Technol. 1992,26, 1737-46. 0003-2700/93/0365-2944$04.00/0
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Representative chromatogram of an ambient air sample containing organic nitrates.
Flgure 1.
use of thermal desorption to remove the sample from the collection media precludes its use in the collection of thermally labile trace gases.l6 Peroxyacetic nitric anhydride (PAN), the most abundant atmospheric organic nitrate, is thermally labile, which precludes solid sorbents as a viable option for our work. Cryogenic techniques, where the sample is passed through a cold region prior to the analytical column, have the distinct advantage of maintaining sample integrity, since the sample does not come into contact with any other surface than it would during normal chromatographic analysis. In cryogenic trapping techniques, a short section at the head of the capillary column or sample loop is cooled while the sample is transferred into the GC. Typically, liquid cryogens such as nitrogen or carbon dioxide are used. The trace gases in the sample stay on the cooled section until the cryogen is removed or until the section is heated, at which time it moves onto the analytical column in a relatively narrow band. Since, mobility of compounds from the capillary column trap generally can be accomplished at lower temperatures than those required by solid sorbents, decomposition of thermally active compounds with cryotrapping is less likely than with a heated solid support. There are several disadvantagesto liquid cryogenictrapping methods. Since the liquids are consumed during the procedure and need to be replenished on a regular basis, the expense and inconvenience limits their usefulness. Furthermore, using expendable liquids in field studies requires that sources be close by and readily available. Safety is also a consideration in the handling and transfer of liquids. These drawbacks to liquid cryogenic trapping methods motivated development of a new method for cryotrapping for organic nitrates that would be portable, reliable, and automated without reliance on liquid cryogens. Different versions of the cryogenic trapping system described herein have been tested in chromatographic systems (16) Roberta, J. M.; Bertman, S. B. Znt. J. Chem. Kinet. 1992, 24, 297-307, and references therein. 0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65. NO. 20, OCTOBER 15, 1993
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Schematic drawlng of current InJectlon configuration. For lntroductlon of the sample onto the analytical column, the sample loop is flushed past the cold zone above the thermoelectric heat pump. Flgure 2.
during four separate field studies. In all cases the system allowed maintenance-free, automated trapping of the atmospheric samples where retention times for the chromatograms obtained were consistent and reproducible to within 0.2 min on a 0.53-mm4.d. capillary column. Figure 1shows an ambient air chromatogram obtained with the new system on a 30-m DB-210 capillary column (J&W Scientific). The chromatogram shows PAN and peroxyproprionic nitric anhydride (PPN)at concentrations of 1.23 and 0.125 ppbv, respectively. In addition, various alkyl nitrates are present at 2-5 pptv. The demands of this particular application led to a trapping system of specific design which could be easily altered for other chromatographic and cryogenic needs.
DESIGN CONSIDERATIONS The system developed and discussed here centers around a single-stage, closed-cycle, Freon refrigerator which cools a heat sink for a thermoelectric (Peltier) heat pump." In our appliciation the thermoelectrics sit on top of a heat sink and provide additional cooling beyond that provided by the refrigerator. Thermoelectrics are small, lightweight, solidstate sandwiches of dissimilar metals that pump heat from one side of the unit (coldjunction) to the other (hot junction), thus cooling the cold junction. The temperature difference between the two junctions is driven by an applied potential. The efficiencydepends in part on how well the heat is removed from the hot junction and on the junction potential as a function of temperature. Since junction potentials approach 0 V at absolute zero, cooling efficiency will decrease with decreasing absolute temperature. The commercial singlestage refrigerator used in this study provides -50-W cooling power at -50 "C. Closed-cycleunits are available that achieve lower temperatures by either cascadingseveralstages together in a single unit or by using different refrigerants such as helium. The costa for achieving lower temperatures with the refrigerator are greater expense, size, weight, and power consumption. The thermoelectric unit provides a means of achieving lower temperatures at a fraction of these costa and also becomes an integral part of the system by virtue of its ability to be cycled quickly and easily to cool or to heat a sample on the top of the unit. Thus, the thermoelectric heat (17) Solid State Coolingwith Thermoelectrics. Electron. Pockag.R o d .
1989, (Nov).
pump provides a built-in heater to release the sample into the GC, eliminating the need for an additional component of the system. Figure 2 shows a schematic representation of the current configuration with a six-port valve and an injection loop for prefocused direct injection of gas samples. A groove was milled in the solid copper heat sink in which the cold finger of the refrigerator was placed, and a pedestal was milled out of the heat sink for the thermoelectric unit to sit upon. The thermoelectric was attached to the surface of the copper pedestal with thermally conductive paste, and the area surrounding the pedestal was insulated, thus further isolating the cold zone and preventing unwanted cooling of any part of the column. The section of column that functions as a cold trap was placed over the top of the thermoelectric unit, and the whole assembly was encased inside a well-insulated aluminum box. The heat-pumping capacity of thermoelectrics decreases as the temperature of the hot junction decreases, thus limiting the cooling capacity of a single unit. Thermoelectrics can be custom designed to take advantage of cascading,which allows for a useful temperature differential even starting at the low temperatures reached by closed-cycle refrigerators. In the current application, a three-stage cascaded thermoelectric heat pump was used which achieved a 40 "C temperature differential with the hot junction starting at -60 "C. Different heat pump designs could result in even greater temperature differentials. Flexibility in the configuration of the thermoelectric heat pump adds versatility for customizingspecific system applications. Several designs have been built and tested and the performances for all are qualitatively similar, but the best performance was obtained with a copper block insulated in a sealed aluminum box with polyisocyanate foam insulation. This system consisted of a Neslab CC-65 (Neslab Instruments, Inc., Portsmouth, NH) immersion cooler and a three-stage thermoelectric heat pump (Melcor,Trenton, NJ) with a 6-V, 3-A dc power supply to drive the thermoelectric heat pump. The total power consumption for the system is -350 W. Operation of the cold trap is straightforward. The entire system is maintained at the minimum temperature of the refrigerator. When the cooling is required, dc power is supplied to the thermoelectric heat pump for the duration of the trapping. To warm the trap and release the sample, the
2946
ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993
IL
80 OC could damage them. Placement of the capillarycolumn
inside the cryogenicsystem enclosure must be done with some care to avoid cold spots that could interfere with analysis.
CONCLUSIONS
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Figurr 3. Representative freeze/thaw cycle for a 5-mln collectlon followed by a 10-min warmup.
polarity of the power to the heat pump is reversed. A thermocouple placed on top of the thermoelectric unit connected to a temperature controller (Omega Engineering, Inc.) allowsthe temperature to be maintained within a narrow range during heating. The temperature control protects both the thermoelectric unit and any thermally sensitive sample which is of interest. Figure 3 shows a graph of time vs temperature for a typical freezehhaw cycle obtained with the current system. The temperature measured at the top of the thermoelectric cascade would routinely reach -101 "C from -60 "C in less than 1 min and heat to 0 "C from the minimum in less than 0.5 min. The reproducibility of the minimum temperature was usually better than *3 O C . The maximum temperature was power limited in our system to -35 OC. Because of the low-meltingBi/Sn solder used in the construction of the thermoelectrica,temperatures greater than
1
The cryogenic trapping system described here provides a relatively inexpensive, reliable, and consistent cold region for on-column focusing of atmospheric gas samples with no need for liquid cryogens. This method would be useful in any application where trapping of a sample is needed to increase the sensitivity for measuring small concentrations. Described here is only one possible configuration which was designed to meet specificrequirements. The system currently ~ ~ 1 ' in1 use~ is1 sufficient for our application but is not necessarily ideal for all applications. Each component of the system is flexible enough in design to allow for significantmodification for specific applications. For instance, different thermoelectric units can be designed and/or different types of refrigerators used to cool the heat sink block. The power supply used to drive the thermoelectrics has some effect on the overall performance of the system as well. In the present application, size and weight were important considerations. Where these conditions are less critical, low enough temperatures to trap very volatile compounds such as the C&S hydrocarbons could be reached.
ACKNOWLEDGMENT The authors thank Bill Kuster and Paul Goldan of the NOAA Aeronomy Laboratory for their helpful discussion at the onset of the system development.
RECEIVED for review March 12, 1993. Accepted July 14, 1993.
