Anal. Chem. 1999, 71, 1474-1478
An Air To Water Bridge: Air Sampling and Analysis Using Tetraglyme J. R. Troost
American Analytical and Technical Services, 11950 Industriplex Boulevard, Baton Rouge, Louisiana 70809
A water-soluble organic liquid is shown to scrub a wide variety of volatile organic compounds from air and gas streams. Gas pulled through impingers containing chilled tetraglyme (an organic solvent utilized in USEPA methods 3050A and 8240) is found to efficiently trap volatile Priority Pollutant, Hazardous Substance List and other organic species. A portion of the tetraglyme is subsequently dispersed into water and analyzed using conventional water analysis methodology. Practical quantitation limits of 100 ppbv have been demonstrated, and a potential to achieve lower limits of detection is clear. The method offers advantages over canister, adsorption tube, or Tedlar bag air-sampling techniques. Attributes include broad applicability, preservation of sample integrity (“plating out” of analytes is eliminated), freedom from water vapor interference, ready inclusion into water analysis methodology, simplicity, and low cost. Environmental laboratories with ordinary water/volatile organic analysis equipment are enabled to perform air-monitoring analyses without specialized hardware or expertise. The measurement of pollutants in water and soil is routine in environmental laboratories. Nearly all laboratories are equipped with purge and trap equipment to analyze for volatile organics in water or soil. Few facilities have developed air analysis capabilities. Bag, canister, or sorbent tube air-sampling techniques (EPA method 18, TO-14, TO-1, etc.) require special devices, handling, elaborate gas reference standards, and expertise foreign to most water and soil analysts. Interest and demand for ambient air analysis has increasing as has the number and diversity of air pollutants of concern. The 1990 Clear Air Act Amendments and California Safe Drinking Water and Toxic Enforcement Act of 1986 require a large number and a wide variety of volatile organic compounds to be monitored. More than one air analysis method may be required to assess a group of target analytes having dissimilar chemical properties. Air analysis techniques currently in use involve using plastic (Tedlar) bags or metal canisters (SUMMA)1,2 to collect and (1) Measurement of Gaseous Organic Compound Emissions by Gas Chromatography. Office of the Federal Register, National Archives & Records Administration, Code of Federal Regulations, Part 60, Appendix A, Method 18, 1998. (2) Winberry, J. T.; Carhart, B. S.; Randall, A. J.; Decker, D. L. Method TO-14, Compendium of Methods for the Determination of Toxic Organic Compounds in Air; EPA-600/4-89-017, U.S. Environmental Protection Agency, Research Triangle Park, NC, 1988.
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transport air samples. Sorbent tubes (Tenax, activated carbon, molecular sieve, etc.) are also used to trap, retain, and transport volatile constituents for analysis. Gas reference calibration mixtures required for some of these techniques are difficult to prepare and may not adequately match samples in terms of moisture and diluent gas content. Volatile compounds have been reported to “plate-out” on the metal surfaces of a canister method.3 Water vapor (air samples collected with varying degrees of humidity) has also been shown to alter air analysis results.4 Many of the sorbent tube methods are limited in applicability and offer “one-shot” analyses. With thermal desorption techniques, there is no reanalysis recourse for analytes exceeding instrument calibration range. Additionally, usually only “grab” samples are able to be taken. While sorbent tubes may be used to collect samples over extended periods of time, bag and canister samples are rarely collected in this manner. The sample may not adequately represent the source. In the study reported here, tetraglyme (tetraethylene glycol dimethyl ester), a water-soluble organic solvent that had been used in the now obsolete USEPA volatile organics gas chromatographic/ mass spectrometric (GC/MS) method 8240 and the associated purge and trap method 3050A,5 has been employed as an impinger fluid and found to be an efficient and versatile scrubbing agent for a diverse group of volatile compounds. Stripping efficiencies from both air and methane gas (landfill gas) streams have been measured. As in method 8240 (superseded by method 8260), tetraglyme, retaining stripped air pollutants, is dispersed into water and analyzed using the water method without modification. Air analyses may thereby be conducted with a water method. Refrigerated tetraglyme has been shown to retain even the most volatile of the organic pollutants for a period of up to nine months, despite headspace, without measurable loss. Detection limits of 100 ppbv were achieved under the study conditions. Lower limits of detection could be achieved by drawing the air (3) Pete, Bruce; Jayanty, R. K. M.; Peterson, Max R.; Evans, G. F. Temporal Stability of Polar Organic Compounds in Stainless Steel Canisters. Proceedings of the 1991 EPA/ALWMA International Symposium on Measurement of Toxic and Related Air Pollutants; VIP-21, Air & Waste Management Associates, Pittsburgh, 1991; pp 375-381. (4) Ogle, L. D.; Brymer, D. A.; Jones, C. J.; Nahas, P. A. Moisture Management Techniques Applicable to Whole Air Samples Analyzed by Method TO-14. Proceedings of the 1992 EPA/ALWMA International Symposium on Measurement of Toxic and Related Air Pollutants; VIP-25, Air & Waste Management Association, Pittsburgh 1992; pp 25-30. (5) Methods 3050A and 8240, 3rd ed.; USEPA Office of Solid Waste and Emergency Response, SW-846, July 1982. 10.1021/ac981316g CCC: $18.00
© 1999 American Chemical Society Published on Web 02/27/1999
Table 1. Volatile Organics Study Target Lista acetone benzene chlorobenzene chloroethane dichlorobenzene dichloroethane dichloroethene ethylbenzene hexanone, 2 chloromethane methylene chloride butanone, 2 a
pentanone, methyl styrene tetrachloroethene toluene trichloroethane trichloroethene vinyl chloride xylene siloxanes: octamethylcyclotetrasiloxane decamethylcyclopentasiloxane
Detection limit goal, 1.0 mg/m3 gas.
sample for a longer period of time, by using more sensitive analysis instrumentation, or by reducing the volume of tetraglyme used to collect the sample. Tetraglyme offers simplicity and low cost to air sampling and analysis while eliminating many of the problems associated with existing methods. The study reported here evolved from a research effort to develop a method to identify and quantify organic and inorganic pollutants in landfill gas. EXPERIMENTAL SECTION A 50 µg/mL reference solution of the volatile organic compounds targeted for study (Table 1) was prepared in methanol from certified stocks and dilutions of pure materials. Tetraglyme was purchased from Aldrich (Milwaukee, WI) and evaluated for purity (absence of volatile organic contaminants) by conventional method 8240 GC/MS analysis (100 µL of tetraglyme dispersed into 5 mL of water). Using an approach as described in method 18, 40 CFR part 60, a heated midget glass impinger was utilized as a vaporization
chamber to feed volatilized targeted organics to a pair of impinger chambers connected in series, each filled with 25-30 mL of tetraglyme. (See Figure 1). Over the course of the experiments, 0.5-1.0 mL of the reference solution was injected slowly (over ∼60-s period) into a heated (100 °C) vaporization chamber at the onset of a purging period. The reference materials were injected through a cylindrical, Teflon-lined septum as shown in Figure 1. Zero grade helium was used as a purge gas in the system and was varied in flow from 50 to 100 mL/min for the experiments described. The tetraglyme liquid impingers were housed inside a small ice chest to facilitate experiments reducing the temperature of the impinger fluid. To achieve a target temperature of -21 °C, rock salt was added on top of the ice in the cooler housing the tetraglyme impingers (ice and salt being readily available in the field). Through all experiments, a constant total volume of 6 L of gas was drawn through the impingers. To accomplish this, the 50 mL/ min flow experiment was conducted for 120 min, the 100 mL/ min study for 1 h. A 6-L draw was chosen to achieve a research study detection limit goal of 1.0 mg/m3 utilizing GC/MS for analysis. A sample calculation is given by eq 1 where Cg is the
Cg )
CtVt Vg(P/760)(273/(T + 273))
(1)
analyte concentration in the gas stream (mg/m3), Ct is the analyte concentration in tetraglyme (mg/L), Vt is the volume of tetraglyme used in the impinger (mL), Vg is the volume of gas stream pulled through the impinger (L), P is the atmospheric pressure at the time of sampling (mmHg), and T is the temperature of gas entering the impinger (°C).
Figure 1. Adsorption study sampling configuration.
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Table 2. Tetraglyme Absorption Efficiency under Various Conditions recovery (%) 4.2 mg/m3 25 mL needle room temp 100 mL/min
4.2 mg/m3 25 mL needle ice 100 mL/min
8.3 mg/m3 25 mL needle ice/salt 50 mL/min
8.3 mg/m3 30 mL needle ice/salt 100 mL/min
8.3 mg/m3 30 mL frit ice/salt 100 mL/min
gas organic
bp (°C)
front
back
front
back
front
back
front
back
front
back
chloromethane vinyl chloride chloroethane acetone 2-butanone methylpentanone 2-hexanone methylene chloride dichloroethene dichloroethane trichloroethene bromodichloromethane 1,1,2-trichloroethane tetrachloroethene chlorobenzene bromoform benzene toluene ethylbenzene xylene styrene octamethylcyclotetrasiloxane decamethylcyclopentasiloxane
-23.7 -13.4 12.3 56.0 79.6 117.0 127.0 40.0 60.0 83.0 87.0 100.0 113.0 121.0 131.0 149.0 80.1 111.0 136.0 144.0 145.0 176.0 210.0
0 0 13 70 76 100 100 77 95 100 100 100 100 100 100 100 86 100 100 100 100 100 100
0 12 30 30 24 0 0 23 5 0 0 0 0 0 0 0 14 0 0 0 0 0 0
0 11 0 100 72 100 100 86 100 100 100 88 100 100 100 100 100 100 100 100 100 100 100
17 21 37 0 28 0 0 14 0 0 0 12 0 0 0 0 0 0 0 0 0 0 0
30 55 75 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
60 45 25 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
46 56 71 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 naa na
54 44 29 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 na na
33 50 73 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 na na
36 40 27 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 na na
a
na, not available.
Another variable in the experiments was the substitution of a medium porosity frit sparger tip for a glass needle sparger tip (see Figure 1). Results are presented in Table 2. Ultimately, the tetraglyme trapping capability was determined at two flows, two tetraglyme impinger fluid levels, with frit as well as needle sparger tips, with different analyte concentration, and with varied fluid temperature. Immediately following each experiment, the tetraglyme from the front and back impingers was placed separately into 40-mL vials and stored in a refrigerator prior to GC/MS analysis. To enhance accuracy of the GC/MS analyses, a calibration curve was prepared bracketing the concentration of the spiked analytes. A constant ratio of tetraglyme to water (100 µL to 5 mL) was maintained throughout for all solutionssstandards, blanks, and samples. Front and back impinger fluids were analyzed separately and in duplicate. A study was also undertaken to examine the holding time of tetraglyme solutions stored in a freezer (with 10 mL of headspace) over a 9-month period. Front and back impinger fluids from a vaporized standard (corresponding to 25 mg/m3), prepared using the impinger train in Figure 1, were reanalyzed following 9 months of storage. The analyses are compared directly with a fresh, pure water (no tetraglyme) standard of the same theoretical concentration (25 mg/m3) and with a fresh, although different lot of tetraglyme. Chromatograms associated with this study are presented in Figure 2. Note that the six largest peaks between 4 and 12 min, most obvious in the bottom two traces, are the internal standards and surrogates of method 8240. Additional evidence of the retaining properties of chilled tetraglyme are provided by the experiments depicted in Table 2. 1476 Analytical Chemistry, Vol. 71, No. 7, April 1, 1999
In each experiment, the standard was introduced within the first minutes of a 1-2-h vigorous purge of the tetraglyme, yet breakthrough was observed into the second impinger for only a few of the lightest, most volatile compounds. RESULTS AND DISCUSSION As observed in Table 2, nearly all of the targeted organics were trapped with good efficiency even at room temperature. Breakthrough occurred with the lowest boiling point componentss chloromethane (-23.7 °C), vinyl chloride (-13.4 °C), and chloromethane (12.3 °C) at room temperature, but was greatly reduced when the tetraglyme was chilled to -21 °C (ice/rock salt mixture) prior to the desorption process. With the ice/salt bath, a 30-mL tetraglyme impinger fluid volume, 100 mL/min flow, and needle sparger tip, all targeted analytes were retained in the front and back impingers with 100% efficiency. All but three of the targeted analytes (those boiling below room temperature) were retained completely in the front impinger. The adsorption efficiency extended equally to all the chemical species studied. Of the compounds, the boiling point was the only analyte chemical functionality found to have a bearing on the results. Little effect was noted by substituting a frit sparger for a needle sparger or by lowering the flow rate below 100 mL/min. The particular lot of tetraglyme used for this study as well as the blank prepared using the impinger train was found to be free of contamination above the targeted detection limit of 1 mg/m3. No measurable loss of even the lowest boiling compounds was detected with the 9-month tetraglyme stability study depicted in Figure 2. This figure illustrates the following: (a) the direct comparison of a conventional water standard to a front impinger
Figure 2
having adsorbed the exact corresponding components and concentrations of that water standards9 months earlier; (b) the striking differences between the front and back impingers in view of the 25 mg/m3 standard vaporized into them; (c) the subtle differences in the nature and level of contaminants found in different lots of tetraglyme. While the original lot of tetraglyme purchased from Aldrich was relatively free of volatile organics. Subsequent lots were found to contain significant levels of several compounds. The tetraglyme is able to be cleaned by heating to 80 °C and purging with an ultrapure gas for several hours. More than a dozen field tests have been conducted with the tetraglyme liquid impinger method. Trials were conducted flowing
landfill gas through chilled impingers, by pulling ambient air through the impingers with a personal monitoring vacuum pump, and by directing the effluent of groundwater air stripping units and incinerators through the impingers. All but one trial met with the success predicted by Table 2. In a field application where the effluent of an incinerator was sampled, a large amount of breakthrough was observed in the second impinger. The reason for the failure was likely twofold: (1) the sampler directed flow through the impingers at five times the highest rate of the Table 2 (i.e., the incinerator effluent was allowed to flow through the chilled impingers at a rate of 500 mL/ min), and (2) the incinerator effluent gas temperature was elevated much above ambient temperature. Analytical Chemistry, Vol. 71, No. 7, April 1, 1999
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While a limited amount of work has been done thus far to explore the boundaries of this tetraglyme method, it is clear that the flow and the temperature of the gas directed through the tetraglyme must be limited. CONCLUSIONS A tetraglyme midget impinger air scrubbing and analysis method is shown to effectively trap, retain, and provide an easy mechanism to analyze for a wide range of important volatile organic air pollutants including certain siloxanes. An impinger kit can be built with little cost from readily available materials. Tetraglyme may be obtained from chemical supply houses although provisions should be made to clean and test the tetraglyme for volatile contaminants prior to use (trip blanks of tetraglyme should accompany front and back impingers fluid supplied to field samplers). Tetraglyme, as a glycol ether, is a suspected carcinogen and should be handled with gloves. Also, as noted by the supplier, it should be checked periodically for a
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potential to form peroxides using EM Quant Test Strips (see method 5030A). Solutions of chilled tetraglyme may be retained for long periods of time even with headspace. Air analyses may be conducted directly with the conventional environmental analysis method 8260. ACKNOWLEDGMENT This study was conducted at Southern Petroleum Laboratories (SPL) Lafayette, LA. U.S. Patent No. 5,529,612, Method and System for Removing Volatile Organics from Landfill Gas, was issued June 25, 1996, and another patent is pending on other aspects of this procedure. Certain patent rights are owned by SPL. Special thanks to Lilian Zepeda for preparing the manuscript and to Herb Brown for permission to publish this work. Received for review December 1, 1998. Accepted January 14, 1999. AC981316G
