Research Communications Method for Determination of Methyl tert-Butyl Ether and Its Degradation Products in Water CLINTON D. CHURCH,† LORNE M. ISABELLE,† JAMES F. PANKOW,† DONNA L. ROSE,‡ AND P A U L G . T R A T N Y E K * ,† Department of Environmental Science and Engineering, Oregon Graduate Institute of Science and Technology, P.O. Box 91000, Portland, Oregon 97291-1000, and U.S. Geological Survey, National Water-Quality Laboratory, Denver, Colorado 80225
An analytical method is described that can detect the major alkyl ether compounds that are used as gasoline oxygenates (methyl tert-butyl ether, MTBE; ethyl tert-butyl ether, ETBE; and tert-amyl methyl ether, TAME) and their most characteristic degradation products (tert-butyl alcohol, TBA; tert-butyl formate, TBF; and tert-amyl alcohol, TAA) in water at sub-ppb concentrations. The new method involves gas chromatography (GC) with direct aqueous injection (DAI) onto a polar column via a splitless injector, coupled with detection by mass spectrometry (MS). DAIGC/MS gives excellent agreement with conventional purgeand-trap methods for MTBE over a wide range of environmentally relevant concentrations. The new method can also give simultaneous identification of polar compounds that might occur as degradation products of gasoline oxygenates, such as TBA, TBF, TAA, methyl acetate, and acetone. When the method was applied to effluent from a column microcosm prepared with core material from an urban site in New Jersey, conversion of MTBE to TBA was observed after a lag period of 35 days. However, to date, analyses of water samples from six field sites using the DAI-GC/MS method have not produced evidence for the expected products of in situ degradation of MTBE.
Introduction The environmental fate of methyl tert-butyl ether (MTBE) has become a subject of renewed interest due to the large quantities of this compound that are now being used to oxygenate gasoline (1). Some of this MTBE is inevitably released to the environment, resulting in detectable concentrations in urban air (2, 3), surface waters (4-6), and some shallow groundwaters (6-12). Whether the resulting contamination will become an important environmental issue depends, in part, on the rates and products of MTBE degradation. There are few data on the degradation of MTBE in the aqueous phase, however, because the rates of degra* Corresponding author e-mail:
[email protected]. † Oregon Graduate Institute of Science and Technology. ‡ U.S. Geological Survey.
S0013-936X(97)00545-2 CCC: $14.00
1997 American Chemical Society
dation are generally slow and the resulting products are not easily detected using conventional methods. Ethers are a class of compounds that are characteristically unreactive over a wide range of industrial and laboratory conditions, so it is unlikely that MTBE will be degraded rapidly in the aquatic environment. The most thoroughly studied degradation pathway is atmospheric photooxidation, where attack by the hydroxyl radical yields primarily tert-butyl formate (TBF) (13-15). In the aqueous phase, reaction of MTBE with the hydroxyl radical can also be important in engineered treatment systems (16), but accumulation of TBF is not observed because it is readily hydrolyzed to tert-butyl alcohol (TBA). In surface waters, soils, and groundwaters, the only expected pathways for MTBE degradation are microbially mediated hydrolysis (17) and oxidation (18, 19). The major product of both biodegradation pathways is TBA. The major degradation pathways for MTBE and TBA are summarized in Figure 1. Of the products and intermediates shown in Figure 1, TBA is the most promising indicator of degradation because it is (i) common to most pathways, (ii) a demonstrated product of biodegradation (20, 21), and (iii) sufficiently resistant to further degradation that it may accumulate as an intermediate before being further degraded (21-23). To date, the use of TBA as an indicator of in situ MTBE degradation has been limited because TBA is difficult to measure at low concentrations in water. To overcome this difficulty, a general purpose analytical method is needed that gives rapid and sensitive detection of TBA and the other likely products of MTBE degradation (such as TBF, methyl acetate, 2-propanol, and acetone) as well as MTBE itself. We have achieved these goals with a method that involves direct aqueous injection (DAI) and gas chromatography (GC) on a highly polar column, with detection by mass spectrometry (MS). Preliminary application of the DAI-GC/MS method to column experiments with aquifer core material has verified that the method is effective for demonstrating MTBE biodegradation in terms of TBA appearance. However, in water samples from a variety of aquifers, we have not yet found TBA that can be attributed to in situ biodegradation of MTBE.
Experimental Section Materials and Methods. Standards were made from 1000 µg/L stock solutions containing MTBE, TAME, TBA, and TBF (Aldrich); TAA, (Fisher); and ETBE (Chem Service). Each compound was obtained in the highest purity that was commercially available (>97%) and used as received. Groundwater and core samples were provided by various collaborators. Standard water sampling techniques for VOCs were used (24) except that samples were not preserved by acidification (to avoid hydrolysis of TBF). Instead, all samples were packed on ice in the field and promptly shipped to the Oregon Graduate Institute, where they were refrigerated at 4 °C and analyzed in less than 2 weeks. For comparison with our method for MTBE, split samples from one site water were taken in the field and sent directly to a commercial laboratory, and split samples from another site water were prepared at the Oregon Graduate Institute and shipped on ice to the U.S. Geological Survey National Water-Quality Laboratory, Arvada, CO. Column studies were performed with core materials from a U.S. Geological Survey study site in Trenton, NJ (25). To
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TABLE 1. Detection Limits for Gasoline Oxygenates and Their Expected Degradation Products (µg/L) analyte MTBE ETBE TAME TBA TAA TBF acetone 2-propanol methyl acetate a
DAI-GC/MS P&T-GC/MS OC-DAI-FID GGC/ASTM (this study) (26, 27) (31) (36) 0.1 0.1 0.1 0.1 0.1 5
