Assessing the Fate of MTBE with the DAI-GC/MS Method

Method

Approach

The current concern over contamination of groundwater by MTBE has increased the need for a better understanding of the processes that control its environmental fate. As part of an ongoing study to characterize the kinetics and mechanisms of MTBE degradation, we have developed a direct aqueous injection (DAI) GC/MS technique that allows us to simultaneously determine for MTBE and its expected products (TBA, TBF, isopropanol, and acetone) at the sub-ppb level in water.

The DAI-GC/MS Method

Currently, most analyses for MTBE in water are performed using conventional purge-and-trap methods. Most of the expected degradation products of MTBE, however, have lower Henry's gas law constants than the parent compound, making conventional purge and trap methods unsuitable for simultaneous analyses of both parent and products at low concentrations. In order to perform simultaneous analyses, we developed a direct aqueous injection (DAI) technique using gas chromatography with a highly polar column and detection by mass spectrometry (GC/MS). Minimum detection limits for the analytes tested to date are shown on the right (in µg/L). (Church, et. al., 1997)

Analyte
MTBE
ETBE
TAME
TBA
TAA
TBF
Acetone
IP
MA

MDL
0.1
0.1
0.1
0.1
0.1
5.0
<10
40
40

Frequently Asked Questions about the Method

Can the DAI-GC/MS technique be used on a benchtop GC/MS? It should be possible to run this technique on a benchtop GC/MS, provided that a valve system for water control like the one reported in our paper is used. This is because the small diameter diffusion pumps on most benchtop instruments are not capable of quickly removing the large amounts of water vapor resulting from a ten 無 injection. Smaller injections may be possible, but the tradeoff is higher detection limits. However, the two other laboratories have tried to use this technique on benchtop instruments and have had little success to date.
What detection limits are likely on a benchtop GC/MS? It is possible to achieve detection limits that are within two orders of magnitude of our detection limits, provided an adequate valve system is used.
How did you define your detection limits? The detection limits we reported correspond to a 10:1 signal to noise ratio, determined from injections of standards. This approach was used, rather than the statistical method favored by many laboratories, because the latter method gives unrealistically low MDLs for the DAI-GC/MS method. This is apparently due to the highly reproducable peaks areas (and subsequent low standard deviation) of replicate injections of concentrations near the detection limit.
Do samples require special preparation or handling? One of the convenient aspects of using this technique is that there are no special extraction steps necessary. We simply inject 10 無 of water directly into the injector. If the vials to be sampled are equipped with open top caps and septa, there is no need to even open the vial. This is highly advantagous when analyzing for volatile compounds as the sample is never opened to the atmosphere, and replicate injections of the same vial can be accomplished with the confidence that no loss of sample constituents has taken place. Large direct injections do have one drawback, however. Samples preserved by acids may not be injected directly onto the column, as it damages Carbowax stationary phase. For cases where biodegradation in the sample vials is probable, we are currently developing preservation methods that do not interfere with the analytical method.

Applications

A series of chromatograms obtained with the DAI-GC/MS method, which demonstrate the advantage of detecting MTBE degradation from product appearance. The data are for an effluent from a soil column before (front) and after (middle) the onset of biodegradation. A 40 µg/L standard (back) is shown for reference. Note that the 44 day chromatogram shows a positive indication of MTBE degradation through the appearance of TBA even though it is not yet apparent from the disappearance data. (Church, et. al., In prep.)

Collaborators with Field Sites

To explore what can be learned from the DAI-GC/MS method, we are collaborating with a variety of research groups who are doing (extensive or intensive) studies of MTBE occurence in the environment. Usually, we analyze for trace levels of MTBE degradation products while others do the parent compound using standard P&T methods. All collaborators to date are shown here. We welcome inquiries from potential collaborators with access to significant study sites.

James Landemeyer, USGS, Reston, VA.
James Barker, University of Waterloo, Ontario, Canada.
Mario Schirmer, University of Waterloo, Ontario, Canada.
Rick L. Johnson, Oregon Graduate Institute, Portland, OR
Arthur Baehr, USGS, Trenton, NJ.
Mary Ann Thomas, USGS, Chicago, IL.
Joseph Domalgalski, USGS, Sacramento, CA
Steve Maddox, Maddox & Associates, La Habre, CA.
John Zogorski, USGS, Rapid City, SD.

Publications

Church, C. D., L. M. Isabelle, J. F. Pankow, and P. G. Tratnyek (1997) Assessing the in situ degradation of methyl tert-butyl ether (MTBE) with a direct aqueous injection technique for products at the sub-ppb level; 213th ACS National Meeting, San Fransisco, CA, Division of Environmental Chemistry, American Chemical Society, Vol. 37, No. 1, pp. 411-413.
Schirmer, M., J. F. Barker, C. E. Hubbard, C. D. Church, J. F. Pankow, and P. G. Tratnyek (1997) The Borden field experiment - Where has the MTBE gone? 213th ACS National Meeting, San Francisco, CA, Division of Environmental Chemistry, American Chemical Society, Vol. 37, No. 1, pp. 415-417.
Landmeyer, J. E., J. F. Pankow, C. D. Church (1997) Occurrence of MTBE and tert-butyl alcohol in a gasoline-contaminated aquifer. 213th National Meeting, San Francisco, CA, Division of Environmental Chemistry, American Chemical Society, Vol. 37, No. 1, pp. 413-415.
Church, C. D., L. M. Isabelle, J. F. Pankow, D. L. Rose, and P. G. Tratnyek (1997) Method for determination of methyl tert-butyl ether and its degradation products in water. Environ. Sci. Technol. 31, 3723-3726.
Buxton, H. T.; J. E. Landmeyer; A. L. Baehr; C. D. Church; P. G. Tratnyek (1997) Interdisciplinary investigation of subsurface contaminant transport and fate at point-source releases of gasoline containing MTBE: Proceedings of the 1997 Conference on Petroleum Hydrocarbons and Organics Chemicals in Groundwater, 12-14 November 1997, Houston, TX, pp. 2-18.
Schirmer, M.; J. F. Barker, B. J. Butler, C. D. Church, and K. Schirmer (1998) Natural attenuation of MTBE at the Borden field site. In Natural Attenuation: Proceedings of the First International Conference on Remediation of Chlorinated and Recalcitrant Compounds, 18-21 May 1998, Monterey, CA; Wickramanayake, G. B.; Hinchee, R. E., Ed.; Battelle Press: Columbus, OH, Vol. 1(3); pp. 327-331.
Landmeyer, J. E., J. F. Pankow, F. H. Chapelle, P. M. Bradley, C. D. Church, and P. G. Tratnyek (1998) MTBE and tert-butyl alcohol (TBA) in a gasoline-contaminated aquifer, 1993-1997. Ground Water Monitoring and Remediation, Fall, in press.
Church, C. D., J. F. Pankow, and P. G. Tratnyek (submitted) Hydrolysis of tert-butyl formate: kinetics, products, and implications for the environmental impact of MTBE. Environ. Toxicol. Chem.
Church, C. D., and P. G. Tratnyek (in prep) Assessing the in situ degradation of methyl tert-butyl ether (MTBE): column studies.

Acknowledgements

Major Funding for this work has come from:

U.S. Geological Survey, Toxics Substances Hydrology Program (1434-WR-96-AG-00003)
U.S. Environmental Protection Agency, STAR Fellowship Program (Clinton Church)
American Petroleum Institute (via the University of Waterloo)
American Water Works Association Research Foundation, National Assessment of MTBE Occurrence in Drinking Water (with the Metropolitan Water District of Southern California, and the U.S. Geological Survey)

Last updated: 21 March 00