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TBA PRODUCTION BY ACID HYDROLYSIS OF MTBE DURING HEATED HEADSPACE ANALYSIS & EVALUATION OF A BASE AS A PRESERVATIVE
Citation:
McLoughlin, P., R. J. Pirkle, D. Fine, AND J T. Wilson*. TBA PRODUCTION BY ACID HYDROLYSIS OF MTBE DURING HEATED HEADSPACE ANALYSIS & EVALUATION OF A BASE AS A PRESERVATIVE. GROUNDWATER MONITORING AND REMEDIATION 24(4):57-66, (2004).
Impact/Purpose:
To observe the results and sensitivity of purge and trap samplers for analysis of fuel oxygenates that are alcohols.
Description:
At room temperature (20°±3°C), purge and trap samplers provide poor sensitivity for analysis of the fuel oxygenates that are alcohols, such as tertiary butyl alcohol (TBA). Because alcohols are miscible or highly soluble in water, they are not efficiently transferred to a gas chromatograph for analysis. To improve the efficiency of transfer, the water in a purge and trap sampler can be heated. Alternatively, the sensitivity for TBA can be improved by preparing the sample in a heated static headspace sampler. In a heated water sample, the acid used as a preservative may cause chemical hydrolysis of methyl tertiary butyl ether (MTBE) to produce TBA. This effect is well illustrated in this paper using data collected by the U.S. Environmental Protection Agency Office of Research and Development on a plume of MTBE in California. Samples were analysed using a static headspace sampler heated to 80°C. The groundwater samples were preserved in the field with HCl to a pH≤2. The extent of MTBE hydrolysis to TBA during sample analysis varied from 19 to 87%; the average extent of hydrolysis was 59%. To confirm and document the importance of acid hydrolysis of MTBE at higher temperatures during sample preparation, the rate of hydrolysis of MTBE was measured at 80°C. At pH=1, the rate of hydrolysis was 1.22/h, while the rate at pH=2 was 0.15/h. Acid hydrolysis of MTBE during sample preparation in a heated headspace sampler can be avoided by preserving the sample with 1% (w/w) trisodium phosphate dodecahydrate (TSP) instead of using HCl, or by neutralizing the acid before analysis. In the presence of an acclimated microbial culture, TSP prevented biodegradation of MTBE, as well as benzene, toluene, ethylbenzene, and xylene compounds, in ground water at room temperature for 66 d. However, in a spike recovery experiment, TSP caused based catalyzed hydrolysis of bromomethane. It is not appropriate as a universal preservative.