Reformulated gasoline, about 30 percent of gasoline sold in the U.S., commonly contains methyl tertiary-butyl ether (MTBE), which has an unpleasant taste and odor and may be a human carcinogen. There are no data as yet on the effects on humans of drinking MTBE-contaminated water, but the U.S. Environmental Protection Agency is evaluating the potential risks.

Because MTBE is more soluble in water than gasoline is, leaking underground gasoline tanks are causing concern about MTBE in groundwater supplies; virtually every state in the United States is now testing for it. The most common testing method uses the purge-and-trap technique, which costs about $150 for each sample. Agilent has developed an alternative: an ambient headspace method that is fast, low-cost, and highly accurate.

Concern for U.S. Groundwater Supply
In the 1970s, the U.S. federal government passed the Clean Air Act to reduce fossil fuel emissions. The 1990 amendments to the Act set specific goals for using reformulated gasoline to increase octane and reduce carbon monoxide emissions from motor vehicles. Reformulated gasoline accounts for about 30 percent of gasoline nationwide; about 85 percent of reformulated gasoline contains methyl tertiary-butyl ether (MTBE), an octane enhancer that promotes better burning of the gasoline. MTBE is plentiful, inexpensive and efficient, but its taste and odor are offensive to most people. In laboratory tests, animals directly injected with MTBE at high levels of exposure developed cancer and non-cancerous symptoms. Because MTBE is more soluble in water than gasoline is, it is finding its way into the country's water supply from tens of thousands of leaking underground gasoline storage tanks across America.

A 20-ppb methyl-t-butyl ether spike identified in water where it has been leaking from underground storage tanks holding oxygenated gasoline.

No EPA-Required Method
There is no EPA-required method for MTBE testing. The only guidance is EPA's non-regulated drinking water advisory (December 1997), which set a level of 20 to 40 ppb. The current method for detecting MTBE simulates U.S. EPA Method 524.2 (or 8260). This method uses the purge-and-trap technique with GC/MS and cryofocusing, an expensive analysis that costs approximately $150 for each sample.

Better Than Purge and Trap
An Agilent-developed ambient headspace method for screening volatile organic compounds has a detection limit for MTBE of 10 ppb and therefore is notably more sensitive than the EPA's recommended level. The method is easily implemented with an Agilent 6890 or 6850 gas chromatography system equipped with a flame ionization detector (FID) or mass spectrometry detector (MSD).

With the MSD, the method employs a mass spectrometer database of locked retention times for 65 volatile compounds from EPA Method 8260. With the FID, the method uses a database of locked retention times for the same volatile compounds. Retention-time locking matches retention time column-to-column, instrument-to-instrument, operator-to-operator. Compounds can be identified quickly by searching retention times collected under locked conditions. Each detected peak is searched against a table of retention times, producing a list of possible identities. The more accurate and precise the retention-time locking, the shorter the list of possible compounds for each peak.

The total GC run time for all 65 compounds, including MTBE, is eight minutes.The ambient headspace method is a fast, low-cost approach that can also be used instead of normal heated headspace for prescreening nonpolar volatiles and for routine, nonregulated analyses.

Click for more information on the Agilent ambient headspace method.