National Water Quality Laboratory

Friday October 19, 2018

USGS Water-Resources Investigations Report 03-4079

Methods of Analysis by the U.S. Geological Survey National Water Quality Laboratory — Determination of Gasoline Oxygenates, Selected Degradates, and BTEX in Water by Heated Purge and Trap/Gas Chromatography/Mass Spectrometry

Donna L. Rose and Mark W. Sandstrom


A method for determination of the alkyl ethers used as gasoline oxygenates [ethyl tert-butyl ether (ETBE), methyl tert-butyl ether (MTBE), diisopropyl ether (DIPE), and tert-amyl methyl ether (TAME)], some of their main degradates [acetone, methyl acetate, tert-butyl alcohol (tBA), and tert-amyl alcohol (tAA)], and BTEX (benzene, toluene, ethylbenzene, and xylenes) at low concentrations (<5 micrograms per liter) in water samples was developed. The compounds are determined using heated extraction to improve purging of polar compounds in a standard gas chromatography/mass spectrometry (GC/MS) method for volatile compounds. Volatile compounds in this method are extracted (purged) from the sample by bubbling helium through a 25-mL (milliliter) sample heated at about 65C. Volatile compounds are trapped on a sorbent and then thermally desorbed into a GC/MS system for identification and quantitation. The calibration range for this method is 0.1 to 200 µg/L (micrograms per liter). Mean gasoline oxygenate recoveries from volatile-grade blank-water samples analyzed at concentrations from 0.5 to 5.0 µg/L were 95 to 105 percent, with relative standard deviations (RSDs) from 1.9 to 3.2 percent. Mean oxygenate degradate recoveries ranged from 88 to 107 percent, with RSDs of 3.2 to 7.4 percent, at concentrations from ito 50 Ig/ L. Mean BTEX recoveries ranged from 91 to 107 percent, with RSDs of 1.1 to 6.6 percent, at concentrations from 0.5 to 10 µg/L. The method detection limits range from 0.035 to 0.0 52 µg/L for the gasoline oxygenates, 0.2 16 to 0.62 µg/L for the oxygenate degradates, and 0.005 to 0.036 µg/L for BTEX. Calculated holding times using American Society for Testing and Materials (ASTM) procedure D 4841-88 indicate that all of the analytes are stable for a minimum of 40 days at pH 2 and pH 7, except for methyl acetate, which is only stable for 7 days at pH 2.

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