U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

A Comparison of Two Analytical Techniques For Identifying and Measuring the Concentrations of Volatile Hydrocarbons in Ground Water

Abstract

Identification of monoaromatic hydrocarbons in ground water is commonly performed in circumstances where petrochemical contamination is suspected. Presence of certain of these species is considered indicative of petrochemical contamination. Most studies of ground-water contamination focus on the fate of benzene, ethylbenzene, toluene, and the xylenes (BTEX). However, a broad range of C₆-C₁₀ aromatic hydrocarbons can be present in ground water contaminated with petroleum products. In this study, results of a comparison of two analytical techniques for the identifying and measuring the concentrations of C₆-C₁₀ monoaromatic hydrocarbons are presented. The techniques investigated are pentane/water microextraction followed by gas chromatography/flame ionization detection (GC-FID), and purge and trap (open-loop) stripping of the sample followed by GC-Ion Trap detection (GC-ITD). Open-loop/GC-ITD allows the identification of 35 C₆-C₁₀ aromatic hydrocarbons, compared to 21 identified by pentane/GC-FID. Compared to the pentane/GC method, higher sensitivity, higher selectivity, but lower precision were observed for samples analyzed by open-loop/GC-ITD. Hydrocarbon concentrations measured by the two techniques were generally in good agreement, within 88.6-99.7 percent for total mono-aromatics. Concentrations of some trace constituents measured by pentane/GC-FID were higher under certain circumstances. For example, difficulty in resolving benzene from interferences by the pentane/GC-FID method sometimes resulted in considerably higher benzene and BTEX concentrations compared to open-loop/GC-ITD. At a sample site where hydrocarbon concentrations were low, mean BTEX concentrations (n=10) were 661 µg/L (micrograms per liter) by pentane/GC-FID compared to 490 µg/L by open-loop/GC-ITD. At a high concentration sample site, mean BTEX concentrations (n=10) were 5,077 µg/L by pentane/GC-FID compared to 5,089 µg/L by open-loop/GC-ITD.