Title

A Gas Sampling Module for a Subsurface Ion Mobility Spectrometer

Publication Date

11-2006

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Civil Engineering

Department

Civil Engineering

Major Advisor

Molly M. Gibb

Abstract

A sampling module was designed for use with a small subsurface ion mobility spectrometer (IMS) sensor system to extract soil gas samples from unsaturated soil (i.e., soil in which there are both gases and liquids in the void spaces) and inject these samples into the IMS. The sampling module, IMS, and electronics are the first system of their kind designed for subsurface use.

To verify that the sampling module worked with the IMS, three experiments were performed. A known concentration of toluene gas was sampled multiple times a day throughout a two-week time period using the sampling module and gas chromatograph coupled with a mass spectrometer to determine the sampling module's repeatability using an ANOVA test on the peak area. The results indicated that the sampling module was repeatable for days 2-4, but not for day 1 at a 95% confidence interval.

In the second experiment, the sampling module's ability to extract tetrachloroethene (PCE) and inject it into the IMS was determined. The spectrum obtained for the PCE samples contained a peak with a Ko value of 1.53 cm2V-1s-1 as compared to the Ko value of 1.52 cm2V-1s-1 obtained by Kanu et al. (2006a) with the same IMS, but without the sampling module.

Finally, the sampling module was tested in sand, lean clay, and clayey sand at moisture contents between 6% and 24%, with and without contamination (PCE and TCE). The spectra for the PCE and TCE experiments had Ko values of 1.81, 1.51, and 1.33 cm2V-1s-l for PCE, and 1.90 and 1.77 cm2V-1s-l for TCE, which are in good agreement with the published values of 1.81, 1.64, 1.52, 1.45, and 1.36 for PCE, and 1.91 and 1.78 cm2V-1s-l for TCE by Kanu et al. (2006a). Two unknown peaks were observed in this work with Ko values of 1.23 (sand only) and 1.26 cm2V-1s-l (PCE and TCE contaminated soils). The peak at a Ko value of 1.23 cm2V-1s-l (contaminated sand) was not present during the uncontaminated soil experiments, and may be the result of chemical interactions with the soil, sampling module, or water clusters. Soil type apparently had an affect on the spectra produced by the IMS. Spectra obtained from samples taken in clayey soils had a peak that appeared prior to the reactant ion peak (RIP) as compared to gas samples extracted from sand.

These tests showed that the sampling module can successfully extract gas samples from various soil types. The sampling module can inject discrete samples of known volume or multiple samples in a continuous repeatable manner.

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