Date of Award

1992

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Environmental Sciences

First Advisor

Edward B. Overton

Abstract

Although the use of portable gas chromatographs is a well established technique, it is still experiencing rapid growth. For continued growth and acceptance, however, it must be a viable alternative over the use of traditional gas chromatographs, in terms of its applicability for identification of important compounds, and its routine use for the analysis of volatile organic compounds (voc) in ambient air. The biggest limitation to the use of portable gas chromatographs is their lack of sensitivity to low concentrations of voc's. Most portable gas chromatographs at present have a detection limit of about 1 ppmv, while the concentration of voc's in ambient air is often below the ppmv range. This dissertation describes two strategies for lowering the detection limit of a portable model M200 microchip gas chromatograph, namely by: (a) introduction of temperature-programming capability, and (b) incorporation of an on-line concentration step through the use of a solid adsorbent. In Chapter One, the M200 gas chromatograph was evaluated. Parameters evaluated include column efficiency, retention time and peak area reproducibility and selectivity. In Chapter Two the introduction of temperature-programming capability to the M200 GC is discussed. This required temperature ramping rates on the order of 1.0$\sp\circ$C/s and resulted in an increase in the peak capacity of the system. Similar to the results for conventional GC, temperature-programming capability with the M200 GC was shown to be an effective solution to the general elution problem. The use of an on-line concentrator to further lower the limit of detection of the M200 GC was discussed in Chapter Three. For maximum efficiency, it was determined that the concentrator should be placed between the sample loop and the switching valve. In Chapter Four, single and dual bed microtraps were installed and optimized onto the commercial portable M200 GC. The dual bed microtraps were shown to result in an overall better concentration effect for both the low and high molecular weight volatile compounds. Finally, using dual bed microtraps, the portable M200 GC was shown to detect most voc's at the 0.1 ppmv concentration level, a ten fold increase in the detection limit.

Pages

237

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