Doctoral Dissertations

Date of Award

3-1988

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Electrical Engineering

Major Professor

E. J. Kennedy

Committee Members

J.M. Rochelle, T. V. Blalock

Abstract

The helium ionization detector (HID) is one of the most sensitive detectors currently available for gas chromatography. The ionization efficiency is estimated to be as high as 5% as opposed to 0.0005% for the flame ionization detector. Despite its sensitivity and a universal response mechanism, it has not been widely used owing primarily to a lack of stability and reliability. Conventionally, the detector has been operated only in a dc mode by applying a constant polarizing voltage across the cell and measuring the output current continuously. In this study, instrumentation has been designed to operate the detector in both a fixed-frequency and constant-current pulsed mode.

Mechanisms in the detector are examined, and an electronic model of the detector is developed as a function of sample concentration. Design and analysis of a high voltage pulser and high speed electrometer for use in both pulsed modes are presented. Novel circuitry to operate the system in a closed-loop, constant-current pulsed mode is also developed. Since the HID exhibits a low background current with high purity helium carrier gas, the highest frequency of operation is obtained at the lowest concentration of an analyte.

Experimental results presented show that useful operating conditions are obtained in both fixed-frequency and closed-loop pulsing schemes. Useful polarity inversions for some noble gases are exhibited in the fixed-frequency mode that may be utilized to eliminate "w"-shaped peaks and to quantify high concentration analyte responses when the dc mode of operation either breaks down or yields unusable results. The closed-loop operation is shown to exhibit comparable sensitivity to dc conditions, yet prevents spontaneous breakdown of the detector and extends the useful dynamic range to 1% mixtures, which extends the upper limit by a factor of 50 over conventional usage.

The overall effect of pulsed operation has been to improve stability and increase the dynamic range. Additionally, the inherent sensitivity of the detector has been maintained and, in certain fixed-frequency cases, increased over dc operation.

U.S. patent numbers 4,705,947 and 4,705,948 have been received for pulsed helium ionization detection in fixed-frequency and constant-current pulsed mode.

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