Design and construction of ionization chamber detector for radio-gas chromatography

Abstract

This system of ionization chamber simplifies the analysis by providing simultameous gas chromatography and continuous radioactivity analysis from which specific activity data may be calculated.
The author manufactured the ionization chamber as a detector for radio-gaschromatography and examined the many characteristics of the equipment. This equipment consists of the ionization chamber, thermostatic oven contained the chamber, and the vibrating reed electrometer. Three kinds of chamber, 270 ml, 150 ml, and 70 ml respectively, were used in the experiment. The system provides a extensive range of operating conditions. Oven temperature of ionization chamber can range from ambient to as high as 300 °C, easily accommodating high boiling compounds such as fatty acid esters, amino acid derivatives and steroids. Activities can be measured from 2 mμ curies ¹⁴C (74 dps) or 10 mμ curies (370 dps) to many millicuries per component.
Excellent resolution is retained by the purge system shown in the system flow passage diagram (refer to fig. 4) in which a purge gas is introduced into the stream prior to its entering the ionization chamber. The purge gas flow may be varied to adjust the turnover time in the chamber. With the 270 ml chamber, the time for complete replacement of the chamber volume can be varied from approximately 1 minute to 20 minutes.
The purge gas system offers an important merit in that one may select a desirable filling gas for the chamber. The variation in specific ionization (electron volts/ion pair) and density of gases affect the amount of current and chamber efficiency respectively. By using a gas of high density having a low specific ionization (e.g. argon, argon +CH₄), the high resolution can be achieved at little or no loss in sensitivity.
The high temperature ionization chamber is specially designed to retain the high sensitivity and precision of the ionization chamber method at elevated temperatures. The 270 ml volume offers the most desirable compromise in size when the properties of both ¹⁴C and ³H, the most common tracer isotopes, are considered. The chamber is constructed of stainless steel and is electrically insulated from the oven by insulators on the gas inlet and outlet lines. The chamber base includes a rigid line connector to and adapter assembly on which the chamber electrode mounts at one end and is connected to the electrometer input at the other. The electrometer input is thermally isolated from the oven by rigid line. The vibrating reed electrometer has the high sensitivity and stability essential for radioactivity measurements of ¹⁴C and ³H which involve detection of currents less than 10¹⁵ amperes. Input resistor of the vibrating reed electrometer used is 10¹² Ω in maximum. Dynamicrange of chamber is 10³. Data are given for steroid and higher fatty acid methyl esters.