Determination of impurities in argon by gas chromatography with a microwave-induced plasma detector

Abstract

A helium microwave-induced plasma at atmospheric pressure is used as a detector for the gas chromatographic determination of gaseous impurities in argon. Either a Porapak N or a 5 Å molecular sieve stationary phase is used to separate H₂, CH₄ and CO₂ from argon. Atomic emission from carbon or hydrogen is monitored at 247.8 or 656.2 nm, respectively. Retention times and elemental contents serve to characterise the gas mixtures qualitatively and peak area is used for mass calibration. At higher concentrations of H₂, CH₄ and CO₂, either stationary phase is adequate. However, the use of the Porapak N column provides a significantly lower background at the carbon emission line, thereby yielding superior sensitivity for CO₂ and CH₄. For the Porapak N stationary phase, linear calibration ranges are observed from approximately 100 p.p.b. to 100 p.p.m. for H₂(defined as mole fractions assuming ideal gases) at 656.2 nm, and from 100 p.p.b. to 300–1000 p.p.m. for CO₂ and CH₄ at 247.8 nm. At the H l emission line, the absolute detection limits for H₂ and CH₄ are 2 and 40 pg, respectively. At the C l line, these limits are 10 and 30 pg for CH₄ and CO₂, respectively.