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