The use of a concentric flow sheath gas of argon or helium for a helium microwave-induced plasma (MIP) shows useful characteristics for gas chromatographic detection, by virtue of minimizing direct contact of reactive products formed in the plasma with the cell wall. Such MIPs show fewer emission bands than the capillary-type MIP, argon sheathing being more effective at much lower flow rates than helium. This plasma has a notable cost advantage, while losing little in elemental sensitivity for the plasma. Comparisons of chlorine detection are made for Ar–He and He–He plasmas.