Abstract: A method for the detection and quantitative analysis of certain selected constituent parts of a gas stream. A gas stream containing the selected constituent is introduced into a reaction zone and contacted with metastable mercury (6.sup.3 P.sub.o) atoms to form an excited complex of the selected constituent and the metastable mercury atom, which decomposes emitting light. The intensity of the light emission is measured and directly correlatable to the concentration of the constituent in the gas stream. The method of the present invention is particularly applicable to the measurement of ambient air containing selected constituents in an amount or concentration in the 1-100 ppb range. The method can be used to measure trace amounts of numerous selected constituents contained in a gaseous stream including such constituents as ammonia, hydrazine, water vapor, alcohols and various amines.