PURPOSE: To stably and quickly make the qualitative and quantitative analysis of alkaloid with high sensitivity by treating a gaseous sample with a silylating agent then separating the same in a gas chromatography, bringing the sample into contact with metallic platinum to decompose the gas and measuring the ionization current of the cracked gas.

CONSTITUTION: The gaseous sample discharged from a sepn. column 2 is mixed with the air introduced from a pipe 5 through a T-joint 3 into the column. The gaseous sample mixture flows from the top end of a quartz nozzle 4 into an ionization chamber 7 where the gas contacts with a heated electrode 15 and is thermally cracked. An amine compd. is ionized to form positive ions. The positive ions are captured by a collector electrode 11 and are amplified by an electrometer 18, by which the alkaloid is detected.