PURPOSE: To decrease optical disturbance by gallium and to decrease the limit for detecting impurities by decomposing the semiconductor by a decomposing and settling liquid to settle and separate gallium then determining quantitatively the impurities in the supernatant thereof.

CONSTITUTION: The decomposing and settling liquid consisting of, for example, 40vol% ammonia water, 14vol% hydrogen peroxide and 46vol% water is added to the semiconductor contg. gallium arsenide doped with impurities such as potassium, chromium and silicon. The gallium arsenide decomposes and the metallic gallium settles when the liquid is held for about 24 hours. The supernatant thereof is taken out into a graphite furnace and is subjected to a flameless atomic-absorption spectro photometry by an ordinary method to make quantitative determination. The greater part of the gallium is already separated by the decomposition and and settling and therefore the optical disturbance is considerably decreased. The limit of detection is thus decreased and the execution of efficient analysis is made possible.