A spark source mass spectrometric system has been developed for trace levels of oxygen and carbon in gallium arsenide using liquid helium cryopumping of the source chamber. Following bake-out, a collector positioned inside the source chamber is cooled using a controlled flow of liquid helium. Residual back-ground gases are more effectively removed than with liquid nitrogen and detectabilities of 0.02 ppma for oxygen and 0.05 ppma for carbon have been obtained. Routine analyses are carried out with a moderate consumption of liquid helium.