Spectroscopic and analytical characteristics of a low power atmospheric pressure mixed argon–helium microwave induced plasma were examined for its usefulness in the determination of As, Sb and Hg by optical emission spectrometry and using a continuous flow chemical vapor generation of these elements (AsH3, SbH3, Hg0) in the reaction with sodium tetrahydroborate. The effect of the concentration of helium in the plasma gas on excitation, vibrational and rotational temperatures and the electron number density in the discharge obtained from argon and mixtures of argon with helium was studied to evaluate changes in excitation conditions and their impact on the analytical performance of the plasma. It was found that the presence of 10–20% v/v of helium in the carrier/plasma gas is very convenient for the excitation of As, Sb and Hg introduced to the plasma via the volatile species generation reaction. In reference to limits of detection of 4.1 (As), 1.8 (Sb) and 1.4 (Hg) ng ml−1, such a plasma has a considerably better analytical performance as compared to the plasma generated only in an argon atmosphere.