A two-chamber electron impact emission sensor effective for monitoring vapor flux of materials in the presence of interfering species is described. The sensor includes two independent electron excitation regions and one photodetector for monitoring emission from excited species from both chambers. Copper vapor flux from an evaporation source was accurately measured in the presence of interfering H2O vapor, and Ga vapor flux from an evaporation source was accurately monitored in the presence of interfering CO2 gas. The invention permits deposition rates to be monitored using electron-impact emission spectroscopy with significantly improved accuracy in the presence of interfering gases at high partial pressures.