Abstract: A device for determining small amounts of impurities in gases comprises a measuring cell in which an arc is generated with a low frequency alternating voltage source between two electrodes. The emission spectrum of the arc is observed by means of interference filters and photosensors and the concentrations of the impurities are determined in the electronic evaluation unit for the spectral intensities typical of the impurity traces. The device is suitable for the continuous measurement of small quantities of gaseous impurities for example traces of nitrogen, argon, neon, water vapor, gaseous hydrocarbon, in helium.