A method for the mass spectrometric quantitative determination, down to the sub-ppb level, of the He-4 content in the gaseous products of cold fusion Pd+D2O electrolytic cells is described. The data collected in an experiment lasting over 1000 h provided a time picture of the phenomena. Measurements of the Ne-20 content have also been performed in order to check air contamination. The methods and controls employed are suitable for use with non-dedicated mass spectrometers which cannot be operated in vacuum static conditions.