Abstract: Apparatus and a method for measuring trace amounts of oxygen in an atmosphere is described. An oxygen free nitrogen gas is used as the carrier gas and nitrogen, ultra high purity, with approximately 10 ppm (parts per million) water is used as the reagent gas of an ionization source is operated at atmospheric pressure. A small electric field of approximately 200 volts per centimeter is applied across the length of the ionization chamber up to a pinhole entrance to a quadrupole mass spectrometer. The voltage is biased to repel negative ions towards the quadrupole mass spectrometer. Measurement of the peak height of the m/e value of 52 in the mass spectrum are integrated over a period of time. The results of the integration are then compared to a calibration chart which indicates the amount of oxygen that is present in the atmosphere being measured.