Abstract: A method for measuring trace amounts of water in an atmosphere. A dry inert gas is used as the carrier gas and a dry gas comprising about 20 percent oxygen and about 80 percent inert gas is used as the reagent gas. The ionization source is operated at atmospheric pressure. A small electric field of about 200 volts per centimeter is applied across the ion drift chamber and it is biased to repel negative ions to a quadrupole mass filter. Measurements at an m/e value of 52 are integrated over a period of time. The results of the integration, when compared to a calibration chart, indicate the amount of water that had been present in the atmosphere being measured.