Measurement of heavy gases and vapors by means of a conventional RGA

A conventional residual gas analyzer (RGA) has been utilized for the measurement of gaseous substances of molecular weights far beyond the range of the instrument. The novel method, termed CPC (“crack-product calibration”), is based on the evaluation of just one or few selected crack products within the range of the RGA. Calibration is achieved by guiding a quartz-microbalance-controlled molecular beam of the substance under consideration through the RGA's ion source and, at the same time, admitting a noble gas with density defined by a Bayard-Alpert gauge (BAG) - thus anchoring the calibration of the RGA to the readings of the more stable total pressure gauge. In this way, the RGA's sensitivity can be characterized and corrected with an accuracy that is essentially limited just by the reproducibility of the BAG readings. By means of a twin-gas standard, even effects due to drift of the RGA's field axis potential can be corrected to a large extent. The basics of the method and the apparatus are described with special attention to the measurement of silicone oil with a mean molecular weight above 100,000 g/mol, and to the discussion of accuracy concerns.