Methods for in situ QMS calibration for partial pressure and composition analysis

The expectation of users of small quadrupole mass spectrometers (QMS) is that the partial pressures reported can be related to actual partial pressures present. To achieve this, a calibration of the QMS is needed followed by verification over time by analyzing a test mixture appropriate to the process. Four in situ calibration methods are presented: 1. For UHV base pressure and low pressure processes, an open ion source QMS is recommended with a local viscous-flow reference mixture pumped by the process vacuum system; this gives a repeatable composition of partial pressures for calibration or verification. 2. For XHV RGA an ultra-low flow rate reference gas is proposed. 3. For processes where pressure reduction by sampling system is needed, a closed ion source (CIS) QMS with a similar viscous-flow reference mixture flowing directly into the CIS is recommended for calibration. 4. For processes near atmospheric pressure, capillary sampling with a molecular leak to the CIS is recommended plus a separate sampling capillary to a reference mixture. The gas dynamics to assure known compositions is presented together with data showing QMS accuracy and stability. Repeated measurement of the reference mixture over days generates a quality assurance plot revealing changes in calibration and provides the basis data for adjustment of QMS calibration.