Paper VT-MoM6
Sampling System Design to Predict Mixture Composition at a Quadrupole Mass Spectrometer Ion Source

Monday, October 21, 2019, 10:00 am, Room A213

The use of gas mixtures to measure the sensitivity and fragmentation ratios for calibration of a quadrupole mass spectrometer (QMS) simplifies the process and enables insitu calibrations. A necessary factor in a mixture calibration method is knowledge of the composition present in the QMS ion source and at the reference ion gauge. Under the conditions of molecular flow of the mixture into the ion source and molecular pumping of the outlet flow, the composition at the ion source is equal to the stated values of the mixture. This knowledge enables a composition-weighted correction of the ion gauge reading to get the true ion source pressure due to different gauge sensitivities of each species. The partial pressures of components in the ion source are calculated using the mole fraction of the mixture’s species times the corrected ion gauge pressure. The sensitivity for a specific gas species is the ratio of the ion current representing the species to the calculated partial pressure at a common time.

The gas dynamics of very low flow rate gas into the ion source from an end crimped capillary or small pore size frit from a high-pressure mixture is presented. The low flow rate produces molecular effusion and the stated mixture composition is established in the ion source. Another model for low pressure (< 10 Torr) gas introduction through an orifice predicts molecular flow into the ion source and a (correctable) species dependence of the mixture composition as a function of time as the species deplete from the sample volume. Results of QMS calibrations using these gas sources and methods are presented together with composition analyses of unknown gas samples.