A GAS INLET SYSTEM FOR QUANTITATIVE MASS-SPECTROMETRY

A gas inlet system for quantitative mass spectrometry is described. The principle is to make the gas pass through the ion source using tubing terminating close to the ion source. With molecular flow conditions in the tubing a directed, reasonably collimated beam is achieved giving rise to a local pressure increase in the ion source. This local pressure increase is due to molecules that have not interacted with the chamber walls, and can easily be obtained for each gas species of interest by taking signal differences or by lock-in techniques if a shutter/chopper is positioned between the gas sample tubing and the ion source. The traditional plagues of quantitative mass spectrometry, namely background gases and wall gas-exchange effects are thus eliminated. For stable gas species this gas inlet system offers essentially the same advantages as a molecular beam gas inlet, but at much lower cost and complexity and requiring much smaller gas samples for analysis. For example, no differential pumping is required. The properties and performance of the gas sampling system are analysed theoretically and demonstrated by experiments. Good quantitative agreement is obtained between theory and experiment. It is shown theoretically, and verified experimentally, that the ratio, R, between the local pressure rise in the ion source, due to the directed beam, and the average mass spectrometer (MS) chamber pressure varies as R almost-equal-to S M1/2 where S is the pumping speed and M the molecular mass number. As a specific example it is demonstrated how the water vapour content of a gas sample can be correctly measured even in the presence of a high water background in the MS vacuum chamber. It is also shown how the gas inlet device can be used as a simple tool to measure pumping speeds in vacuum systems.