Paper VT-WeA1
Vacuum Pumping Requirements for Miniature Mass Spectrometers

Wednesday, October 17, 2007, 1:40 pm, Room 618

A trend in mass spectrometers (MS) for gas analysis is miniaturization to occupy less volume when attached to vacuum chambers and additionally to decrease weight and power consumption for portable and space probe instruments. This presentation identifies the pumping system requirements for different MS applications. When sample impurities like H₂, H₂O, CO, N₂, O₂ and CO₂ are being analyzed, a lower base pressure (e.g. <10^-8 Torr) is required for low background ion currents from MS outgassing resulting in a low detection limits for these species in the sample. For analyzing hydrocarbons and other species with masses greater than 44, a higher base pressure (e.g. 10^-6 Torr) can be tolerated if the hydrocarbon background is kept low by initial cleaning and operating methods. Factors dictating the operating pressure of the MS and the resulting gas throughput are presented together with scaling rules for miniaturization. Given the pumping requirements, a high vacuum pump can be selected or adapted. A comparison is made of throughput and capture pumps for MS applications focusing on the end use of the MS as a process monitor or a field portable instrument. Examples from literature are given. Finally the pumping system for gas sampling and pressure reduction to the MS is addressed.