Metrological characteristics of a group of quadrupole partial pressure analyzers

Abstract

Linearity and stability of the pressure response of five different commercial quadrupole partial pressure analyzers and the influence of ion source parameters (emission current, electron energy, and ion energy) on the response has been investigated over the range 10−7–10−1 Pa for He, N2, and Ar. In the 10−3–10−1 Pa range, each instrument developed a maximum (as large as a factor of 100 in one instrument) in the sensitivity versus pressure relation when operated with ‘‘low’’ ion energy (about 2–3 eV). All but one of the instruments also showed significant low-pressure nonlinearity, down to pressures as low as 10−7 Pa in one instrument, when operated at ‘‘high’’ ion energies (greater than about 7 eV). These results bring into question the often-assumed linearity of such instruments at low pressures. The dependence of the signal developed from a constant pressure (10−6 Pa) trace gas as a function of the pressure of another gas (the matrix) was studied using He and Ar as the trace and matrix, and vice versa. The results demonstrated an apparent correlation between the magnitude of this dependence, and an instrument’s nonlinearity as a function of pressure with the matrix gas alone. Brief exposures to certain active gases (O2, C3H8, CO2, CO) were found to cause shifts as large as 10% in the instruments’ inert gas sensitivities (He, N2, Ar). Tens of hours were required to return to pre-exposure sensitivities. Absolute argon sensitivity, monitored over a period of 220 days, for a standard set of operating parameters and with Faraday cup ion detection, showed initial one-directional changes as large as a factor of 2 for one instrument and a scatter of about ±20% in all the instruments. The very large differences among the five test instruments with regard to sensitivity, linearity, and stability, as well as the influence of one gas on the sensitivity to another, point out the need for careful characterization and calibration of such instruments for all except the most qualitative applications.

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