MEASUREMENT OF POSITION-DEPENDENT DETECTION EFFICIENCIES FOR 2 QUADRUPOLE MASS SPECTROMETERS WITH ELECTRON-IMPACT ION SOURCES

Measurements have been carried out of the position-dependent detection efficiencies of two quadrupole-type commercial mass spectrometers with electron-impact ion sources. Contours of constant detection efficiencies for different ion source parameters were determined by scanning a collimated argon beam of 1-mm diameter across the ion source. The measured results show clearly how different ion source settings influence the detection efficiency and the size of the active volume within the ion source. Furthermore it is found that the optimal performance of the instrument in the molecular-beam mode as well as in the residual gas mode occurs at the same ion source settings. The absolute value of the detection efficiency, for this particular ion source setting, is D(max) = 0.88 x 10(-5). A simple model is developed connecting the conventional sensitivity factor with the measured position-dependent detection efficiency contour plots. The agreement between the measured and calculated sensitivity factors is good.