Mass spectrometer combination and ion source subcombination therefor. The mass spectrometer has a magnetron ion source, a quadrupole ion analyzer and an ion collector with digital counting read-out and this is useful for residual gas analysis, partial pressure analysis and chemical composition determination and is particularly suited for ultra-high vacuum usage yielding high sensitivity and improved signal-to-noise ratio, consistent with low weight and bulk. The ion source is constructed so that photons are not produced at those loci which can be viewed through the ion exit aperture thereof. Thus, the spectrometer collector is free from impingement thereon of light energy and the signal-to-noise ratio of the instrument is improved thereby.