GB1149426A - Improvements relating to mass spectrometry - Google Patents

Abstract

1,149,426. Mass spectrometers. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. 24 Nov., 1967 [1 Dec., 1966], No. 53876/66. Heading H1D. In a double-focusing mass spectrometer metastable mass peaks are recorded utilizing ion decompositions occurring both in the ion source 2 and in the region A between the ion source and the electrostatic analyser 5 by producing a scanning variation of the voltage ratio between the ion accelerating voltage and the electrostatic analyser deflection voltage, e.g. by producing a scanning variation of the ion accelerating voltage unaccompanied by complementary variation of the electrostatic analyser deflection voltage. The varying ion collector output is recorded in time correlation with the scanning variation. The collector output and also the voltage ratio may be periodically sampled and presented in digital form (Fig. 3) and a peak top sensing unit may be included (Fig. 5). The recording circuit may include a threshold detector responsive to the collector output rising above a set value towards a peak (Fig. 6). In a fully automated system the magnetic field of the magnetic analyser is scanned, preferably by decreasing the magnetic field exponentially, and as each peak is detected an accelerating voltage scan is made (Fig. 9). Each peak is arranged to have a flat top where a change in magnetic field produces little or no change in collector ion current, e.g. by suitable adjustment of the source and collector slits 4, 7. Provision may be made for discontinuing the magnet field scan for the duration of each voltage scan without interrupting the magnet current by use of a flux stabilizing arrangement (Fig. 10). A mass marker may also be included presenting mass identification in digital form. Examples are given of specific circuitry for suitable threshold detectors for the voltage sweep generator and for the accelerating voltage supply (Figs. 11 and 12).