GB1248761A - Improvements in or relating to spark source mass spectrometry - Google Patents

Abstract

1,248,761. Ion sources; mass spectrometers. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. 8 April, 1969 [8 April, 1968], No. 16742/68. Heading H1D. [Also in Division G3] In a method of mass spectrometry which comprises vibrating at least one of two electrodes in the ion source of a spark source mass spectrometer, e.g. as described in Specification 1,161,431 (Division H1), and in which an ion-producing discharge current is caused to flow across a gap between the two electrodes such that the mean value of the size of the gap is varied over an optimum range by adjusting the relative mean positioning of the electrodes, there is derived from the discharge current a smooth electrical signal representative of the mean value of the discharge current, from which is determined an optimum adjustment of the electrode positioning to provide a gap size within the optimum range. The discharge current is detected by a ferrite ring and sensing coil arrangement and the induced voltage signal is applied to a half-wave rectification and smoothing circuit, Fig. 4 (not shown), the resulting smoothed output voltage V varies as the electrode gap is varied and peaks at the optimum gap size. This voltage, or a fraction of it, may be displayed on a voltmeter, for example, to guide the operator of a remote manual control, which may superimpose a direct voltage bias on the coil controlling the vibrating electrode. Thus the electrode gap, which varies with time due to electrode erosion, may be adjustably maintained within the optimum range. Alternatively, the gap size may be automatically controlled by a circuit arrangement (see Division G3) which detects variations of the voltage V from the maximum and produces a suitable bias signal for applying to the coil of the vibrating electrode. Preferably in this arrangement the bias signal is caused to vary cyclically so that the mean electrode position sweeps repeatedly across a range within the optimum gap range. The sweep time may be of the order of 20 sees. for a complete cycle. Details of both a manual and an automatic method of starting an analysis, in which an optimum range sweep is used and which ensures that the initial direction of sweep brings the mean gap size within the optimum range, is also given.