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A method of ratio recording for lead isotopes in mass spectrometry Stacey, John Sydney

Abstract

Ratio recording methods in mass spectrometry have hitherto been successfully used in analyses involving only a few of the lighter elements. In all these cases a great improvement in precision has resulted. This thesis describes a means whereby grid ratio recording can be extended to the precise measurement of complex spectra in the high mass range. In particular, lead isotope studies in geophysics provide a great and continuing need for increased analytical precision. The project has involved the construction of a complete mass spectrometer which incorporates two main features which have not been used before in mass spectrometry. The first of these is a scanning mechanism which moves the collector across the stationary ion beams. It is this innovation which has made possible the development of grid ratio recording. Since the total ion beam is stationary, variations in its intensity during the scanning process can be monitored accurately. A servo-divider system is arranged to make the measuring system Insensitive to variations in the total ion beam by recording the individual ion beam currents as fraction of the total Ion beam. The ratio system will effectively reduce large variations in the total Ion beam due to changes in source conditions by a factor of fifteen. Smaller variations are reduced to below the noise level existing in the recording system itself. The second principle which is believed to be new to mass spectrometry is the method by which the results of an analysis are presented by the instrument. An expanded scale system of recording has been developed whereby the amplitude of an ion current is presented as a digit on a counting tube, with the remainder displayed as a peak on a chart recorder. It is demonstrated that the expanded scale arrangement will record a known signal with a standard deviation of less than 0.02%.

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