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Digital analysis of mass spectra Weichert, Dieter Horst

Abstract

The purpose of this thesis is twofold. A special type of signal is studied and methods for its reduction investigated. The results of this work are then applied to the automatic reduction of the trimethyllead mass spectrum with the aim of improving the information recovery from this signal, using modern data processing techniques. The precision measurement of amplitudes must always provide some form of averaging or filtering. It is demonstrated that the results become increasingly sensitive to irregularities in the abscissa of the record, as the width of the averaging function increases. The continuity of the transition from a purely height sensitive calculation to an area sensitive calculation depends on the shape of the impulse response of the measuring arrangement. For all physical shapes, a least squares fit will lead to area sensitive amplitudes. These results have not been known in mass spectrometry or, to our knowledge, in other fields. The mass spectrum is regarded as the convolution of an ideal line spectrum and a peak shape which is determined by the characteristics of the mass spectrometer and the associated electronics. The reduction presents essentially a problem in deconvolution. Although occassionally suggested in the literature, methods involving transformation to the frequency domain were not found to be useful. Thus it is necessary to carry out the analyses entirely in the time domain. Even then, methods requiring the use of derivatives of the spectrum, which have been successfully applied in special cases, usually suffer seriously from noise. Correlation techniques are especially valuable for signals with a high noise content. Correlation is an integration in the time domain and corresponds to a general filtering process in the frequency domain. A close relationship between the correlation technique and a least squares method is emphasized. The development of a practical procedure for this laboratory was part of the study. The trimethyllead group of the tetramethyllead spectrum is recorded in digital form on paper tape. At a constant sampling rate of two per second, about fifty readings per mass interval are recorded. The reduction of the paper tape record to relative isotopic abundances of lead has been automated, applying the experience gained from the general study. The saving in reduction time over former standard methods is substantial. There seems to be a real reduction in the standard deviations of individual peak measurements, but this does not exceed a factor of two.

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