Fig. 4.12 The main steps in PyMS (redrawn from Goodacre, 1994).
PyMS mass spectra provide relatively crude differentiation of micro-organisms if limited to fragment mass/charge ratio (m/z) and intensity. Typically, a range of fragments with a m/z between 50 and 150 provide a quantitative spectral fingerprint (see Fig. 4.13). However, interpretation is not easy and the spectra are frequently qualitatively similar for different genera or species. The small but significant differences in mass intensities require sorting and 'reduction' using multivariate statistical techniques such as principal components analysis (PCA) together with the closely related, canonical variates analysis (CVA). These treatments enable samples to be grouped and differentiated between. Further analysis enables the construction of a 'percentage similarity matrix' and hierarchical classification to form dendrograms (Fig. 4.14) that enable the relationships between different strains to be visualised.
Ion separation Electron multiplier
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