The SISCOM Procedure

The SISCOM procedure developed by Henneberg and Weimann (Max Planck Institute for Coal Research, Miilheim a. d. Ruhr, Germany) was targeted to structural determination in industrial MS laboratories. The procedure is commercially available in the form of MassLib in association with various libraries. Its primary goal is to give the most plausible suggestion for the structure of an unknown component and offers information to support the interpretation of the spectra from a comprehensive knowledge base, that is, the collection of spectra which have already been interpreted the Search for Identical and Similar Compounds. 

The libraries used for the SISCOM search are divided into 14 sections of ion series. The assignment of a reference spectrum to one of these sections is based on the strongest peak in the ion series spectrum. Ion series spectra consist of 14 peaks. Each of these peaks (or ion series) represents the sum of all the intensities of the original spectrum and with a mass of m/z modulo 14 = . Thus, for example, the ion series 1 corresponds to the sum of the intensities of the masses 15, 29, 43. The strongest ion series in the spectrum ofthe unknown substance decides which section of the library is to be used for further searching. As the ion series spectra of related compounds (similar structures) are almost identical (even when the intensity ratios in the spectra differ), the ion series spectra are particularly suitable as a filter for a similarity search. 

NC The number of common characteristics contained in both the unknown spectrum and the reference spectrum. High NC values indicate similar structures. 

NR The number of characteristics from the reference spectrum which are not present in the unknown spectrum. NR is a measure in relation to NC of the extent to which the reference spectrum is represented in the unknown spectrum or is part of it. 

NU The number of characteristics in the unknown spectrum which are not present in the reference spectrum. NU gives the part of the unknown spectrum which is not explained by the reference spectrum. Contamination or a mixture may be present. 

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Figure 3.36 Automatic peak purity control (deconvolution) sho ing the relevant mass chromatograms ith m/z values on co-elution using the SISCOM procedure (MassLib, Chemical Concepts).

SISCOM provides a powerful automatic mixture correction. The subtraction procedure is based on the assumption that specific ions exist for a substance which is removed. These ions disappear completely if the exact percentage proportion is removed. If larger proportions are removed, negative peaks are formed. The mixture correction uses an iterative procedure based on these, which is orientated towards the sum of the negative intensities. 

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