Spectral Similarity Search Techniques

MassLib is not only applicable to spectrum similarity searches. The system contains an MS-specific coding technique for chemical structures based on 160 structure descriptors. This enables the spectroscopist to apply chemometric techniques to analyse the results of spectral similarity searches in the structure space. 

The various match factors calculated by the matching program are listed in Table I. The overall match factor (PT) is a combination of forward and reverse searching techniques. It takes into account the deviations in intensity of the sample spectrum peaks with respect to the candidate spectrum peaks and vice versa for all peaks in both spectra. The pattern correspondence match factor (PC) is a forward searching match factor which takes into account the intensity deviations of sample spectrum peaks with respect to the candidate spectrum peaks for peaks common to both spectra. This factor detects structural similarities, such as substructures, based on common spectral patterns. NC, NS, and NR give an indication of the number of peaks upon which the match was based and in which direction it was most successful. IS and IR indicate the magnitude of the ion current unmatched in each direction. These match factors are similar to those proposed by Damen, Henneberg, and Wiemann (9). 

Finally, the hit quality index does not provide any absolute measure of the probability that the sample actually is the same as the library sample. The arbitrary scale of the hit quality values (0-1) does not give a very good statistical measure of the similarity of the spectra. In short, using only a single training spectrum to represent all possible samples in the future does not give the analyst any statistical assurance that the spectra are truly the same or different. It provides only a relative measure for all the library samples. For anyone who has tried simple library search techniques for spectrally similar samples, this result is all too obvious. 

Cosine correlation analysis (CCA) is a multivariate technique that assesses similarity in spectral datasets and image datasets [52]. Other similarity analysis techniques include Euclidean distance [49] and spectral angle mapping [53] which have been used for spectral library searching and image contrast generation in remote sensing applications, respectively. 

In an identity search, a similarity metric that best discriminates between identity and nonidentity should be chosen, However, it should be noted that even if the compound whose structure is to be verified is present in the reference database, an exact match of spectral data may be unrealistic. There are intrinsic instrumental limitations, possible contaminants in the compound under study, differences in sampling techniques, limitations in the discriminatory power of the similarity metric used, and, possibly, unreliable reference data. Thus, the matching tolerance requirement for assigning structure must reflect these circumstances. Although no sharp boundary exists between an identity and a similarity search, in practice they are considered to be different and serve different purposes (see Section 3.2). 

Finally, for routine applications, our software provides a database management system called BASIS for storage and manipulation of chemical information. BASIS can access generally available spectral libraries from three different spectroscopic techniques (MS, H-NMR and F13C-NMR, IR), and permits the creation of new libraries. For structure elucidation and substructure search of unknown compounds, library search algorithms allow the retrieval of identical and structurally similar spectra. 

Since the quantitative composition analysis of a copolymer relies on the use of relative peak intensities, the mass spectra pattern variation can clearly alter the composition results. The EO/PO copolymer examined in these investigations is composed of structurally similar monomers, but for other types of copolymer, with greater variability in monomer structures, even larger spectral variations are expected. It is clear that a better understanding of how experimental parameters affect the mass spectra pattern will facilitate the search of conditions under which reproducible results can be obtained. Once the reproducible spectra can be obtained, it is then possible to explore the use of other techniques such as NMR to correlate the relative peak intensities in MALDl mass spectra with actual copolymer composition. The validated MALDl method should provide a means of accurate and rapid analysis of copolymer composition. 

All these spectrometers are suitable for spectral scanning and searching for resonances of unknown frequency. If the frequency is known then pulse techniques, similar to those used in NMR, may be employed [19]. These give accurate line-shapes and, as for NMR, can be used also to measure relaxation times. 

Various infrared spectral databases or libraries are available, which contain collections of the infrared spectra of a number of specific chemical species. Spectral search or data retrieval is a technique enabling one to identify a material of unknown origin by comparing its spectrum with library spectra, or to make a guess at the chemical structure of the unknown material from the similarity of its spectmm to some library spectra. 

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