Library search techniques

Qian, K. Killinger, W.E. Casey, M. Nicol, G.R. Rapid Polymer Identification by In-Source Direct Pyrolysis Mass Spectrometry and Library Searching Techniques. Anal. Chem. 1996, 68, 1019-1027. 

Chromatographic interfaces are based on three common approaches the flow-through cell (light pipe) and solvent elimination with either matrix isolation or cold trapping [2,198,201]. Flow-through cells provide a simple and convenient interface for GC-FTIR, since typical mobile phases are transparent in the mid-infrared region. Mobile phase elimination interfaces are used primarily to increase sensitivity, and to obtain high-resolution or condensed phase spectra, for improved confidence of identification by library search techniques. Vapor phase spectra have characteristic broad absorption... 

K. Qian, W. Killinger, and M. Casey, Rapid polymer identification by in-source direct pyrolysis mass spectrometry and library searching techniques. Anal. Chem., 68 1019-1027 (1996). 

Library searching techniques are of two kinds identity searches and similarity searches. Identity searches (i.e., searches requiring an exact match between the query and the reference) are successful only if the sought spectrum or structures are already in the reference library. If not, the search fails completely. In contrast, similarity searches depend on the metrics introduced into the representation of the spectra or structures. The metrics and consequently the distance or the similarity measure between the query object and the reference depend on the problem being studied hence, the results obtained on the basis of similarity are not directly comparable when the problems are from different fields. 

In addition, the potential of combined GC mass spectrometry (GC-MS) for determining volatile compounds, contained in very complex avor and fragrance samples, is well known. The subsequent introduction of powerful data acquisition and processing systems, including automated library search techniques, ensured that the information content of the large quantities of data generated by GC-MS instruments was fully exploited. The most frequent and simple identi cation method in GC-MS consists of the comparison of the acquired unknown mass spectra with those contained in a reference MS library. 

In addition, the spectral library search technique is sensitive only to the general spectral shapes and patterns and not to very subtle variations... 

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. 

In the past, PTRC screening was mainly based on gas chromatography-mass spectrometry (GC-MS) [116]. The choice of GC-MS was based on a number of good reasons (separation power of GC, selectivity of detection offered by MS, inherent simplicity of information contained in a mass spectrum, availability of a well established and standardized ionization technique, electron ionization, which allowed the construction of large databases of reference mass spectra, fast and reliable computer aided identification based on library search) that largely counterbalanced the pitfalls of GC separation, i.e., the need to isolate analytes from the aqueous substrate and to derivatize polar compounds [117]. 

Access to the largest library catalog, that of the Library of Congress, is available at http //catalog.loc.gov. This page explains the different kinds of catalogs and searching techniques available. 

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. 

The spectrum is examined first. Before starting any interpretation, it is strongly recommended that a computer or a manual library search is performed to check whether this spectrum belongs to an existing collection. Identification of an unknown compound in this way depends directly on the quality and comprehensiveness of the collection used. However, only libraries of electron ionization spectra are efficient. Other ionization techniques yield spectra that are much too dependent on the instruments and experimental conditions. 

There has been a rapid proliferation of commercial search interfaces, such as DataStar and STN International. STN may deserve special mention since it is a science information service operated by Chemical Abstracts Service (CAS) in North America, Fachinformationszentrum-Karlsruhe (FIZ-K) in Europe, and Japan Association for International Chemical Information (JAICI) in Japan. It is available as online subscriber access to 220 databases (STN Express) and as web access (STN on the Web for experienced searchers and STN Easy for infrequent searchers). See www.cas.org/stn. html and/or www.stn-international.de/. Since Internet search techniques depend on the interface available, no detailed explanations are given here. There are several published guides to database searches for chemical and physical properties, but the situation is evolving rapidly and it is recommended that you consult a reference librarian for information about the current situation concerning search interfaces at the library you use. 

The desk top computer, via the Internet and such US compilations as delphion.com, or the various national patent office compilations, notably www.patent.gov.uk/, esp cenet and www.uspto.gov/patft/index.html/, provides access to patents, greatly aided in the author s case by the British Library paper and electronic systems. Patents provide useful literature references and access to the inventors basic thinking, right or wrong Patents, the author has found, are full of mistakes, inconsistencies and contradictions, which enable the reader to learn by comparison, and to sort out the quintessential information. See Appendix C, for an elementary patent search technique via the US Patent Office. 

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