Spectral library searching systems

The spray paint can was inverted and a small amount of product was dispensed into a 20 mL glass headspace vial. The vial was immediately sealed and was incubated at 80°C for approximately 30 min. After this isothermal hold, a 0.5-mL portion of the headspace was injected into the GC/MS system. The GC-MS total ion chromatogram of the paint solvent mixture headspace is shown in Figure 15. Numerous solvent peaks were detected and identified via mass spectral library searching. The retention times, approximate percentages, and tentative identifications are shown in Table 8 for the solvent peaks. These peak identifications are considered tentative, as they are based solely on the library search. The mass spectral library search is often unable to differentiate with a high degree of confidence between positional isomers of branched aliphatic hydrocarbons or cycloaliphatic hydrocarbons. Therefore, the peak identifications in Table 8 may not be correct in all cases as to the exact isomer present (e.g., 1,2,3-cyclohexane versus 1,2,4-cyclohexane). However, the class of compound (cyclic versus branched versus linear aliphatic) and the total number of carbon atoms in the molecule should be correct for the majority of peaks. 

The common procedure to generate silylated pyrolysates is to perform pyrolysis in a filament system followed by off-line derivatization with BSTFA. The chromatographic separation was done on a DB-5 column (60 m long, 0.32 mm i.d., 0.25 pm film thickness) using a temperature gradient between 50° C and 300° C with detection by mass spectrometry. The compounds identified by mass spectral library search in the pyrograms from Figures 12.3.3 and 12.3.4 are listed in Table 12.3.2. 

Tentatively identified compounds (TICs) Compounds detected in samples that are not target compounds, internal standards, system monitoring compounds, or surrogates. TICs usually consist of up to 30 peaks that are greater than 10% of the peak areas, or heights, of the nearest internal standard. They are subjected to mass spectral library searches for tentative identification. A client may specify the number of unknown peaks in its samples it wishes the laboratory to tentatively identify. 

Two separate teams, those of Wolfgang Bremser and Henk van t Klooster, were chosen to cover the subject of spectral databases, computer-aided library search systems and expert systems for structure analysis. 

These spectra allowed the characterisation of the additives by spectral library search and/or spectral interpretation. The usefulness of the spray-jet interface system for the coupling of size exclusion chromatography(SEC) and FTIR spectrometry was examined on the basis of the analysis of a PS standard mixture. Representative IR spectra of the SEC effluent indicated that the SEC-FTIR system could be used for the determination of compositional changes across the polymer MWD. 25 refs. 

Woodruff and co-workers introduced the expert system PAIRS [67], a program that is able to analyze IR spectra in the same manner as a spectroscopist would. Chalmers and co-workers [68] used an approach for automated interpretation of Fourier Transform Raman spectra of complex polymers. Andreev and Argirov developed the expert system EXPIRS [69] for the interpretation of IR spectra. EXPIRS provides a hierarchical organization of the characteristic groups that are recognized by peak detection in discrete ames. Penchev et al. [70] recently introduced a computer system that performs searches in spectral libraries and systematic analysis of mixture spectra. It is able to classify IR spectra with the aid of linear discriminant analysis, artificial neural networks, and the method of fe-nearest neighbors. 

A computer file of about 19,000 peak wavenumbers and intensities, along with search software, is distributed by the Infrared Data Committee of Japan (IRDC). Donated spectra, which are evaluated by the Coblentz Society in collaboration with the Joint Committee on Atomic and Molecular Physical Data (JCAMP), are digitized and made available (64). Almost 25,000 ir spectra are available on the SDBS system developed by the NCLI as described. A project was initiated at the University of California, Riverside, in 1986 for the construction of a database of digitized ffir spectra. The team involved also developed algorithms for spectra evaluation (75). Other sources of spectral libraries include Sprouse Scientific, Aston Scientific, and the American Society for Testing and Materials (ASTM). 

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. 

In the analysis of environmental samples, optoelectronic image devices allow for real time spectral acquisition and rapid identification by comparison tspectral libraries will be available for identifying HPLC eluates by computer search routines similar to those presently in use with MS and FTIR systems. 

Mass spectral quality is an important consideration in performing a successful GC-MS analysis. The quality of the mass spectra depends on (1) the concentration of the constituents in the sample solution, (2) the GC operating conditions used to resolve the peaks, and (3) excessive pressure fluctuation in the MS unit of the GC-MS system leading to distortion of the mass spectrum, especially an El mass spectrum, as reflected in the relative abundance of the ion peaks. The implication of (3) is that distortions of this type could lead to misinterpretation of the spectrum even though a library search is performed. 

Although individual laboratories find it useful to compile their own reference library files, access to very large collections of mass spectra and to published data [55] is essential. A compilation of many thousands of spectra by the Aldermaston Mass Spectrometry Data Centre and the Division of Computer Research and Technology at the National Institutes of Health [56-58] has been made available commercially. The file can be searched in a number of ways using an interactive conversational mass spectral search system via a teletype and acoustic link over telephone lines. 

The MST/EPA/NIH Mass Spectral Library 1998 database ( www.nist.gov/ srd/analv.htm) is the product of a muftiyear, comprehensive evaluation and expansion of the world s most widely used mass spectral reference library, and is sold in ASCII or Windows versions. It contains 108,000 compounds with electron ionization spectra, chemical structures, and molecular weights. It is available with the NIST MS Search Program for GC/MS deconvolution, MS interpretation, and chemical substructure analysis. The NIST chemistry WebBopk ( http //webbook.nist.gov) is a. free online system that contains the mass spectra of over 12,000 compounds (this Standard Reference Data Program also has IR and UV-Vis spectra). 

A.K.M. Leung, F.T. Chau. J.B. Gao and T.M. Shih. Application of Wavelet Transform in Infrared Spectrometry Spectral Compression and Library Search, Chcmometric Intelligent Laboratory System. 43 (1998). 69-88. 

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