Mass Spectral Search System

Heller, S. R., and Milne, G. W. EPA/NIH Mass Spectral Search System (MSSS), A Division of CIS. Washington, DC U.S. Government Printing Office. An interactive computer searching system containing the spectra of over 32,000 compounds. These can be searched on the basis of peak intensities as well as by Biemann and probability matching techniques. 

NIH/EPA Chemical Information System (1983) Carbon-13 NMR Spectral Search System, Mass Spectral Search System, and Infrared Spectral Search System, Arlington, VA, Information Consultants... 

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. 

Instrument companies have added spectral search systems as a spectrometer accessory for modest-sized files. The largest mass spectral files are accessible through the NIH-EPA Chemical Information System (CIS), and the largest IR on-line search system is available on Tymshare (196). 

Peak identification is based on the comparison of normalized spectra representative for the peak with spectra of one or several standard compounds run in the same separation system and stored in a spectral library [107,116]. This approach is less powerful than for mass or infrared spectral searches due to the rather broad and featureless bands that typify absorption spectra. Absorption spectra of similar compounds and compounds with a chromophore well separated from the variation in molecular structure are often virtually identical. Also, spectral changes dependent on the experimental conditions (pH, mobile phase composition, temperature, etc.) occur frequently. For this reason user prepared local libraries tend to predominate over general libraries, in contrast to common practices in infrared and mass spectral searches. A favorable spectral match for an absorption spectrum by itself is not acceptable for absolute identification. 

Part Mass Spectrometry by A. N. Davies gives an overwiew from a user s point of view. Commercially available mass spectral databases and software products for library searches are characterized. A statement from section 24.1 is repeated here because it seems to be essential not only for mass spectral database systems "... let the people who will be working with the systems have a major say in the testing and selection of the product to be purchased. .. . 

Working on large databases it is often desirable to restrict searches to a subset of the data. The HTSS system can perform operations on such subsets, which are defined by the serial numbers of the compounds to be included in a subtree . If these numbers are the result of a previous substructure search, all molecules in the subtree will contain this structural feature if the numbers are the results of a mass spectral retrieval system, the compounds in that subtree have a common spectral property. This feature of the HTSS system facilitates correlation studies between chemical structure and other properties. 

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. 

Figure 5.9. Spectral search at Spectral Database Systems (SDBS). The infrared (IR), nuclear magnetic resonance H-NMR and 13C-NMR), electron spin resonance (ESR), and mass (MS) spectra of organic compounds and common biochemical compounds can be viewed/retrieved from SDBS.

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. 

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. 

Part of moat mass spectral examination involves the subtraction of background spectra or of known Interferences. The data system permits subtractions in two ways. The first is a subtraction in display only, while a given data set is being examined, while the second also alters the data stored in RAN. As a valuable assist in identifying spectra, we have an NIH/EPA library and attendant search routines on-line. This library is expandable and can provide as many as twenty likely matches for an unknown spectrum. 

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). 

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. 

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