Spectral Library Searching

Visually comparing unknown and reference spectra to each other to aid in interpretation is part of infrared spectroscopy. In the days before personal computers, collections of reference spectra called spectral libraries were plotted on paper and 

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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 three closest matches from spectral library searching are shown below each sample spectrum. The library search results indicate that the clear outer film is an aromatic polyester. This is most likely PET or an ethylene terephthalate/ethylene isophthalate copolymer. No significant spectral... 

The importance of an appropriate transformation of mass spectra has also been shown for relationships between the similarity of spectra and the corresponding chemical structures. If a spectra similarity search in a spectral library is performed with spectral features (instead of the original peak intensities), the first hits (the reference spectra that are most similar to the spectrum of a query compound) have chemical structures that are highly similar to the query structure (Demuth et al. 2004). Thus, spectral library search for query compounds—not present in the database—can produce useful structure information if compounds with similar structures are present. 

Stein, S. Scott, D.R. Optimization and Testing of Mass Spectral Library Search Algorithms for Compound Identification. J. Am. Soc. Mass Spectrom. 1994, 5, 859-866. 

Mass Spectral Library Searches. J. Am. Soc. Mass Spectrom. 1994, 5,316-323. 

The study of fragmentation processes has led to semi-empirical rules used for compound identification. Interpretation from first principles is also used to validate results obtained by spectral library searches. 

Volatile profiles of raw and cooked-beef flavor samples, prepared by the procedures of Figure 1, were obtained after capillary GC and FPD. Although the identification of these sulfur containing compounds is as yet incomplete, the chromatograms demonstrated that there were a number of new sulfur compounds produced on cooking that were not present in the raw beef. Three prominent sulfur compounds were identified as markers in subsequent meat flavor deterioration experiments, namely, methional (13.2 min), methyl sulfone (13.8 min), and benzothiazole (25.3 min). Each compound produced an adequate mass spectrum for spectral library search and positive identification. 

The peak identification in Figures 7.1.1 and 7.1.2 was done based on mass spectral library search. A list of some characteristic mass peaks, most of them found in both chromatograms, are given in Table 7.1.1,... 

Some differences are also seen in the El spectra for the same type of compound but generated from a different initial monosaccharide. However, these differences are sometimes more difficult to interpret, as the identifications rely on mass spectral library searches. The spectrum for tri-TMS 1,6-anhydroglucopyranose was shown in Figure... 

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. 

A list of peak identifications for the chromatogram shown in Figure 16.2.3 is given in Table 16.2.7. The identifications were done mainly using mass spectral library searches. 

When the monomer ratio in a copolymer increases, the contribution to the pyrolysate also increases. However, the yield of different pyrolysis products depends on the nature of the polymer. In addition to quantitative information, as shown in Chapter 4, structural information can be obtained from pyrolysate. One example in this direction is that of a poly(ethylene-co-methyl acrylate), CAS 25103-74-6, (with butylated hydroxyethyl-benzene inhibitor). A sample with 21.5% wt. methyl acrylate (MAc), with M = 79,000 and Mn = 15,000, pyrolyzed at 600° C in He with the separation on a Carbowax column generates the upper trace of the two pyrograms shown in Figure 6.1.11. The lower trace, displayed for comparison, is that of polyethylene. The peak identification for the pyrogram of poly(ethylene-co-methyl acrylate), with 21.5% wt. methyl acrylate, shown in Figure 6.1.11 was done using mass spectral library searches only, and Is given in Table 6.1.7. 

The peak identification for the chromatogram shown in Figure 6.1.25 was done using MS spectral library searches. This identification is not always possible, since most compounds with a higher MW are not found in the commercial mass spectral libraries (such as NIST 2002, Wiley 7. etc.). The similarity between the spectra in each series of compounds can be used for peak identification, even when the compound is not found in the mass spectral library. This is exemplified in Figure 6.1.26, which shows the spectra of the B series of compounds shown in Table 6.1.10. 

A pyrogram for this polymer is shown in Figure 6.1.27. The pyrolysis and the separation of the pyrolysate were done in identical conditions as for the other polymers previously discussed, using 0.4 mg material pyrolysis at 600° C in He with separation on a Carbowax column (see also Table 4.2.2). The peak identification for the chromatogram shown in Figure 6.1.27 was done using MS spectral library searches and is given in Table 6 1.13. 

The peak identification for the chromatogram shown in Figure 6.2.10 was done using MS spectral library searches only and is given in Table 6.2.7. 

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