Library matching

The mass spectrometer should provide structural information that should be reproducible, interpretable and amenable to library matching. Ideally, an electron ionization (El) (see Chapter 3) spectrum should be generated. An interface that fulfils both this requirement and/or the production of molecular weight information, immediately lends itself to use as a more convenient alternative to the conventional solid-sample insertion probe of the mass spectrometer and some of the interfaces which have been developed have been used in this way. 

Figure 30 Infrared spectrum of the injector contaminant ( black goo ) along with the three best library matches.

Si element ATR-FTIR spectroscopy was used to analyze this residue, and its spectrum, along with the closest library matches, are shown in Figure 41. The absorbance of this residue is low as a consequence of the thin layer present on the plate. This makes matching the sample spectrum with a reference spectrum somewhat difficult. The closest matches extracted from the library interrogated are to ester-based plasticizer materials, which is consistent with a phthalate-plasticized PVC. A more specific identification could have been made with further testing such as subjecting the residue to GC-MS analysis, but the information suggested by the ATR-FTIR analysis was, in this case sufficient. 

Figure 41 Si ATR spectrum for the residue on the polished metal plate (top) along with the three closest library matches. 

The ATR-FTIR spectrum of the middle opaque polyethylene layer of the "bad" sample is shown in Figure 70. This spectrum was acquired from the fracture surface where the outer polyester film and tie layer delaminated from the polyethylene layer. The highest-scoring library match in Figure 70 indicates that the middle layer is a polyethylene with a low branch content, most likely a HDPE or a LLDPE, although a much more detailed spectral analysis would be required to confirm this. 

Of course one may employ automated library searches ( library percent reports ) to check for compound identities, but algorithms for library matching are not infallible, and mass spectral libraries are not exhaustive, thus some compounds of interest will likely not be identified. Additional dilemmas are presented by mere reliance on retention times and library percent reports to ascertain the presence of common or unique peaks from among multiple mass spectral data files. As illustrated in Table 2.1, the TICs from the GC-MS of urine from four elephants evidence a peak at essentially the same retention time, but the library search results are inconclusive as to their common identity or lack thereof. As will be seen below, our novel macros can assist in making such decisions for a large number of peaks. 

In Figure 8.14, the Cold El mass spectrum of corticosterone in methanol solution is shown in the upper trace, and is compared with the standard NIST 98 El library mass spectrum shown in the lower trace. Note the similarity of the library mass spectrum to that obtained with the SMB apparatus. All the major high mass ions of m/z 227, 251, 269, and 315 are with practically identical relative intensity and thus good library search results are enabled with the NIST library-matching factor of 829, and the reversed matching factor of 854% and 86.5% confidence level (probability) in corticosterone identification. In addition, the molecular ion at m/z 346 is now clearly observed while it is practically missing in the library (very small in the shown mass spectrum and absent in the other three replicate mass spectra). 

FIGURE 8.14 A comparison of the cold El mass spectrum of corticosterone obtained with the supersonic LC-EI-MS system and its fitted NIST library mass spectrum, including the NIST library-matching factors and the probability of identification. Note the enhanced molecular ion (m/z = 346) exhibited. The NIST library-matching factors and the probability of identification are included. Corticosterone was flow injected using a 200pL/min methanol solvent flow rate with a 1 ng/pL corticosterone sample concentration. (Reproduced from Amirav, A. et al, Rapid Commun. Mass Spectrom., 15, 811, 2001. Copyright 2001. With permission from Elsevier.)... 

Figure 15.6 shows the separation achieved for the essential oil of Coriandrum sativum using GCxGC. The identity of the compound was elucidated and confirmed primarily from the MS library matches as well as by comparing the first-dimension retention index with reference libraries. GCxGG-TOFMS allowed the identification of 81 compounds, compared with only 41 compounds identified by conventional GG-qMS. 

FIGURE 15 GC-MS chromatograms of the static ethanol extracts of Santoprene tubing materials (underivatized). The chromatograms from these two Santoprene materials were quite different from those of the silicone materials (Figure 14). IS = internal standard (dimethyl phthalate). See Table 39 for the tentative peak identifications. Only those peaks with recorded spectral library matches are noted for each sample, although retention times and patterns may suggest some additional peak identifications [78]. 

Fig. 6 El spectrum from an unknown elastomer extractable (A), with a best fit computerized library match (B).

With the DAD, spectra can be acquired automatically for each peak during the analysis. The spectra can be compared with those stored in a hbrary, either interactively on the computer display or mathematically with the help of microprocessors. In Fig. I, the spectral library matching is illustrated. The standard sample is analyzed and the spectram of the target compound is registered in the library. The spectram of the unknown sample is compared to standard spectra in the library. 

The advantage of GC/MS over LC/MS is that extensive libraries of El data are available for searching (see Section 9.10.4.3.4). Where necessary, this can be complemented by molecular weight data from Cl. Library identification then requires confirmation by comparing the column retention time and MS and MS data with that of a standard. If no library match is found, then a similar process of determining elemental formulas (see Section 9.10.4.3.3) and interpretation of the fragmentation data from first principles must be followed (see Section 9.10.4.3.4). 

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