Spectral Peak Identification

These chromatographic examples demonstrate the utility of PLOT columns for the resolution of the various chemical constituent fractions of shale oil and their compatibility with interfaced IR and mass spectral peak identification. The major advantage of the higher column peak capacity of the PLOT columns makes the latter measurements more feasible particularly for the minor components in the chromatogram. 

Wallace, W.E., Kearsley, A.J., and Guttman, C.M. (2004) An operator-independent approach to mass spectral peak identification and integrahon. Anal. Chem., 76, 2446-2452. 

Table I. Mass Spectral Peaks Employed for Identification of Nitrosamines...

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. 

With the introduction of diode-array technology in the 1980s, a further dimension is now possible because coupled LC-UV with diode array detection (DAD) allows the chromatographic eluent to be scanned for UV-visible spectral data, which are stored and can later be compared with a library for peak identification. This increases the power of HPLC analysis... 

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]. 

The first positive identification of the product apparently was made by Bryce and Ingold.65 They thermally decomposed (CH3)2Hg in the presence of NO and bled the reaction mixture through a pinhole into a mass spectrometer, where they observed a mass spectral peak at... 

The capillary HPLC separation from a selected protein spot provides a base-peak profile shown in Figure 6.2A. The base-peak profile is similar to a total ion current (TIC) profile, but it contains only the most abundant mass spectral peak in each scan. The chromatogram is simplified and the contributions from background ion abundances are eliminated, resulting in an enhanced signal-to-ion ratio for an improved visualization of data. The molecular mass for each component is labeled along with corresponding amino acid residues. This format provides a comprehensive approach for peak selection and peptide identification. 

The wavelengths at which absorption or emission spectral peaks occur are characteristics of the particular atom or molecule giving rise to the peaks, and thus may be used for qualitative identification. In quantitative instrumental methods of analytical chemistry, we try to measure some property of atoms or molecules which varies linearly with the concentration of the species of interest. What parameter should we measure if we wish to exploit atomic absorption ... 

Some difficulties were encountered in the absolute confirmation of configurational and structural isomers where mass spectral peak intensities and mass values of characteristic fragment ions were too similar under the conditions of the analysis. In those instances, however, identification was readily made on the basis of gas chromatographic retention time data. The estimated lower limit of detectability of organochlorine pesticide residues by this analytical scheme and instrument configuration was determined to be approximately 0.05 to 0.1 ppm. 

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,... 

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. 

Peak Identification with a Diode Array Detector SPECTRAL LIBRARY FILE... 

The mode of peak identification that the diode array detector provides allows identification of unknown peaks, not only by retention time but also by their UV spectra. The UV spectmm is very reproducible and its full shape is much more compound-specific than is generally assumed. It is a powerful tool that, beyond other applications, it can also be used as a scouting technique to find out the possible identity of an unknown sample. However, spectral identity is a necessary but not a sufficient precondition for compound identification. The information that is acquired by the comparison of an unknown and a known spectrum can be used as strong indication of compound similarity or confirmation of identity. Combinatorial estimation of retention time and spectral comparison results will give us a more definite idea. 

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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