Library Directed Peak Searches

The position can be determined either from information derived from the peak location or by taking account of library data which indicate which peaks to expect within the particular multiplet. Both have their advantages and disadvantages. Obviously, unless a gamma-ray is in a library then it will not be taken account of and so a simple library directed approach cannot cope with the unexpected. On the other hand, small peaks within a multiplet and very close multiples may not be resolved by the peak search and incorrect peak areas may again result. 

Seymour, R.S. and Cox, J.E. (1991). Library-Directed versus Peak-Search-Based Gamma-Ray Analysis, Radioact. Radiochem., 2, 10-18. 

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. 

Gas Chromatography-Mass Spectrometry. A VG Trio-2 mass spectrometer was directly coupled with the Hewlett-Packard 589 R gas chromatograph, equipped with a 30 m x 0.32 mm DB-WAX capillary column (J and W. Scientific bonded polyethylene glycol phase). The carrier gas was He at 2.3 ml/min. The injector and transfer temperature was programmed as follows 30 C (2 min isothermal), to 38°C at l°C/min, then to 180°C at 2°C/min. The instrument was operated in the electron-impact mode at an ionization voltage of 70eV. Quantitative analysis by peak area, plots of chromatograms, and library search results were obtained. 

The success of a library search in analytical chemistry depends very much on the data representation. Different kinds of data (numerical, textual, spectral, structural) require appropriate treatments. Usually, spectral data collected from the instruments are not in a form suitable for direct input either into a collection or as a query for a library search. Once the spectrum is in the computer, a number of preprocessing steps (e.g., smoothing, baseline correction, normalization, peak and intensity detection, reduction of the measurement space, autocorrelation, deconvolution, shape tracing, etc.) should be applied in order to bring the data into a standardized format... 

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