Reconstruction of Chromatograms

The end result of this process is that over 1000 ab-sorbance spectra, corresponding to the contents of the light-pipe measured at approximately 1-s intervals throughout the entire chromatogram, are stored at the end of the run. Many of these spectra contain no useful information as they were measured when no component was present in the light-pipe; thus the next step in a GC-IR analysis is to determine which of the stored spectra contain useful information. To achieve this, a chromatogram must be reconstructed from the spectroscopic data. This is usually achieved in two ways, the first of which is known as the Gram-Schmidt (GS) vector orthogonalization method. Here, short, information-rich regions of the interferograms are treated as vectors and the vector distance between this part of each interferogram measured during the chromatographic run and several interferograms that were acquired when nothing except the helium carrier gas was flowing through the light-pipe (known as the basis set) is calculated. When an analyte elutes from the column, the magnitude of the vector difference is approximately proportional to the quantity of this material in the light-pipe. Because only a short region of the interferogram is examined. calculation of the GS 'signal' can be achieved in a few milliseconds. Furthermore, since all compounds besides monatomic and homonuclear diatomic molecules have at least one band in their IR spectrum, GS chromatograms are very nonselective. Some compounds yield much stronger IR spectra than others, however. For example, the spectra of most nonpolar compounds are rather weak, whereas the spectra of very polar compounds are usually much stronger. As an example, the detection limits for GS chromatograms of polycyclic aromatic hydrocarbons (which have very low absorptivities over most of their IR spectra) are about 20 times greater than the corresponding values for the barbiturates (which are very polar and have several strong IR absorption bands in their spectra).

The other commonly used algorithm by which chromatograms are constructed from the IR data involves calculating the integrated absorbance in one or more specified spectral regions. These regions are usually chosen to correspond to the characteristic absorption frequencies of functional groups present in the class(es) of molecules of interest. The chromatograms generated by this approach have been called by a variety of names including Chemi-grams™, functional group (FG) chromatograms and selective wavelength (SW) chromatograms. FG chromatograms are, of course, far more selective than GS chromatograms, but are rarely completely selective as many molecules have weak overtone and combination bands over much of the fingerprint region of the spectrum. For compounds with functional groups giving rise to intense absorption bands, such as the C=O stretching mode of carbonyl compounds, the limits of detection of FG chromatograms may be less than those of the corresponding GS chromatograms, but the two algorithms often have comparable sensitivity. A useful way to detect the presence of a particular functional group is to compare the relative heights of peaks in the GS and FG chromatograms. If the ratio of the peak heights in the FG and GS chromatograms is large, the presence of that functional group in that component is indicated: if the ratio is small, there is a much smaller probability that the analyte contains that functional group.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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