Spectral Searching

Once the chromatography has been completed, the spectra of those components of interest can be displayed. (In fact, several GC-IR software packages allow the spectra to be displayed while data acquisition is still in progress.) Each component generating a peak in the GS chromatogram with a signal-to-noise ratio greater than about 10 can usually be identified by comparing its spectrum to a library of vapour-phase reference spectra. The unknown and reference spectra are first scaled so that the most intense band in each spectrum has the same absorbance (usually 1.0). By treating the spectra as vectors, the Euclidean distance between the unknown and each reference spectrum is calculated. This distance is usually called the hit quality index (HQI); the smaller the HQI, the better is the spectral match. The highest probability for the identity of the unknown is that of the compound in the reference library yielding the smallest HQI. Some software scales the Euclidean distance and then subtracts it from, say, 1000 to give the HQI; in this case, of course, the larger the HQI, the better the spectral match. However unequivocal identifications cannot be made on this basis alone, for several reasons. The reference spectrum of the authentic analyte may not be present in the spectral library. If the spectrum of the unknown is noisy, the value of the

HQI may be determined more by noise than by the true absorption spectrum. The reference spectrum may have been measured with the sample at a different temperature from the light-pipe, measured at a slightly different resolution, or computed with a different apodization function. Finally, some members of homologous series can have very similar spectra, so it is not uncommon for compounds of the same type (e.g. methacrylate esters) to give similar HQI values.

It is always recommended that the user should make a side-by-side comparison of the GC-IR spectrum and the reference spectra of the top few 'hits' to get a good idea of the probability that the structure of the top hit, or by one of the other close hits, or whether there is enough similarity between the GC-IR spectrum and all of the closely matching reference spectra that unequivocal identification is impossible. In this case, the simultaneous application of MS may be necessary to yield an unequivocal identification.

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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