Limits of Detection and Identification

The limit of detection (LOD) for an acceptable GC-IR response for most compounds is between about 1 and 20 ng (injected) per component, the actual value depending on the chemical nature of the analyte. The LOD is often defined with respect to the strongest band in the spectrum. Most bands in the IR spectra of nonpolar compounds are fairly weak and so these compounds tend to have the highest LODs, but even these compounds usually have at least one band in the spectrum with a high absorptivity. Examples include the C-H stretching bands of alkanes and the aromatic C-H out-of-plane deformation bands of polycyclic aromatic hydrocarbons. Detection limits also depend on the width of the GC peak; the wider the peak, the more dilute the analyte and the higher the LOD.

The amount of a given component that must be injected into the chromatograph to yield an identifiable spectrum, often known as the minimum identifiable quantity (MIQ), depends not on the strongest band, but on the signal-to-noise ratio of the most characteristic bands in the spectrum. For an analyte with a spectrum that is very different from any other spectra in the reference database, the MIQ may be only slightly higher than the LOD. On the other hand, there are often only very subtle differences between the spectra of members of this class of compounds. If the analyte is a member of a homologous series and several reference spectra of members of this series are contained in the library, the signal-to-noise ratio of the spectrum must be high, and hence the MIQ will be much greater than the LOD if the analyte is to be correctly identified.

If a minor peak is present in a chromatogram measured with a conventional GC detector such as a flame ionization detector (FID), but is not observable in the GS or FG chromatogram, it may be possible simply to inject a greater volume of the sample into the chromatograph. Even if the major components overload GC column in this case, the minor components will not. However, sometimes the major peaks will broaden to the point that they start to overlap a neighbouring minor peak. In this case, it may become necessary to subtract the spectrum of the major peak (linear in absorbance) from the spectrum measured in the region of the minor peak, to identify the minor component. This procedure is needed because of the relatively low sensitivity of light-pipe-based GC-IR instruments. Two other approaches that have led to increased sensitivity for GC-IR measurements are described here.

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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