Examples of useful near-infrared absorption bands

4 Examples of useful near-infrared absorption bands 

The principal saturated hydrocarbon functional groups of concern are methyl, methylene and methyne (—CH3, —CH2—, = CH—). The spectra of typical hydrocarbon mixtures (for example as in gas oil and gasoline) are dominated by two pairs of strong bands in the first overtone and combination regions (5900-5500 cm-1 and 4350-4250 cm-1). These are predominantly methylene (—CH2—). The methyl end groups typically show up as a weaker higher-frequency shoulder to these methylene doublets. 

Olefinic, unsaturated hydrocarbons have CH groups adjacent to C=C double bonds. The C=C vibrations do not show up directly except as weaker contributions in the combination region, but the nucleophilic effect of the C=C bond tends to shift nearby CH groups to significantly higher wavenumber, and this is indeed seen in hydrocarbon spectra. 

If NIRS can be demonstrated to have sufficient hydrocarbon speciation capability to reproduce the analysis with the same precision as the GC method, but in a fraction of the time, then a useful process analytical goal will be achieved. 

The ester group C=0 functionality makes a fleeting and weak appearance in the NIR spectrum only as the second overtone at around 4650 cm-1, where it may well be hidden by overlapping aromatic C—H features if the sample mixture also includes aromatics. 

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