Two-spectra comparison

More than spectrum characterisation, a comparison of two spectra can often be of great interest. 

The previous examples deal with the spectra of real samples. Another way to proceed is to subtract, from the real spectrum, a spectral contribution corresponding to the presence of a given absorbing compound. For example, in natural water, the presence of nitrate can hide the optical response of organic compounds, even if the water has been filtred with a very low cut-off membrane (Fig. 7). The determination of nitrate concentration (by conventional analytical methods or by UV method see Chapter 5) allows to subtract the part of the spectrum related to nitrates from the initial spectrum. 

This basic handling makes possible the revelation of the UV response of the organic matrix. The presence of carboxylic acids at low concentration can explain the denitrified 

This data exploitation is less used but interesting if a comparison between spectra is required, in order to check the general quality and origin of wastewater, for example. For each wavelength, the absorbance value of one spectrum is plotted against that of the second spectrum. The initial spectra shape is lost, but the obtained graphical relation is relevant (Fig. 8). 

Considering the result, a straight line or a curve, the studied spectra can have the same shape (homothetic) or be very different. Depending of the curve type, information 

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