Hi Morteza, 

I have not worked with FTIR spectrum. I am not sure but think that the basics are same with NIR and VIS spectrometry. What we normally do is to correlate the spectra with specific features we want. For example, if you want to measure Pb as a heavy metal in sewage sludge, you need to measure Pb with a reference method such as Atomic absorption etc. then try to find which band of the spectra is linked to Pb. For this purpose, you can use multivariate models. 

I do not know if I could answer your question or not. You can have a look at my recent paper or ask a more specific question, then I know clearly what to explain.

Cheers,  

Iman