Infrared spectroscopy

please help me i have answered all my other questions on infrared spectroscopy but i dont know how to answer this last one question
discuss what is meant by subtraction of spectra and consider the use of a background scan and any other situation where you may be required to carry out a subtraction

Simple example... Suppose you have two materials in the sample, one for which the composition is known and you know the spectra that, theoretically, is coming from this material. The other material is unknown. Therefore, you could subtract that known spectra, using the relative intensities that, theoretically, predict what the relative peak heights should be. This will leave you with the intensity spectra that is coming from the other component.

I hope this helps