Systems for protein quantitation using a Fabry-Perot interferometer. In one arrangement, a quantitation device includes an infrared source, a sample holder, and a Fabry-Perot interferometer positioned to receive infrared radiation from the source passing through a sample on the sample holder. A band pass optical filter sets the working range of the interferometer, and radiation exiting the interferometer falls on a detector that produces a signal indicating the intensity of the received radiation. A controller causes the interferometer to be tuned to a number of different resonance wavelengths and receives the intensity signals, for determination of an absorbance spectrum.