Is the interferogram signal stable?? If not, the problem might be attributed to the laser source... Also might be a question of the sample...
As the polystyrene standard passes, the main problem is associated with your sample. It seems that the sample is a mixture of some components. Each component contributes a little with the entire spectrum. For the low absorption peaks there may be an automatic increase in the amplifier to make the signals visible. The increase in the amplification will result in unstable signal. For me I'll try to wash the sample to remove some of the components. The washing may be done with organic solvent to remove some of the organic components and see the FTIR of the remaining solid. regards
Thank you for your precious informations. Actually the signal without any sample is not stable and ADC converter (as I remember :)) error message appears, so I think it is hardware issue and I was trying to find easier solution if possible, so I appreciate your expected valuble contributions.