I'm measuring the IR spectra trought FTIR-ATR of the same film multiple times. There is no chemical reactions occurring (as far as I Know). The IR spectras are different from one measurement to another by an offset. This does not happen with other substances and, as the substance do not change with time, I'm guessing that the problem (the variation; the offset) is probably due to sample preparation, but I'm not sure.
In ATR-FTIR, the signal quality very much depends on the contact area between sample and ATR crystal. Might it be that your polymer film has different from your other samples physical properties which may influence this contact area? If you run a background scan with a cleaned and dry crystal and then measure some liquid, is your baseline at 100% or do you observe some (reproducable) offset? I would suspect an old or scratched ATR crystal if yes.
Anyway, I would assume contamination if peaks appear or disappear during repeated measurements of the same sample. Besides all advantages ATR offers, contamination or memory effects are the worst issues to cope with.
Dear Andre, first of all it would be interesting to know whether the observed offset has a positive or negative direction and second whether the magnitude of the offset changes contineously with time. I do not know the optical configuration of your instrument and I also do not know the answer to the questions above, but in my opinion one should first check if the offset phenomenon is due to changes of your sample (drying?, temperature change?, etc.) or because of an unstable optical configuration.
In ATR-FTIR, the signal quality very much depends on the contact area between sample and ATR crystal. Might it be that your polymer film has different from your other samples physical properties which may influence this contact area? If you run a background scan with a cleaned and dry crystal and then measure some liquid, is your baseline at 100% or do you observe some (reproducable) offset? I would suspect an old or scratched ATR crystal if yes.
Anyway, I would assume contamination if peaks appear or disappear during repeated measurements of the same sample. Besides all advantages ATR offers, contamination or memory effects are the worst issues to cope with.
Let me explain a little better: The offset on the IR spectrum is sometimes positive and sometimes negative. If I measure the the IR in a specific spot in the film several times, the IR spectrum do not change, but if I measure the IR spectrum in a different spot (let's say 2 cm do the left), the IR spectra changes. The film is applied to another polyester film that seems quite rigid, not really moldable. Can it be the polyester film the cause?
Dear Andre, having read your answer, I would guess that the most simple explanation of the observed offset phenomenon is true: to me it appears that your ATR system is not optimally aligned, i.e. the overall light intensity reaching the detector changes with the measurement position. Perhaps there is some variation of the angle of incidence, or the IR beam is partially blocked (particularly in case of multiple internal reflections). Please carefully check the IR bands (intensity, frequency positions, ratios) to disprove this hypothesis. These parameters shouldn't change much!
IR is a little different to other absorption spectroscopies. Baseline shifts are due to changes in energy. For example if you baseline during instrument warm-up ,as the lamp increases in energy you can see a progressive change in offset. The other common cause for a baseline change between samples is the position of the crystal prism, this often means it has come loose in its top plate. This is easy to diagnose by measuring the 100%T line first with the presser head released and then with it in contact with the prism. The 100%T line should remain the same for both measurements.
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