in the most of IR spectrum, there is a lot of disruptive interference in consequence of internal reflectance as a noise, in the thin film sample holder of FT-IR spectrometer (etalon effect),
is there a way to decrease this problem?
Actually interferences are your friends, as they allow you to determine when the Beer-Lambert law breaks down and how thick your sample is. You can avoid them by wedging your sample. Take a look at
https://www.researchgate.net/post/How_avoid_interference_in_IR_spectra_of_thin_samples
1) Do a FT of the absorbance spectrum, find & delete the sharp spike in the interferogram, do then the inverse FT.
2.) If this does not work, you could try to work with slightly wedged spacers or wedged windows which can be used to reduce (or even to eliminate) fringing patterns in your spectra.
3.) Fringing can be also effectively reduced by lowering delta(n), i.e. the difference of the refractive indices of the sample (sample solution?) and the optical material used in your IR cell (CaF2, ZnSe, KRS-5, etc.). For example, we found it helpful to work with aqueous solutions in CaF2 cells instead of working with aqueous solutions in ZnSe cells.
thanks, dear peter
cloud you show me an article interrelate with the first approach
Dear Amirhossein,
Sorry, but I do not have a reference to the literature regarding approach #1. Instead of this I have attached a scan from an old project documentation that illustrates the suggested procedure. Please ignore the top panel and pay attention only panels labeled as (1)-(4). Do not hesitate to contact me directly by email in case of questions.
Panel (1): original IR absorbance spectrum showing the fringing pattern
Panel (2): Fourier transform (FT) of (1) with the spike indicated
Panel (3): FT of (1) with the spike removed
Panel (4): Inverse FT of (3) with the fringing pattern removed
Best regards, -Peter
Peter Lasch, Thank you so much! Very useful information!
I mastered the removal of interference in Origin Pro!!!
Peter Lasch A word of caution: Removing interference fringes by FT is not a good idea! Why? You only remove one of the symptons, but not the disease, and by doing so you rob yourself of the chance to cure it.
What am I talking about? If you see interference fringes in a spectrum then you know that the Beer-Lambert law is no longer valid, i.e. absorbance is non-linearly depending on the thickness.
Why? The fringes are an indication of standing waves at certain ratios of wavelength/layer thickness. At these wavelengths the average electric field intensities are high, whereas at other wavelengths they show a minimum. Absorption is proportional to the electric field intensities! So you have higher absorption at certain wavelengths compared to others even if the attenuation constants would be the same. Read the full story here:
Article The Electric Field Standing Wave Effect in Infrared Transmis...
Article The electric field standing wave effect in infrared transfle...
But you can use the fringes! Here you are told how (and we are working on transfering the method to spectra of layers on a transparent substrate):
Article Removing interference-based effects from the infrared transf...
https://www.researchgate.net/project/Fast-and-reliable-Determination-of-the-true-Absorbance
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