Dear Researgate Community,
I hope you are all doing good. I am using caffeine as an additive in my experiment. I have run FTIR spectroscopy to analyze the purity of the sample. FTIR spectra showed a shift in carbonyl group stretching vibrations frequency. In literature, these vibrations are reported at 1699 cm-1 for carbonyl group and 1652 cm-1 for C--C. Whereas, in my sample, these peaks appeared at 1690 cm-1 and 1644 cm-1. Please give your valuable opinion on this peculiar change in the position of stretching frequencies. Please also note that in the literature the transmittance data is reported whereas FTIR spectra were obtained in absorbance mode.
Thank you for your time!
Dear Abdul Sattar , I know that people talk about an absorbance mode, but absorbance is a virtual quantity based on the Beer-Lambert approximation (with conventional equipment like FT-IR spectrometer you are usally measuring transmittance or reflectance; new instruments like O-PTIR may be able to measure absorptance, but never absorbance).
If you perform, e.g., ATR measurements, you measure the reflectance and assume that -Log10(R/R0) = Absorbance. However, since this is only an approximation, the peaks in ATR spectroscopy are redshifted, the stronger their oscillator strength is. If you want to know more about the background, I suggest reading
Preprint Wave optics in Infrared Spectroscopy
Article The Bouguer-Beer-Lambert Law: Shining Light on the Obscure
Have you used an ATR accessory? If not, how did you measure your sample
Thomas Mayerhöfer Thank you for your valuable comments. I really appreciate it.
Abdul Sattar My pleasure! Out of curiosity - is this an ATR spectrum or how did you obtain the spectrum?
Thomas Mayerhöfer You got it right. This spectrum is obtained using the Diamond ATR module!
Thanks! If your spectrometer software includes an advanced ATR correction, you can apply it. Afterwards, you can compare the peak positions and deviations could be interpreted it with regard to "chemical" changes - at least for pure substances. In mixtures, even in ideal ones, local field effects need to be considered. In these cases, you can still expect peak shifts of up to 5 cm-1 that cannot be interpreted as changes of the structure of the molecule. In addition, it might well be that also the transmittance spectra you used as reference might need correction. In other words, I would not interpret any peak shifts that are smaller than about 5 cm-1 for stronger peaks.
Dear Abdul Sattar thank you for your interesting technical question. For more detailed information about the IR spectrum of caffeine please have a look at the following relevant article:
FT-IR Study of the Hydration of Caffeine, Sucrose, and Their Mixtures in Water
Book FT-IR Study of the Hydration of Caffeine, Sucrose, and Their...
This book chapter is freely available as public full text on ResearchGate.
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