This question might not be very technical or might be very naive. I have a solution with multiple components. I have recorded the spectrum (ATR-FTIR), my question is, if I increase the concentration of one of components, will I be able to see linear change (beer-lambert law) in multiple wavenumbers? Is it also possible that with increasing concentration, the absorbance peak decreases in a wavenumber?
Can you be a little bit more specific, or, maybe, show us even spectra? The Beer-Lambert law is a limiting law, like, e.g., the ideal gas law, and works best for vanishing band (= oscillator) strength. This means it can work well for organic and biological materials in the infrared. But even in the absence of changing chemical interactions, like for (quasi-)ideal mixtures, deviations can be observed. We recently investigated such mixtures also with ATR-FTIR and found this confirmed (Article Beyond Beer’s Law: Quasi-Ideal Binary Liquid Mixtures
), but we did not find that a peak of a component the concentration of which increases decreases in intensity. This could be a hint towards the occurence of a chemical reaction...
Hi Thomas
Thank you for your time. let me be more clear:
"
The spectra of NaDCC solution in deionized water for various NaDCC concentrations is shown in this figure. The amounts represent the percentage of NaDCC in a sample containing 20 grams of powdered tablet. In wavenumbers such as 1400, 1550 the absorption is decreasing regardless of the increase in amount of NaDCC. What I think is, excipients show the same absorbance peaks in these wavenumbers. Since the amount of the sample is constant, 20 grams, while the concentration of NaDCC is increasing the excipients concentration is decreasing. According to A=εbc, if the collective molar absorptivity (ε) of the excipients is higher than NaDCC it will lead to a decrease in absorption peak, while the excipients are decreasing regardless of NaDCC increasing."
Hi Hamzeh,
Thank you, this is very helpful! Indeed, it looks like the peaks at 1400 and 1550 cm-1 belong to the excipients. Somewhat strange is the peak at about 1300 cm-1, since it is no peak for 4 grams of NaDCC. If you subtract the spectrum of 4 grams of NaDCC from all the others it looks like that Beer's approximation could hold. The main band at about 1650 cm-1 obviously does not follow Beer's approximation since the peak value for 4 grams of NaDCC is certainly not half of that for 8 grams etc. The reason for this is the water band of the H-O-H deformation vibration. It would be good to have also the spectra for 2 and 0 grams of NaDCC. If you subtract the latter spectrum (weighted subtraction so that the absorbance becomes zero at the lowest value of the spectrum), you usually would generate a set that does approximately follow Beer's approximation (which is never completely valid). Can you do this and show us the result?
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