I've measured three different ethanol samples in my IR spectrometer, whose spectra are attached (one in absorbance and another in transmittance units). The measurement was made with liquid ethanol with a liquid cell (ZnSe windows). They have no similarities to the spectra you find in the literature (an example was also attached). The background was measured using an empty liquid cell. Any ideas why?
Hi,
Your samples seem to be way too concentrated so you completely saturate your signal, your transmittance intensity should not exceed 100%, same thing for absorbance values, they should not exceed 1 in the best scenario. Thus you have two option, either dilute your sample or choose IR cells with way shorter pathlengths, so that your intensity decreases. You should see the bands between 500 and 2000 cm-1 and between 2500 and 4000 appear. The bands you observe are most probably present in the example from the literature, just way less intense than the ones besides them.
Let me know if it works best!
Some additions: If you want to measure all liquids in pure form, you indeed need to choose a cuvette with much smaller thickness. In that case, standing waves (interference effect) will be generated inside your cuvette and you must not choose an empty cuvette as reference. Instead you choose the empty spectrometer for your reference measurement. After the measurement you have to correct your spectra for the intensity changes and the fringes caused by interference. See e.g. Article The control of errors in i.r. spectrophotometry---III. Trans...
We have recently improved the algorithm for transflection measurements, but these improvements can be taken over for transmission measurements with some small adaptions:Article Removing interference-based effects from the infrared transf...
One last thing, a small correction to what Benjamin Martial wrote: Your transmittance should not fall under 10 % (your transmittance values are higher than 100 % most probably because you used as reference the empty cell, which certainly does not make sense )
Thank you Benjamin and Thomas for the replies. I managed to obtain reasonable spectra by removing the spacer between the windows and inserting the liquid manually (I put a few drops of ethanol solution on one window, cover with the second one and then proceed to close the cell). I guess this way I could reduce the width of the ethanol layer enough, without the need of doing a dilution. Maybe not the best way to do it, but I needed the IR spectra for identification purposes only, and so far I think it will work just fine.
Hi Mikhail Pokryvaylo Lvovich Please add your latest IR spectra of ethanol. I never heard to take spectra like this way.
Yeah. Please send the IR spectra of your diluted ethanol sample.
Putting a drop of a liquid between two IR transparent substrates is a well-known technique since ages. Only thing to take into account is that the reference sample is one substrate with the thickness of both taken together. The downside is, as already mentioned, that you don't have control of the thickness of the layer, which makes the correction of the interference effects in the spectra (in particular peak heights and sometimes additionally removement of fringes), in some cases more difficult.
Is it possible to use the data as reference without the control of thickness of the layer ? Thomas Mayerhöfer
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