In the 1000-1200 cm-1 chitosan, chitin and other polysaccharides have a very intensive broad band of the stretching C-O vibrations. If the film thickness is large enough (and it can be seen in the spectrum presented), their first overtones appear in the region of 2000-2300 cm-1.

Is the peak a strong one? A weak /medium one may well be due to a combination of lower lying fundamentals, or overtone of vibration around 1100 cm-1.

And I guess it isn't a deuterated sample or substrate?

Dear Davide,

Could you upload your spectra here? I think we can discuss more if we can take a look on at a spectra.

I identified in my earlier work the band at 2361 and 2344 Cm-1 as evident for CO2 formation (attached file).

It can be also overtone of a peak at 1100-1150 Cm-1

Yes, peaks of CO2 (from air) change intensity by second spectra registration.

I think the broad peak might not come from CO2. Usually, CO2 peak is around 2350 cm-1 of wave number, and not a very broad peak. In this case, the author of this question mentioned that is a broad peak in 2000-2300 cm-1 region. So, maybe this strange peak come from other substance. Anyway, it is very hard to conclude without a real spectrum.

@Mehri Sanati: thank you for uploading a very informative paper.

2350 cm-1 is CO2. I would go for CH2. remember the structure is chiral - you may have more complex effects. Search in the litterature, the IR spectra is already probably discussed somewhere...

This is one of the spectra. They all show this broad peak. Even if it is CO2, I guess my peak is too high. In litterature a peak around 2100cm-1 is attribuited to impurity but it is still very low compared to mine ("The preparation and characterization of low molecular and water soluble free amine Chitosan", M. K. Jang et al.).

Moreover there is this area from 2250 to 3250 that is supposed to be flat but it is not. I can may flatten it using a better baseline correction.

Thanks for your disponibility.

The uploaded spectrum looks a single beam or transmission spectrum. In that case the spectral peaks are those pointing downwards, so the region around 2200-2300 cm-1 is characterised by a lack of absorption as expected.

The baseline of this spectrum does seem odd - suggestive of a single beam spectrum which has not had a background subtracted or else suffering from scattering?

No it is an absorbance spectrum, The peaks are pointing upwards.

If it can help, my samples are very thin and transparent. What do you mean "suffering from scattering"?

There are various types of spectral artifacts and baseline effects that result from scattering. See for example Paul Bassan's PhD thesis (and related papers):

https://www.escholar.manchester.ac.uk/api/datastream?publicationPid=uk-ac-man-scw:131823&datastreamId=FULL-TEXT.PDF

Can I ask how the samples are being measured. Is it by transmission, reflectance or ATR? If the latter it could be from the diamond substrate.

As its transmission B.Lokshin's comment is probably your answer.

They were measured in transmission mode.

In the 1000-1200 cm-1 chitosan, chitin and other polysaccharides have a very intensive broad band of the stretching C-O vibrations. If the film thickness is large enough (and it can be seen in the spectrum presented), their first overtones appear in the region of 2000-2300 cm-1.

between 2000 and 2300 cm-1 may occur signal coming from conjugated effect like carbon-di-imide, not especially pure nitriles C=N=N, N=C=N, C-O-O, N=C=O etc…

The interpretation depends on the wavenumber you find, and the vibrations in the other parts of the spectrum.

in your spectrum I see essentially absorption around

700 (perhaps), 1100 ( localized), 1480 + 1600, 1800 ( typically C=O), 3850 ( perhaps)

around 2300 it is very small;

You have to learn to read the IR spectra in a good way. No director or teacher?

Hi, Davide

First of all, the spectrum you've posted looks as if you (your machine) have forgotten to devide the sample spectrum by the reference spectrum. The last always should be run before acquiring the sample spectrum. On some machines it is imperative, and at attempting to run sample spectrum without the reference one you will be warned about this. Some machines are not so clever. Some permit to download (from a database in memory) a reference spectrum which has been acquired long time before (and, in many cases, that is almost useless). In any case, the best practice is to run the reference spectrum just before running the sample one. I don't know the exact reason, but brood over those named above.

Secondly, yet another imaginable scenario leading to the result you've posted: you have run the reference spectrum on fully open beam (we work in transmission - right?), but trying to record the sample spectrum for your film (right?) you've discovered that it can not cover the full filmholder aperture (half an inch frequently), and so had to use a pinhole diaphragm. It is a bad practice either.

Thirdly, please check the attached PDF where the three necessary (for any FTIR measurement) spectra are overlayed. Black - raw machine reference spectrum, red - raw machine sample spectrum and blue - the desirable product, that is, the neat sample spectrum obtained after (the machine) divided red spectrum by the black one. By the way, the blue line is a typical FTIR spectrum of a chitosan presented in absorbance units. (A note: on the enclosed picture i've reversed the abscissa axis as in your spectrum. Generally speaking, it is not traditional for FTIR spectra (as compared to Raman), but i used this "just to be on the same page with you").

Finally, i wish you good luck. Your response concerning where I was wrong in my analysis would be greatly appreciated.

P.S.: Almost forgot. The absorption band in the range of 1900-2300 cm-1 indeed exists in chitosan spectra. But, as you may see, it is hardly the most informative one from the whole chitosan spectrum. Yes, Prof. Lokshin seems to be correct about the origins of this absorption band.

Good morning Mr Shchegolikhin and sorry for the delay in my answer.

Your suggestions have been useful and running the reference leads to a slightly improvement of the signal. Do you work on chitosan right? Have you ever acquire a spectrum of chitosan treated with acid?

Thank you