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I was studying Infrared Spectroscopy and i analyze different polymers. When i prepared paracetamol making a pill mixing 2mg of it with 200mg of $\ce{KBr}$ on a vacuum-connected press when i put it in the Infrared the equipment returned me this signal. Buck 500, paracetamol

The equipment is infrared spectroscopy BUCK500. On the spectrum we can see starting from 1500, in 1700 the carbonyl bond $\ce{C=O}$ in 2350 some lens damage or dust and in 2900 de alkane $\ce{C-H}$ bond. According with the Paracetamol structure must be at 3500 signal according to its -OH group and $\ce{alkene C=C}$ at 1600. My question is if this IR is correct or is something wrong.

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  • $\begingroup$ Yes i'll edit it. My question is if this IR spectre is correct or not if it could be something in the procedure to not appear the -OH and the C=C group $\endgroup$
    – Andreas
    Feb 11, 2022 at 23:54
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    $\begingroup$ I searched for “paracetamol IR spectrum” and found spectra. I suggest doing the same and looking at ones from authoritative sources: this is just acetaminophen (Tylenol, as one of the OTC trade names), so I would expect it to be used in an introductory IR lab experiment. $\endgroup$
    – Ed V
    Feb 12, 2022 at 0:06

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Go to NIST website NIST and check reference spectra of acetaminophen. Do not expect a 100% match in peak shapes because of instrument differences. The peak positions should match.

When you suspect that the IR instrument is not behaving well, it is a good idea to run a instrument health check with a polystyrene film. It does not require any sample preparation, the sheet is directly inserted in the light path with a window made from cardboard. You should get a very nice textbook like spectrum of polystyrene.

As per the manual of your instrument, polystyrene film is provided with the instrument. https://www.bucksci.com/pages/buck-m500-infrared-spectrophotometer

It is amazing to see that there are still scanning IRs in use! Anyway, there are no lenses, you will only find high quality super-expensive mirrors and gratings there. It is hard to damage them because it is all protected. So the artifacts appear to be of electronic nature rather than due to optics.

a) Make sure that KBr and paracetamol are "bone" dry! And warm up the instrument for at least an hour.

b) I see some abnormal artifacts. There is a sharp drop in the transmission spectrum around 700 cm$^{-1}$. This has to be addressed.

c) There is an artifact around 2350 cm$^{-1}$. This is from incorrect baseline correction. This is perhaps due to carbon dioxide. Hopefully not many students are breathing closely near the instrument or poking their nose.

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