The logical thing is to think that the sample has changed (it has reacted, degraded, etc ...) Knowing what sample it is would help to see what may be the cause of this spectrum change ...

Hi Francisco Manuel Marquez for your respond!

The sample is a mineral oil aka paraffinic oil, in this case, it was new oil and not getting any contamination, reaction or something else (i'm pretty sure).

The strange peak also occur not only in this sample, but in all of my tested sample, in the same wave number range. So i think its not because of the sample, but related to instrument issue.

Appreciate your comment!

nb: I use Pike Miracle ZnSe ATR window.

Hi Daniel, as I work with Perkin Elmer FTIR too, this instrument is very good to detect every spectrum difference. If your FTIR instrument already verified and with the same condition, its very possible that your current spectrum is different. Hope it helps

Try to see if there is a possibility of contaminant, such as silica (clay is one of them). Silica has a very strong and broad band around 1100 cm-1.

Agree with Hatsuo on Inorganic contamination/modification. Simple spectral profile and a few broad bands – that is inorganic. Silica or Clay? Could be, although some bands are missing, maybe because of the sample processing history(?). Silica has the strongest band ~1072 cm-1, while Clay ~1026-1003 cm-1. I would also consider some simple Silicate. There is a rather broad shoulder on the left of the peak, ~1200 cm-1, generally expected for Silica/Clay, but not sure why it is so broad. Note also those tiny bands at ~3600 cm-1. They are characteristic to, I think crystalline water bound to the Si-O- species.

Oxidation is unlikely to be the reason for the 1300-1000 absorption, as one would expect to see rather strong absorption at ~1720-1710 cm-1 first.

Thanks Febry Ardiana , Hatsuo Ishida ,and Aleksandr Tarasyuk for your idea!

Looking back at my previous recorded measurement, I do ever done measurement for some compounds which contain silica, likely a casting release agent, rubber, and lubricant.

In case of silica contamination in my window, how can I clean my ZnSe window to remove the silica contamination safely?

Appreciate your comment!

Try wiping with acetone. But before doing that, take a background spectrum and see what happens after wiping. If you are successful in removing a contaminant, you will see "negative" absorption. Wipe again, until you see a stable response, i.e. no more changes in spectra after wiping..