I am familiar with the process of IR spectroscopy and the preparation of IR samples.

However, I am curious as to why you cannot use hygroscopic or protic solvents for IR spectroscopy. Is it just because it dissolves the cell (KBr/NaCl), or does it give an OH peak, or are there other reasons?

And are there any protocols that allow you to use water as a solvent for obtaining an IR spectrum?


I applaud this question. I have not really tried to find any solution, but I definitely recognize the trouble with water... nonsense/worthless reading. As far as why... easy; water absorbs like, all of the IR spectrum. Below is the IR spectrum of liquid water (from NIST's Chemistry Webbook).

IR spectrum of water

I actually have an ATR IR Spec, which is (supposedly) meant for reading aqueous solutions...

enter image description here

...but it doesn't really work... it can't be used for figuring out dilutions (well maybe, give or take 20%). In other words, 50% water + sample looks a lot like 95% water + sample... which looks a lot like any sample + water. Water basically drowns the reading- so it all looks alike.

Don't get me wrong... it's super nice that I don't need to oven dry my sample or deal with KBr. Basically, I just put the sample on the top and I get a decent reading, but if the sample has a lot of water in it (like greater than 10%), it loses a lot of definition.

  • $\begingroup$ @benNorris That's what a sample of say anything plus 50% water looks like $\endgroup$ – Ben Welborn Jul 26 '16 at 19:03

Its a continual problem. Usually its due interfering absorptions (often total absorption) from water so try using D$_2$O instead, the frequency shift may be enough for you to be able to use it. Also change to using ATR (if possible), we use this in the teaching lab for several experiments on liquid samples without the need for any cell. Signal to noise is not so good though. You can also try subtraction via a water only spectrum against water plus sample. It will be messy but better than so data at all.


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