[DoctorDomo]

I want to switch over to the thin film technique since KBr discs are too time consuming, but I was wondering how the backgrounding works. I've been cleaning the salt disc and running a background scan on that, but I've seen other people using air. Am I right in thinking you should use the disc as your background? My thinking is that if the disc happens to have any IR active impurities that I couldn't wash off, then they won't show up on the spectrum, but if I background with air then any problems with the disc itself will change the spectrum.

[Dan]

I always run the background with the disc.

[marquis]

I like to do both.

Even though you clean the KBr discs, they can still be
dirty.  I'll run a background in air and then run the disc
as an unknown.  If the disc comes back clean, rerun the
disc as the background.  Then add your sample to the
disc and run.

[DoctorDomo]

Lets say I want to run an IR on my crude reaction mixture, if I were to cover the plate in my reaction solvent, then run the background on the plate, would I be right in assuming that will make my solvent invisible in the IR spectrum? For example, lets say I have a volatile product dissolved in DMSO, I can't just dry off the DMSO with the nitrogen tap because I dry off the product too. In this case, could I smear the NaCl disc with DMSO, run a background on it, then smear my reaction mixture (diluted a bit first of course) on the NaCl plate, and obtain an IR spectrum that doesn't show S=O, S-C or any other peaks that DMSO would ordinarily produce?

[marquis]

Generally, this didn't work for me.

The liquid generally absorbs all of the IR radiation in some part of the
spectrum.  This means you can't get a complete IR spectrum from your sample.

If you are going to do this, you need a fluid cell.  These aren't terribly expensive.
Try to get one that allows you to change spacers. 

Good luck. 

[marquis]

Thought about this a little more.

There are a couple of techniques that might work.
Both involve ATR (or MIR).  That's  attenuated
total reflectance (ATR) or multiple internal reflectance
(MIR).  These techniques work on the surface of the
substance only.

Microscope FTIR ATR is your best bet. The problem
is that this equipment is expensive and hard to come by.

Another option is a liquid HATR (horizontal ATR)
cell for a conventional FTIR.

Might not work, but it is worth investigating.

Good luck

[DoctorDomo]

Cheers for the info marquis. Ah yeah I see what you mean about the solvent absorbing radiation in some part of your spectrum, so backgrounding with the solvent will result in an incomplete spectrum. Fluid cells sound interesting, I can probably get a hold of one of them. The other things you mentioned are well over my head at the moment.