[mushaboom]

I have taken several IR spectra of compounds where you would expect an OH peak, but one is either not present or not very clear. My TA's and professors have said that it's because we are using solid samples that have not been prepared in any way. What is the reason for this? I've tried to look it up online but have not had any luck.

[TheUnassuming]

I've done IR with many sugar containing polymers I've made, and the OH peak was always present.  This was true whether I used the old school KBr pellet or with an ATR.
Its possible I suppose that in the solid samples/crystals, the hydroxyls are engaged in hydrogen bonding interactions that alter or otherwise dampen the OH stretch you would expect.  This doesn't make much sense to me in the long run though because water shows up just fine and it is highly hydrogen bonded and liquid. 
If you get a good answer from your professors post it here so we can all learn something new .

[marquis]

I've done IRs on solid samples by microscope FTIR.
But it's not usually easy.

You are always looking for the thinnest place in the
sample for an IR when you do microscope.  It's not
always easy to find.  Much prefer solvent cast,
pyrolysate, KBr pellet, nujol mull, etc.

As mushaboom said, please let us know what you find out.

Thank you.