I won't try FTIR to begin with!  Remember, 'weak radiation, weak response'.

Try HPLC equipped with a PDA/DAD and set at 210 nm.  The LOQ will be about 50 ppb.

I have gone down to 0.02 ug/ml of Indolicidin for FTIR. It worked for me.

Best

0.1 g maybe but you MAY only see the tertiary structure.  Most proteins look alike mainly because they have amide bonds, and there are 20 L-amino acids.