For residual Bio-Process Contaminants,
HPLC is not the "right tool for the job".
1) it is not sensitive enough
2) you can not be sure you could detect or identify BSA subunits, that could still be immunoreactive.
Most people use some type of ELISA for Process Residuals (BSA, Serum Proteins, Insulin, Host Cell Protein, residual Protein A, etc.)
Cygnus Technologies sells ELISA Kits for all of these. I have had good luck with them, and I feel they are well accepted in the industry.
Their BSA kit has a limit of detection of <250pg/mL.
see :
http://www.cygnustechnologies.com/index.html
Good Luck and Have Fun,