The easiest (and cheapest) way would be to tag the BSA prior to spiking the samples, which is easily accomplished with an ITC-Fluorophore. Just make sure you quantify the average amount of tagging and avoid oversaturation as this will dampen the signal on your Fluorescence Spec.

Vincent Paul Schmitz thank you. I've read that buffers containing amine groups cannot be used during the FITC labelling process. However, if I keep the labelling process amine-free and use the labelled BSA (after purification to remove unreacted dye) on a surface containing amine groups, will this be a problem?

Seems like a sound strategy, I usually just use PBS as a buffer and use molecular weight cutoff filters (~3 kDa) to remove residual dye (at which point you can do buffer exchange simultaneously). Fair warning: I've realized from doing MALDI-ToF/ToF that pretty much all cutoff filters have residual detergents that can form micelles with your protein samples. I usually run 8M urea through them a few times, followed by ddH2O, to ensure sample purity/integrity.

Vincent Paul Schmitz Great, thanks for the help.