FTIR compared to protein quantitation using absorbance at 280 nm (A280)

The UV-Vis-based method of protein quantitation relies on absorbance at 280 nm by a protein’s aromatic amino acids, predominantly tryptophan with minor contributions from tyrosine. Therefore, protein extinction coefficients can vary widely (greater than two-fold difference between extinction coefficients of albumin and immunoglobulin G). Those proteins that do not contain tryptophan, such as Protein A, are very difficult to quantify accurately based on 280 nm absorbance. Also, secondary, tertiary, and quaternary structure all affect absorbance, therefore factors such as pH, ionic strength, etc. can alter the absorbance spectrum. In contrast, FTIR quantitation detects amide bonds, which are present in all proteins and whose IR spectrum is independent of protein structure.
What this means is that you can treat proteins like BSA like a more universal standard curve without being biased by the effects of protein sequence on absorption and thus affecting the slopes of different proteins at varying concentrations.