The present application provides a method of quantifying an amount of a derivatized peptide displayed on a phage by phage display, the method comprising: providing a phage containing a target peptide thereon; reacting the phage containing the target peptide with a first reagent to derivatize the target peptide to form a derivatized peptide, reacting the derivatized peptide with a capture agent comprising a detection marker, thereby incorporating the detection marker within the derivatized peptide; and determining an amount of the detection marker, thereby quantifying the amount of the derivatized peptide dis-played on the phage. A kit comprising a capture agent compris-ing a detection marker for quantifying the phage displayed derivatized peptides is also provided.