Labeled peptide Sequence #of aa MW (Av.), Da HC-3-1024 mAb IS Acetyl-NH-VKQRPGQG-[L(13C6;15N)]-EWIGEINPSSGR-COOH 21 2357.6 HC-1-992 mAb IS NH2-EVQLQPGSE-[L(13C6;15N)]-VRPGASVKLS-CONH2 21 2228.5 HC-3-total IS Acetyl-NH-YRVVSVLT-[V(13C5;15N)]-LHQDWLNGK-COOH 18 2175.5

Table 1: Primary sequence of the stable isotopes (internal standards).