A typical isotopic pair of biotinylated peptides from WM-115 (light, “L”) and WM-266-4 cells (heavy, “H”) used for quantification by ESI-MS. The identities of peptides were confirmed by MS/MS acquired in data-dependent scan mode. (A) ESI mass spectra of an isotopic peptide pair, IGDLQAFQGHGAGN-LAGLK#GR ([M + 3H]3+ ion, m/z 807.2) and IGDLQAFQGHGA-GNLAGLK@GR ([M + 3H]3+ ion, m/z 810.0) of the solute carrier family 3 protein from WM-115 and WM-266-4 cells; (B and C) the product-ion spectra of the biotinylated peptides carrying a light (B) and heavy (C) lysine.