Nano-LC/ESI-MS reconstructed ion chromatogram of the holo-siderocalin sample. The siderocalin complex (1 μL) was injected onto the trap column at 10 μL/min, desalted, and eluted with a 5-min linear gradient from 10 % to 90% solvent B in 5 min (Solvent A = 5% acetonitrile, 0.1% formic acid; solvent B = 95% acetonitrile, 0.1% formic acid). The chromatogram was reconstructed using the signals corresponding to the most abundant charge states present in the ESI-MS spectra of apo-siderocalin (+17) and holo-siderocalin (+10). NGAL, neutrophil gelatinase-associated lipocalin (siderocalin).