Determination of urinary hepcidin-20 by comparison to stable isotope labelled hepcidin-20. All urine samples were diluted to 20 μg protein/ml and spiked with labelled hepcidin-20 prior to desalting on C8 beads and MALDI mass spectrometry. Figure 9a shows spectra of 5 urines with increasing levels of endogenous hepcidin-20 (m/z 2192) spiked with 80 ng/ml labelled hepcidin-20 (m/z 2202). Figure 9b shows the correlation between the hepcidin-20 concentrations determined from spiked experiments and the peak intensities in the original MALDI-DS experiment. Non-cancer controls are indicated by solid triangles and cancer patients by open squares.