iMALDI workflow. Stable-isotope labelled internal standard (SIS)-Ang-I (green) is spiked into plasma and the sample is incubated with anti-Ang-I-antibody conjugated beads. After immunoprecipitation of endogenous (blue) and SIS-Ang-I the beads can be placed directly on a MALDI target. To elute the analytes and permit MALDI-MS analysis, CHCA matrix is applied. The relative abundances of endogenous to SIS-Ang-I are used for quantitation.