tandem mass spectrometry electrospray ionisation tandem mass spectrometry CHMO:0000577 ESI-MS/MS electrospray ionization tandem mass spectroscopy electrospray ionisation tandem mass spectroscopy ESI-MS2 Mass spectrometry involving multiple mass-selection steps, with some form of fragmentation occurring between each stage. Sample ionisation is achieved by forcing a solution (usually in an organic solvent) of the sample through a small heated capillary (at a flow rate of 1–10 uL min-1) into an electric field to produce a very fine mist of charged droplets.