 Electrospray ionization allows a wider range of substances to be analysed than other ionization methods, due to its ability to ionize samples without thermal degradation.In 2017, Prof. Frank Crespilho and coworks introduced a new approach to operando DEMS, aiming the enzyme activity evaluation by differential electrochemical mass spectrometry (DEMS).NAD-dependent alcohol dehydrogenase (ADH) enzymes for ethanol oxidation were investigated by DEMS.