Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions using an electrospray in which a high voltage is applied to a liquid to create an aerosol. It is especially useful in producing ions from macromolecules because it overcomes the propensity of these molecules to fragment when ionized.Recent development of the nanospray techniques, applied at ultralow flow-rates, allowed a sensitive analysis of compounds at femto/attomolar level.