Desorption electrospray ionization is an ambient ionization technique which is a form of ionization where ions are formed in an ion source outside the mass spectrometer without sample preparation or separation  and ions can be formed by extraction into charged electrospray droplets.
Desorption electrospray ionization (DESI) is an ambient ionization technique that can be coupled to mass spectrometry for chemical analysis of samples at atmospheric conditions. Ambient ionization is a form of ionization in which ions are formed in an ion source outside the mass spectrometer without sample preparation or separation. Ions can be formed by extraction into charged electrospray droplets, thermally desorbed and ionized by chemical ionization, or laser desorbed or ablated and post-ionized before they enter the mass spectrometer. 
