The development of ambient ion sources is the only approach to fabricate miniaturized-MS systems that can analyze relevant analytes at trace to ultra-trace levels. Accordingly, many efforts have been made to develop ambient ion sources for mass spectrometer, such as desorption electrospray ionization (DESI) , probe electrospray ionization (PESI) , atmospheric pressure chemical ionization (APCI) , direct analysis in real time (DART) , matrix-assisted laser desorption/ionization (MALDI) and desorption atmospheric pressure chemical ionization (DAPCI) , electrospray-assisted laser desorption/ionization (ELDI) , dielectric barrier discharge ionization (DBDI) .