Ambient Ionization MS
Different types of desorption/ionization sources are classified according to their basic methods of operation, namely spray-based, laser-based, or plasma-based. Ambient ionization mass spectrometry techniques, such as direct analysis in real time (DART), desorption electrospray ionization (DESI), and extractive electrospray ionization (EESI), and similar related techniques have emerged in the past few years. Desorption atmospheric pressure photoionization (DAPPI) is the DESI analog of atmospheric pressure photoionization. DESI works better with polar analytes that are easy to protonate or deprotonate, whereas DAPPI is often used to ionize nonpolar or neutral compounds. DART produces relatively simple mass spectra dominated by protonated molecules in the positive ion mode or deprotonated molecules in the negative ion mode. EESI is an ambient ionization source similar to DESI, but it has two colliding aerosols: a sample spray and an electrospray. The LTP ionization source was based on dielectric barrier discharge that created a low-power plasma.