DART ion source
DART (Direct Analysis in Real Time) is an atmospheric pressure ion source that instantaneously ionizes gases, liquids and solids in open air under ambient conditions. It was developed in 2005 by Laramee and Cody and is now marketed commercially by JEOL and IonSense. It was among the first ambient ionization techniques not requiring sample preparation, so solid and liquid materials can be analyzed by mass spectrometry in their native state. Ionization can take place directly on the sample surface, such as currency bills, tablets, bodily fluids (blood, saliva and urine), glass, plant leaves, fruits & vegetables and even clothing. Liquids are analyzed by dipping an object (such as a glass rod) into the liquid sample and then presenting it to the DART ion source. Vapors are introduced directly into the DART gas stream. DART ionization combines the techniques of thermal desorption and Penning ionization.
The ionization process involves an interaction between the analyte molecule (S) and electronically excited atoms or vibronically excited molecules (metastable species – M*):
Upon collision between the excited gas molecule (M*) and the surface of the sample, an energy transfer takes place, from the excited gas molecule (M*) to the neutral analyte molecule (S). This causes an electron to be released from the analyte molecule, producing a radical cation. The molecular cation is then ejected from the sampling surface and travels to the mass analyzer along with the gas stream (typically N2 or Ne). The process presented in the above equation is called Penning ionization. For this ionization process to take place, the energy of the excited state gas molecule must be higher than the ionization potential of the neutral molecule.
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