Since ionization occurs in the sample gap, both the dry DART gas stream and analyte are exposed to open air. Different ionization mechanisms occur depending on the type of sample being analyzed (and its concentration), the nature of the carrier gas used and the polarity of the ions formed. A brief summary of mechanisms for both positive and negative ion formation, when helium is used as the carrier gas, is given below.
The metastable helium atoms formed in the source react with atmospheric water to produce ionized water clusters:
The He(23S) electronic excited state has an energy of 19.8 eV and a reaction cross-section of 100Å for water ionization. The protonated water formed after reacting with the excited-state helium metastable can then react with the analyte to form a protonated molecule.
Metstable helium atoms can react with a neutral (N), such as the grid electrode, or another neutral species to form electrons through Penning ionization:
The electrons formed are rapidly thermalized by collisions with atmospheric gases (G) and react with gaseous oxygen to produce ionized oxygen anions. These oxygen anions can then react with sample molecules (S) to produce analyte negative ions:
where: S is assumed to be a sample that contains hydrogen.
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