Reactant Ion Formation in PTR-MS

When water vapor is passed through the discharge of a hollow-cathode ion source it is ionized by primary electrons, basically by EL Due to ion-molecule reaction sequences the product ion mixture forms HsO ions with high selectivity of up to 99.5% (Chap. 7.2.5) [51,52]. Even or ions from nitrogen rapidly react with water molecules in the source region to result in more HsO ions. Thus, a high (typical count rates of 10 counts s ) and almost pure flux of hydronium primary ions is provided that may then serve as reactant ions in a PICI process  

Fortunately this proton transfer reaction can generally occur for all VOCs as their PAs exceed that of water (FAwater = 697 kJ mol ) allowing for their exothermic protonation. 

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