Formic Acid as a Dopant for Atmospheric Pressure Chemical Ionization for Negative Polarity of Ion Mobility Spectrometry and Mass Spectrometry

Formic acid (FA) is introduced as a potent dopant foratmosphericpressure chemical ionization (APCI) for ion mobility spectrometry(IMS) and mass spectrometry (MS). The mechanism of chemical ionizationwith the FA dopant was studied in the negative polarity using a coronadischarge (CD)-IMS-MS technique in air. Standard reactantions of the negative polarity present in air are O-2 (-)& BULL;(CO2)( n )& BULL;(H2O)( m ) (m = 0, 1 andn = 1, 2) clusters. Introduction of the FA dopant resulted in theproduction of HCOO-& BULL;FA reactant ions. The effectof the FA dopant on the APCI of different classes of compounds wasinvestigated, including plant hormones, pesticides, acidic drugs,and explosives. FA dopant APCI resulted in deprotonation and/or adduction formation, [M - H](-) and [M + HCOO](-), respectively. Supporting density functional theory(DFT) calculations showed that the ionization mechanism depended onthe gas-phase acidity of the compounds. FA dopant APCI led to theimprovement of detection sensitivity, suppression of fragmentation,and changes in the ion mobilities of the analyte ions for analyteswith suitable molecular structures and gas acidity.