Deprotonation of trinitrotoluene by dichloromethane in atmospheric pressure chemical ionization mass spectrometry

RationaleTrinitrotoluene (TNT) and its derivatives are found in the environment. Depending on the analysis conditions, TNT may be detected as a deprotonated molecule or a molecular ion through atmospheric pressure chemical ionization (APCI). Dichloromethane (CH2Cl2) is used as an extraction solvent for TNT from environmental samples and as an ionizing agent to generate chlorinated species. APCI mass spectra of TNT dissolved in CH2Cl2 revealed the presence of only the [TNT - H](-) ion. This phenomenon was investigated by reactions with the CH2Cl2-related reactant ions. MethodsTNT in CH2Cl2 was ionized through APCI, and CH2Cl2 and acetone were used as the eluent. The vaporizing temperature was varied from 200 to 350 degrees C. TNT-related ions and reactant ions were analyzed. Detection limits for TNT were determined under different conditions. Energy-minimized structures of the product ions were used to interpret the analysis results. ResultsSignificant [TNT - H](-) ion was observed, but the [TNT + Cl](-) and TNT center dot- ions were not detected. The [HCl + Cl](-) and [3acetone + Cl](-) ions were detected as reactant ions when using CH2Cl2 and acetone as eluent, respectively. Other potential reactant ions such as CH2Cl2 center dot- and [CH2Cl2 + Cl](-) were not observed. The detection limit improved as the vaporizing temperature increased. The favorability of [TNT - H](-) formation was explained on the basis of the heats of reactions. ConclusionsThe [TNT - H](-) ion was primarily produced by the deprotonation reaction between CH2Cl2 center dot- and neutral TNT. This reaction was determined to be significantly exothermic. CH2Cl2 can be helpful for the analysis of TNT in environmental samples through APCI mass spectrometry.