Occurrence of radical molecular ions in atmospheric pressure chemical ionization mass spectra of heterocyclic compounds.

It is well-known that the use of atmospheric pressure ionization (API) techniques, such as electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI), is associated with the formation of even-electron ions,1 for example, protonated molecules [M C H]C, adducts with alkali metal ions [M C Na]C and [M C K]C, adducts with ammonium ions [M C NH4]C in the positive-ion mode and deprotonated molecules [M H] and some adducts with small inorganic anions in the negative-ion mode. The relative abundances of fragment ions are generally low to negligible and mostly they have also even number of electrons, otherwise even-electron rule would be contradicted. Odd-electron ions do not typically occur in ESI/APCI mass spectra, but some exceptions are known,2 for example, radical molecular ions M ž formed by an electron capture3 or charge exchange mechanisms4,5 in the negative-ion APCI mode, and radical molecular ions MCž formed by the charge exchange mechanism in case of nonpolar polyaromatic hydrocarbons.6,7 In the present communication, we would like to highlight few well-pronounced examples on the formation of radical molecular ions both in the positive-ion and negative-ion APCI modes using an ion trap Esquire 3000 (Bruker Daltonics, Bremen, Germany) or a single quadrupole Platform (Micromass, Manchester, UK) under the following experimental conditions: the mass range m/z 50–1000, the target mass m/z 250 and the compound stability 20% for alkyl series from C2 to C4, the target mass m/z 350 and the compound stability 100% for alkyl series from C5 to C10, pressure of the nebulizing gas 15 psi, the drying gas flow rate 4 l/min, temperature of the ion source 350 °C and temperature of drying gas 300 °C. The samples were dissolved in acetonitrile and delivered into the system by an infusion pump at the flow rate of 5 μl/min. In some experiments, different solvents (methanol, benzene, or water) were used to compare results with the measurements in acetonitrile. Unlike the conventional electron ionization, the formation of odd-electron molecular ions is quite unusual in API mass spectra. The preference of radical ion formation depends on the structure, the ionization mode, and the experimental conditions, for example solvents used for the analysis. In ESI, the occurrence of molecular radical ions is very rare and it is usually attributed to the electrochemical processes on the electrospray needle or different oxidation states of metal ions. On the other hand, the presence of radical molecular ions in the latest API technique, atmospheric pressure photoionization (APPI), is relatively common due to the different ionization mechanism.5 The presence of molecular radical ions is not typical for APCI, but some examples have been reported for polyaromatic hydrocarbons without other functionalities in the positive-ion mode6,7 or for compounds with the high electron affinity and low gas-phase proton affinity in the negative-ion mode.8 The formation of radical molecular ions MCž in the positive-ion API mode can be explained by a charge exchange mechanism2: