Investigation of ion cluster formation in a pulsed ion mobility spectrometer operating in the negative mode

Ion mobility spectrometry (IMS) is a well-known technique for fast trace gases detection. Employing atmospheric pressure chemical ionization in IMS, ion clusters, e.g. protonated monomer or proton bound dimer analyte ions such as MH + (H 2 O) n or M 2 H + (H 2 O) n form in the positive mode, where the chemical ionization is based on positively charged reactant ions H + (H 2 O) n . In the negative mode, where the ionization is based on negatively charged reactant ions O 2 - (H 2 O) n , similar cluster formation is possible but less common. In this paper, we investigate 2-chlorophenol, formic acid and toluene-diisocyanate as single substances and in mixtures, showing their different behavior regarding the formation of symmetric and asymmetric clusters and thus the presence and absence of additional peaks in the ion mobility spectra. Quantum-chemical calculations regarding the stabilization energy can explain the absence of certain cluster signals quite well when based on the assumption of ionization by electron capture in contrast to the typically expected ionization by proton abstraction.