A new online electrochemistry/liquid sample desorption electrospray ionization mass spectrometry (EC/LS DESI MS) system with a simple electrochemical thin-layer flow-through cell was developed and tested using N,N-dimethyl-p-phenylenediamine (DMPA) as a model probe. Although oxidation of DMPA is observed as a result of ionization of LS in positive ion mode LS DESI, application of voltage to the online electrochemical (EC) cell in EC/LS DESI MS increases yields of oxidation products. An advantage of LS DESI MS is its sensitivity in aqueous electrolyte solutions, which improves efficiency of electrochemical reactions in EC/LS DESI MS. In highly conductive low pH aqueous buffer solutions, oxidation efficiency is close to 100%. EC/ESI MS typically requires mixed aqueous/organic solvents and low electrolyte concentrations for efficient ionization in MS, limiting efficiency of electrochemistry online with MS. Independently, the results verify higher electrochemical oxidation efficiency during positive mode ESI than during LS DESI.

The ionization of LS samples in desorption ionization mass spectrometry (LS DESI MS), supplied continuously through a LS interface separated in space from the spray emitter, was investigated in this work. The role of electrochemistry (EC) in the ionization process was addressed. The visual (observation) of the operation of the LS DESI MS system showed a thick spray plume generated by the electrosonic spray ionization (ESSI), forming a liquid cone at the LS interface. When the LS interface was grounded the cone collapsed and the MS ion signal was lost, indicating that the LS was carried to the MS inlet by the spray that emerged from the cone. Ion signals in a new in-line LS DESI MS system, in angled LS DESI MS, and in electrospray ionization (ESI) MS, which produced the most intense ion signals from methanol/water solutions, and in ESSI MS, of dopamine (DA), tyrosine (Tyr) and N,N-dimethyl-p-phenylenediamine (DMPA), were evaluated using methanol/water and aqueous (aq) solutions. In addition, the effect on ion signals of geometric parameters and the LS and the spray solution flow rates was tested in in-line LS DESI MS. Of the methods tested, the analysis of aq LS containing electrolytes was simplest by LS DESI MS. The signal intensity was higher in in-line than in angled LS DESI MS. In online electrochemistry (EC)/LS DESI MS, when 0 V was applied to the EC cell Tyr ion signal was detected only at low pH (2).