Chemistry in electrospray mist: red-ox reactivity of nitrohalogenated aromatics during negative ion production

Electroactive neutral compounds have been submitted to electrospray mass spectrometry analysis (ESI/MS) and their chemical reactivity has been studied by running preparative experiments in the spraying conditions of ESI/MS, with the aim of understanding mechanisms of ionization. In the negative mode, the electrospray mass spectrum of 9-bromo-10-nitroanthracene (BNA) showed only bromide anions. By spraying a solution of BNA under similar conditions onto a metallic plate, the whole residue could be collected then analyzed by GC/MS. The major neutral components were 9,10-anthraquinone (AO) and its monooxime derivative (ANOH), the chemical yield increasing with increasing dilution of the analyte in the bulk solution and being quantitative for submicromolar solutions. The selective transformation of BNA (9-bromo-10-nitroanrhracene) into bromide anion and neutral oxidized derivatives suggests that preliminary electron transfers from metal to analyte at the capillary rip are followed by chemical transformations leading to products which are drawn within droplets to the collector. Compared to the time-scales successively involved in the electrospray process, the lifetime of the formerly produced anion radical (1 mu s) is inferior by at least three orders of magnitude, suggesting that chemical transformations are likely to occur inside the electroactive capillary rip, before the expelling of the solution. Thus, on the basis of the known electrochemical behavior of BNA (3-bromo-10-nitroanthracene) in a neutral liquid phase solution, the result of its ESI/MS analysis could be accounted for. (C) Academie des sciences/Elsevier, Paris.