ELECTROSPRAY AS A CONTROLLED CURRENT ELECTROLYTIC CELL - ELECTROCHEMICAL IONIZATION OF NEUTRAL ANALYTES FOR DETECTION BY ELECTROSPRAY MASS-SPECTROMETRY

In this paper an electrospray ion source is shown to be a controlled-c urrent electrolytic now cell which, when operated so that three key re quirements are met, can be used for efficient neutral analyte ionizati on (i,e,, complete analyte electrolysis) and sensitive gas-phase detec tion (i,e., minimized gas-phase signal suppression) in electrospray ma ss spectrometry (ES-MS), These three requirements are as follows: (1) the magnitude of the ES current, i(ES), must be sufficient for the oxi dization of the molar equivalent of all species available for reaction in the ES capillary that are as easily or more easily oxidized than t he targeted analyte, including all of the analyte; (2) the analyte mus t be available for reaction at the metal/solution interface in the ES capillary; and (3) the steps taken to ensure the first two requirement s must not inhibit the formation of gas-phase ions from the ions gener ated electrolytically in solution. The means to meet these requirement s are discussed, including the addition of an appropriate electrolyte to the electrosprayed solutions (e,g,, lithium triflate), the use of s lower now rates (e.g., 5.0 vs 40 mu L/min), and the use of a platinum capillary in the ES device, rather than the more commonly used stainle ss steel capillary. Neutral metallocenes, metalloporphyrins, and polyc yclic aromatic hydrocarbons are used as the model compounds, Operation of the ES ion source in the manner described expands the neutral comp ound types amenable to low level detection by ES-MS to include even th ose that are relatively difficult to oxidize (i,e,, E > 1.0 V vs SCE) and, therefore, also expands the universality of ES as an ionization s ource, From the electrochemical point of view, this operation of the E S ion source might be viewed as a means to provide molecular weight in formation, and possibly the structure, for the ionic products formed d uring a controlled-current electrolysis experiment.