Enhanced desorption ionization using oxidizing electrosprays

Abstract

A signal enhancement of two orders of magnitude was achieved when reactive desorption electrospray ionization (DESI) was used to investigate copper(II) dibutyl dithiocarbamate, Cu(II)(bu 2 dtc) 2 , found in a specialized polymer. Cu(II) was oxidized to Cu(III) during the DESI experiment by oxidants in the spray solvent. Such oxidants could be present or formed during electrospray (e.g., O 2 ) or deliberately added to the spray solvent (this approach is called reactive DESI). When a strong oxidizing agent (e.g., iodine) was added to the spray solvent, the signal increased by two orders of magnitude relative to the pure solvent spray. The correlation between the standard reduction potential of the oxidant and the signal intensity and signal to noise ratio of the product ion for various reagents, was tested and discussed. The observed DESI enhancements in rates of oxidation are not observed in homogeneous solution. The major peaks in the collision induced dissociation (CID) . DESI is a heterogeneous process involving charged solution-phase droplets interacting with a condensed phase sample to yield gas-phase ions. It is therefore straightforward to add solution-phase reagents to the spray solvent to mediate the interfacial chemistry. This capability, sometimes called reactive DESI, has been used to perform selective ligand/protein binding [1], to stereoselectively generate covalent cyclic boronates from 1,2-diols [11] and to perform Eberlin transacetalization of acylium ions (Eberlin, M. N., unpublished results). These instances emphasize the analogy between DESI and chemical ionization (CI), even though the former employs solution-phase rather than gas-phase reagents as is done in the conventional CI experiment.We demonstrate in this paper that redox reactions can be used to advantage in DESI analysis (as they have previously been so used in electrospray ionization [12,13]). We report a very large enhancement in the efficiency of ionization of a particular metal complex present in a specialized layered ethylene/propylene/ diene polymer [14] by using redox spray reagents to assist in ionization. The analyte investigated is the neutral copper(II) dibutyl dithiocarbamate complex, Cu(II)(bu 2 dtc) 2 , which is ionized to [Cu(III)(bu 2 dtc) 2 ] ϩ (m/z 471) during the DESI experiment (eq 1). Electrochemical processes accompanying electrospray ionization (ESI) were first recognized by Kebarle et al. [12] and thoroughly investigated by Van Berkel and co-

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