Anal. Chem. 2004, 76, 1493-1499
Controlling Analyte Electrochemistry in an Electrospray Ion Source with a Three-Electrode Emitter Cell Gary J. Van Berkel* and Keiji G. Asano
Organic and Biological Mass Spectrometry Group, Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6131 Michael C. Granger
ESA, Inc., Chelmsford, Massachusetts 01824
The inherent electrochemistry occurring at the emitter electrode of an electrospray ion source was effectively controlled by incorporating a three-electrode controlledpotential electrochemical cell into the controlled-current electrospray emitter circuit. Two different basic cell designs were investigated to accomplish this control, namely, a planar flow-by working electrode and a porous flow-through working electrode design, each operated with a potentiostat floated at the electrospray high voltage. Control of the analyte electrochemistry was tested using the indole alkaloid reserpine, which is often used to test the specifications of electrospray mass spectrometry instrumentation. Reserpine was relatively easy to oxidize (Ep ) 0.73 V vs Ag/AgCl) in the acidic electrospray medium (acetonitrile/water 1:1 v/v, 5.0 mM ammonium acetate, 0.75 vol % acetic acid) and was oxidized when the conventional electrospray emitter was used at low solution flow rate. With the proper cell auxiliary electrode configuration and adjustment of the working electrode potential, it was found that reserpine oxidation could be “turned off” at flow rates as low as 2.5 µL/min as well as at flow rates as high as 30-40 µL/min. Just as important, it was also possible to “turn on” essentially 100% oxidation of reserpine in this flow rate range. The area of the auxiliary electrode along with flow rate, which affect mass transport of analytes to this electrode, were found to be critical in controlling the electrochemical reactions in the emitter cell. Such control over analyte electrochemical reactions in the emitter has been difficult or impossible to achieve with a conventional electrospray emitter. This control is paramount in obtaining experimental results free from electrochemically generated artifacts of the analyte or in exploiting electrochemical reactions involving the analyte to analytical advantage. Electrochemistry is inherent in the operation of the electrospray ion source used in electrospray (ES) mass spectrometry * To whom correspondence should be addressed. Phone: (865) 574-1922. Fax: (865) 576-8559. E-mail:
[email protected]. 10.1021/ac035240m CCC: $27.50 Published on Web 02/04/2004
© 2004 American Chemical Society
(MS).1,2 The basic electrochemical behavior of the ES ion source is that of a galvanic cell: a two-electrode controlled-current electrochemical (CCE) flow cell.3 A CCE cell does not operate with a fixed interfacial potential at the working electrode. Rather, the interfacial potential is a complex function of the current density (determined by the magnitude of the current and the effective electrode area) and the flux of the various species in solution to the electrode, along with their various electrochemical properties. This is in contrast to a three-electrode controlled potential electrochemical (CPE) flow cell in which the interfacial potential of the working electrode can be controlled relative to a reference electrode with the use of a potentiostat.4 Thus, at the ES emitter electrode, the interfacial potential and the electrochemical reactions that occur can change, even though the current and voltage drop between electrodes do not, if the concentration or identity of the reacting species changes. As a result, it can be difficult to control or even determine what reactions are taking place at any given time at the emitter electrode.5 This lack of control over the electrochemistry at the emitter electrode may have analytical consequences. Under certain operational conditions, the ongoing electrochemistry can significantly alter the composition of the solution being electrosprayed.1,6 As such, the electrochemical processes can cause a change in the nature and distribution of ions that are observed in the ES mass spectrum. The influence of the electrochemistry is most obviously observed when the analyte is directly involved in the electrochemical reactions, resulting in a change in analyte mass or charge. Involvement of the analyte in the electrochemistry can be a good thing, for example, when one wishes to ionize or modify (1) Van Berkel G. J. In Electrospray Ionization Mass Spectrometry; Cole, R. B., Ed.; Wiley: New York, 1997; Chapter 2, pp 65-105. (2) de la Mora, J. F.; Van Berkel, G. J.; Enke, C. G.; Cole, R. B.; MartinezSanchez, M.; Fenn, J. B. J. Mass Spectrom. 2000, 35, 939-952. (3) Van Berkel, G. J.; Zhou, F. Anal. Chem. 1995, 67, 2916-2923. (4) Laboratory Techniques in Electroanalytical Chemistry; Kissinger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1996. (5) Van Berkel, G. J. “Insights into Analyte Electrolysis in an Electrospray Emitter from Chronopotentiometry Experiments and Mass Transport Calculations.” J. Am. Soc. Mass Spectrom. 2000, 11, 951-960. (6) Van Berkel, G. J.; Zhou, F.; Aronson, J. T. Int. J. Mass Spectrom. Ion Processes 1997, 162, 55-67.
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electroactive species to enhance their ES-MS response.7,8,9 Alternatively, one may use the inherent electrochemistry occurring in the emitter to activate analyte-labeling chemistries, as recently demonstrated by Rohner et al.10 In the analysis of unknowns, however, the change in mass or charge, or the spreading of charge among several molecular species, caused by direct or indirect analyte involvement in the electrochemistry may be detrimental.11 Thus, a major portion of the research on the electrochemistry of ES has been focused on the means to enhance or to minimize analyte electrolysis.12-14 Many examples of research in both these areas are documented in the recent review by Diehl and Karst.15 One means to gain control over the electrochemical reactions of the analyte at the emitter electrode is to incorporate this electrode as the working electrode of a CPE cell. This intertwining of a CPE cell into the CCE cell of the ES source produces a circuit in which a potentiostat and the auxiliary, reference, and working electrodes of a three-electrode cell are parallel to the counter electrode of the ES ion source. The counter electrode of the circuit is usually the atmospheric sampling aperture plate or inlet capillary and the various lens elements and detectors of the mass spectrometer.16 This combined circuit provides a means to control the emitter (working) electrode potential and, thus, determine which reactions are possible at this electrode. Incorporation of a CPE cell into the circuit should also overcome limitations in electrolysis efficiency caused by the relatively low level current (
