Expanded use of a battery-powered two-electrode emitter cell for electrospray mass spectrometry
Vilmos Kertesz
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Gary J. Van Berkel
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Published online May 12, 2006 Address reprint requests to Dr. V. Kertesz,
Organic and Biological Mass Spectrometry Group, Chemical Sciences Division, Oak Ridge National Laboratory
, Oak Ridge,
Tennessee 37831-6131, USA
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Organic and Biological Mass Spectrometry Group, Chemical Sciences Division, Oak Ridge National Laboratory
, Oak Ridge,
Tennessee, USA
A battery-powered, controlled-current, two-electrode electrochemical cell containing a porous flow-through working electrode with high surface area and multiple auxiliary electrodes with small total surface area was incorporated into the electrospray emitter circuit to control the electrochemical reactions of analytes in the electrospray emitter. This cell system provided the ability to control the extent of analyte oxidation in positive ion mode in the electrospray emitter by simply setting the magnitude and polarity of the current at the working electrode. In addition, this cell provided the ability to effectively reduce analytes in positive ion mode and oxidize analytes in negative ion mode. The small size, economics, and ease of use of such a battery-powered controlled-current emitter cell was demonstrated by powering a single resistor and switch circuit with a small-size, 3 V watch battery, all of which might be incorporated on the emitter cell. (J Am Soc Mass Spectrom 2006, 17, 953-961) 2006 American Society for Mass Spectrometry
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Eoperation of an electrospray ion source as
typilectrochemistry is an inherent part of the normal
cally configured for electrospray mass
spectrometry (ES-MS) [1, 2]. Oxidation reactions in positive ion
mode and reduction reactions in negative ion mode are
the predominate reactions at the emitter electrode
contact (i.e., the working electrode in this system) that
supply the excess of one ion polarity in solution
required to maintain the quasi-continuous production of
unipolar charged droplets and subsequently gas-phase
ions.
Our interest in this electrochemical process is
aimed towards better understanding the process to
devise means to control it for analytical advantage.
The electrochemical reactions at the emitter electrode
alter the composition of the solution being
electrosprayed, and they can also directly involve the
analytes being investigated. Thus, under certain
conditions, with particular types of analytes, the
electrochemical reactions in the ES ion source can
have a significant influence on the identity and
abundance of ions observed in an ES mass spectrum
[1, 3, 4]. In particular, we have been interested in
controlling direct heterogeneous electron-transfer
chemistry of the analytes under study and their
potential homogeneous chemical follow up reactions.
Basic principles of electrochemistry dictate [5] and
ES-MS experimentation and calculation [3] have shown
that varied degrees of control over the electrochemical
processes involving the analytes can be achieved by
managing one or more of three basic parameters, viz.,
mass transport to the ES emitter electrode, the magnitude
of the current (more precisely, current density) at the ES
emitter electrode, and the ES emitter electrode potential.
In our most recent research efforts, we have developed a
porous flow-through (PFT) electrode emitter [6, 7],
replacing the standard capillary electrode emitter, to provide
very efficient mass transport of analytes in solution to the
electrode even at flow rates approaching 1 mL/min. With
this emitter electrode design all of the analyte in solution
will contact the surface of the PFT electrode on passage
through the emitter and very efficient oxidation or
reduction of analytes can be achieved as long as the reactions
are not current limited, limited by the interfacial electrode
potential, or limited by other reaction rate considerations.
A PFT electrode emitter enhances the ability to
directly involve the analytes under study in the
electrochemistry of the ES process. However, one would like
to use this basic electrode configuration as a general ES
emitter so that it need not be replaced for experiments
in which analyte electrochemistry is not of interest or is
to be avoided. Control over which reactions can take
place at the emitter electrode, and the rate at which they
take place, is achieved by controlling the interfacial
potential of the electrode. This can be accomplished to
some degree with a single electrode system by limiting
the magnitude of the current at the emitter electrode
through adjustable parameters like solution
conductivity or ES voltage drop. Lower current magnitudes either
current limit the reaction (Faradays law) or lower the
current density at the electrode so that the potential at
the electrode drops to a level lower than that required
for the analyte reaction (but still sufficient for another
reaction, e.g., solvent electrochemistry, to provide the
required current). Another means to control the
potential with a single electrode emitter is through the (...truncated)