the solution flow rate, the electrode material, geometry, and area,
and any other parameters that affect the flux of reactive species
to the electrode surface.1,3-5 The current magnitude limits the
extent or efficiency of the reactions. We have investigated the
experimental means to alter ES current (e.g., by addition of
electrolyte) and have investigated the effect of different types and
lengths of metal tubular electrodes (e.g., stainless steel, platinum,
and copper tubes) on the extent of oxidation of various analytes
in the ES emitter.3,10,11,14,15 However, the basic tubular electrode
design presents limited means to alter the electrode material and
area and, more generally, mass transport to the electrode. In
tubular electrode emitter designs, the emitter electrode and
emitter spray tip are one. The inner and outer dimensions of the
tube, and therefore, electrode geometry, area, and mass transport
distances, are restricted to a small range that provides a quality
spray. In addition, a limited number of conductive materials are
commercially available in tubular form of the appropriate dimen-
sions. This limits the materials that can be used as electrodes.
Moreover, tubular electrodes are easily plugged or damaged and,
therefore, depending on the material, can be an expensive
consumable (e.g., platinum capillaries).11
We describe in this paper a thin-channel, planar electrode ES
emitter device in which the electrode and a nonconductive emitter
spray tip are separated. This configuration allows electrode area,
electrode material, electrode covering, and flow path channel
height (or width) to be rapidly and conveniently changed. This
provides greater opportunity to control mass transport to the
electrode surface while more fully exploiting the known role of
electrode material in influencing electrochemical reactions,16
without negatively impacting spray quality. The data presented
demonstrate for nonelectroactive analytes that the analytical
performance of this ES ion source is very similar to traditional
ES ion sources that use tubular electrodes. For more easily
oxidized analytes, the device is shown to provide the means to
easily alter the extent of analyte oxidation from near 100% to an
insignificant fraction of the total amount of material flowing
through the system. Furthermore, the data demonstrate the major
influence electrode material and ES current magnitude can have
on the extent of analyte oxidation.
ES-MS. ES-MS experiments were performed on either a PE
Sciex API 365 triple quadrupole or API 165 single quadrupole mass
spectrometer (MDS Sciex). Solvent flow through the ES systems
in all cases was provided by a syringe pump or gas displacement
pump. Sample was introduced to the ion source by either
continuous infusion or by flow injection using a variable-volume
loop injector (Rheodyne model 7125, Cotati, CA). Experiments
with a normal capillary ES emitter were performed with a
TurboIonSpray ES ion source using a 3.5-cm-long stainless steel
capillary (MDS Sciex, 0.4 mm o.d. × 0.1 mm i.d.). No “turbo”
gas was used. In other experiments, the TurboIonSpray ES ion
source was removed and replaced with a thin-channel, planar
electrode, ES emitter device (Figure 1). This device used the
source housing and x-, y-, z-positioner of a Protana NanoES source
(Protana A/ S, Odense, Denmark) designed for use with the Sciex
instruments. With this arrangement, all instrument safety inter-
locks remained active. The planar electrode device was electrically
isolated from upstream components using a grounded stainless
steel union. The ES current was measured by grounding the
curtain plate (normally 1.0 kV) of the mass spectrometer through
a Keithley model 610C electrometer (Cleveland, OH) and lowering
the emitter electrode voltage by 1.0 kV. The spray capillary was
moved laterally beyond the sampling orifice so all of the charged
droplets impacted the curtain plate.
The planar electrode ES emitter device was composed of three
major components, viz., an inlet/ outlet block fashioned from
PEEK, a Teflon spacing gasket that defined the channel height
(and width), and a PEEK electrode block in which was embedded
a disk electrode. The latter two items were the same components
as those used in the cross-flow, thin-layer electrochemical flow
cell that is commercially available from Bioanalytical Systems, Inc.
(BAS, West Lafayette, IN). The PEEK inlet/ outlet block was
fashioned largely after the stainless steel auxiliary electrode block
of the BAS cell, including electrode and gasket alignment pins
and the hand-tightened screw that provides a means to secure a
leak-proof seal. However, our inlet/ outlet block was fashioned with
smaller bore through holes (250 µm) and did not contain a
reference electrode port. The particular electrodes used in this
study were disk electrodes offset from center (electrode edge at
block center line): 6.0-mm-diameter glassy carbon (GC); dual
6.0-mm GC; 3.0-mm GC; 6.3-mm platinum; 6.0-mm silver; 6.0-mm
copper; 6.0-mm zinc; and 6.0-mm 316L stainless steel. Spacing
gaskets used were 13, 51, and 127 µm thick providing cell volumes
above a 6.0-mm-diameter electrode of approximately 0.5, 1.9, and
4.9 µL, respectively. Solution exited the device and was sprayed
through a 3.5-cm length of 50-µm-i.d., 360-µm-o.d. fused-silica
EXPERIMENTAL SECTION
Samples and Reagents. Bovine insulin (Sigma, St. Louis,
MO), dopamine (Aldrich, Milwaukee, WI), N-phenyl-1,4-phenyl-
enediamine (Aldrich), and the polypropylene glycol tuning solution
(MDS Sciex, Concord, ON, Canada) were used as received.
Analyte solutions were prepared just prior to analysis to minimize
any oxidation by exposure to air and light. N-Phenyl-1,4-phenyl-
enediamine and dopamine solutions were prepared in a 1/ 1 (v/ v)
mixture of water (Milli-RO 12 Plus, Bedford, MA) and methanol
(Burdick and Jackson, Muskegon, MI) containing 0.75% (v/ v)
acetic acid (HOAc, PPB/ Teflon grade, Aldrich) and 5.0 mM
ammonium acetate (NH4OAc, 99.999%, Aldrich). The bovine
insulin solution was prepared in 1/ 1 (v/ v) water/ acetonitrile
(Burdick and Jackson) containing 0.1% (v) HOAc.
capillary with a TaperTip (New Objective, Inc., Woburn, MA). This
1
capillary was held in place using a
/ 16-in.-o.d. Teflon sleeve, a
PEEK ferrule, and the nebulizer tube from the TurboIonSpray
source. A small-gauge tube was welded to the nebulizer tube to
allow connection of the nebulizing gas supply to pneumatically
assist the ES process. Prior to all experiments, the electrodes were
freshly polished. In some experiments, a cellulose ester 5000 Da
molecular mass cutoff membrane (Sialomed, Inc., Columbia, MD)
was used to cover the working electrode.
(14) Kertesz, V.; Van Berkel, G. J. J. Mass Spectrom. 2 0 0 1 , 36, 204-210.
(15) Van Berkel, G. J.; Kertesz, V. J. Mass Spectrom. 2 0 0 1 , 36, 1125-1132.
(16) Laboratory Techniques in Electroanalytical Chemistry; Kissinger, P. T.,
Heineman, W. R., Eds.; Marcel Dekker: New York, 1996.
RESULTS AND DISCUSSION
P lanar Electrode Emitter Setup and Operation with
Nonelectroactive Analytes. A schematic illustration of the thin-
5048 Analytical Chemistry, Vol. 74, No. 19, October 1, 2002