Tools II Voltammetric Detectors

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Tools II Voltammetric Detectors

• Methods Stripping, Metals
• DPV
• LC EC
• Rat Brains

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• Anodic Stripping Voltammetry
• General Process
• Hanging Mercury Drop Electrode (HMDE)
• Mercury Film Electrode
• Non-mercury Electrode

• Some electrode surface that forms an alloy with
  metals,OR, which catalyzes deposition
• Preconcentrate metal by reducing from large
  volume into the alloy
• Strip with an anodic sweep to oxidize from the
  alloy

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Deposition current And time of deposition
Concentration in the drop
HMDE, spherically shaped
ro
Compare to the Randles-Sevich Equation
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For a Mercury Film Electrode the rate of arrival
of the metal atoms at the surface For oxidation
is altered due to the geometry of the electrode
Compare to the peak for voltammetry at a thin
layer electrode
Because the current at a thin film Electrode is
of the same class as Current at a thin film, the
peaks drop To baseline, resulting in better
deiscrimination of Metals with similar potentials
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Figure 11.8.6 bard and faulkner
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Tool II Voltammetric Detectors

1. Methods Stripping, DPV
2. Metals
3. LC EC
4. Rat Brains

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Goal to use the electrstatic attraction for
accumpulation
Similar goal in using Nafion also anionic
material easily manipulated
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6 (a) Hickner, M. A. Ghassemi, H. Kim, Y. S.
Einsla, B. R. McGrath, J. E. Chem. Rev. 2004,
104, 4587. (b) Haile, S. M. Acta Mater. 2003, 51,
5981. (c) Grot W. Chem. Ing. Tech. 1972, 44, 167.
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Note the selectivity in potential and the
discrimination between various metals
Makes use of the enhanced microscopic surface
area and electric fields of the Msall particles
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JChem Ed 2007
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Tool II Voltammetric Detectors

1. Methods Stripping, DPV
2. Metals
3. LC EC, LC EC MS
4. Rat Brains

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Many organics are easily oxidized
Most solid metal electrodes do not oxidize in
this range and so have low background current
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Many others are easily reduced
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All are candidates for voltammetric analysis
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Signal to Noise in Voltammetric Analysis
The limit of detection (LOD) is often defined as
3xpeak to peak signal due to noise

• Noise in voltammetric methods is the result of
• Competing or background electron transfer events
  originating in the
• Solvent
• Surface
• 2. Currents arising from charging of the
  electrode surface

The limit of detection is also affected by
distortion of the signal due to IR error
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Solutions to the IR and Capacitive Current
Problems
Signal
Background
These are on two different time scales! Modulate
the time domain of the experiment to discriminate
against capactive currents.
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Square Wave Voltammetry
1 cycle
E
tp
1
2
Estep
t
2?Epulse
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Kissinger
Bard
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AC Voltammetry
Ac perturbation
DC sweep
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AC Voltammetry
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Differential Pulse Voltammetry
Want
t
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Summary
AC voltammetry
DP Voltammetry
SW Voltammetry
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GCE
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LC EC
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Control xD at a fixed distance Rotating Disk
electrode, RDE Wall Jet Electrode, WJE
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Glassy Carbon Electrode
CFME Carbon Fiber Microelectrode
Poly(3-methylthiophene) (P3MT)
Baredotted lines
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Set as Eapp
Point is that the peak is lost under flow
conditions, so that the selectivity is lost
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Set as Eapp
Point is that the peak is lost under flow
conditions, so that the selectivity is lost
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LC EC MS
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Some analytes are poorly suited to MS because of
the difficulty in the ionization Pathway.
P-chloroaniline (CPA) is one example
Anticarcinogen sulofenur
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Another way to get Ions for the mass Spec is to
allow Electrochemical Reactions with donors And
or acceptors
TMPD can be used as A donor for PAH,
while Dicyanodichloroquinone DDQ is used as an
acceptor
Nice exam question, Why?
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Tool II Voltammetric Detectors

• Methods Stripping, DPV
• Metals
• LC EC
• Rat Brains

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• An example of a unique system of electrochemistry
  which invokes
• Concepts of microelectrodes
• Suppression of capacitance
• Blocking of surfaces to enhance selectivity
• And the diquinone functionality just examined is
• The In vivo detection of dopamine in rat brains
  for nuerochemistry studies

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Diffusion in more than one dimension (Non-linear)
nanorod
Planar Electrode
Planar microelectrode
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Diffusion in more than one dimension (Non-linear)
This equation solved for Cottrell conditions
As t goes to infinity
Planar microelectrode
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C 1 mM r 250 nm, n 2, D 1x10-5 cm2/s
E
Eo
t
t0
The difference in Shape can be used To
simultaneously Determine n and D
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Fitting of the potential step chronoamperometry
(inset) leads to n 2 D 2.7x10-10 m2/s
Paddon, Christopher A., Debbie S. Silvester,
Farrah L. Bhatti, Timothy J. Donohoe, Richard C.
Compton, Coulometry on the Voltammetric
Timescale Microdisk Potential-Step
Chronoamperometry in Aprotic Solvents Reliably
Measures the Number of Electrons Transferred in
an Electrode Process Simultaneously with the
Diffusion Coefficients of the Electroactive
Species. Electroanalysis, 19, 2007, 1, 11-12
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Carbon Nanotubes
Two types of carbon surfaces
nanorod
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Nanotube surface Contains the basal Plane of
layered carbon And the broken edges
59 1/s
170 1/s
The two surfaces Have different e.t. Rate
constants
170 1/s
S. Tsujimura, T. Nakagawa, K. Kano, and T. Ikeda
Electrochemistry 2004 72 437-439
59 1/s
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Desirable as electrode surface because of
enhanced flux (3D solution to Ficks Laws)
Campbell Sun Crooks JACS 1000 121 3779
Multiple walled nanotube
Jun Li, et al JPChem B, 2002, 106, 9299-9305
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Dopamine is a potent neurotransmitter and hormone
in the brain. Can be supplied as medication that
acts on the sympathetic nervous system producing
effects such as increased heart rate and blood
pressure. Deficits in dopamine are linked to
behavioral diseases such as attention-deficit
hyperactivity disorder (ADHD). Abnormally high
dopamine levels have been linked to psychosis,
schizophrenia and cocaine addiction
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Typical cylindrical carbon-fiber microelectrodes
are 5-30 mircometers in diameter and 25-400
micrometers in length. Due to the small size of
the probe, minimal tissue damage occurs during
insertion into the brain
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B. Jill Venton and R. Mark Wightman, Anal. Chem.,
2003, 414A, Psychoanalytical Electrochemistry,
Dopamine and Behavior
Voltammetric detection of dopamine. When
sufficient potential is applied to the electrode,
dopamine is oxidized to dopamine-o-quinone,
donating two electrons that are detected as
current. When the potential is returned, any
dopamine-o-quinone remaining at the electrode
surface is reduced back to dopamine by accepting
electrons, producing current in the opposite
direction. In the example shown, the potential is
applied by fast-scan cyclic voltammetry. With
this technique, the resultant current comprises
time-resolved peaks that aid analyte
identification. These measurements are typically
repeated several times per second.
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InVivo Monitoring of Dopamine Release in Rat
Brain with Differential Normal Pulse
Voltammetry Analytical Chemistry 1984, 58, 3,
573, F. G. Gonan, Florence Navarre, and M .J.
Buda
Measurement of Dopamine (see structure) is in a
background of other Easily oxidizable materials
such as ascorbic acid (AA) 3,4-dihydroxyphenylacet
ic acid (DOPAC). Could not really solve the
problem by Instrumental methods so instead
inhibited the ability of the rat to produce DOPAC
by lesion to the brain.
DOPAC
Dopamine
Ascorbic Acid
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InVivo Monitoring of Dopamine Release in Rat
Brain with Differential Normal Pulse
Voltammetry Analytical Chemistry 1984, 58, 3,
573, F. G. Gonan, Florence Navarre, and M .J.
Buda
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B. Jill Venton and R. Mark Wightman, Anal. Chem.,
2003, 414A, Psychoanalytical Electrochemistry,
Dopamine and Behavior
B. Back ground current attributed to the charging
of the double layer (rearrangement of charged
species around the electrode) and is proportional
to the scan rate and capacitance of the
electrode. C. Resulting background-subtracted
cyclic voltammogram. Indicates that change in
current is attributable to oxidation of dopamine
and reduction of the electron form quinone back
to dopamine. Dopamine has an oxidation peak of
.6V
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Shows the response as a carbon-fiber
microelectrode is lowered through the nucleus
accumbens in 150 micrometer increments, while
dopamine fibers in the medial forebrain bundle
were electrically stimulated. In the release from
a single recording site in the nucleus accumbens
varied while the stimulating electrode was
incrementally lowered through the dopamine fiber
pathway. With these methods, an area w/ high
release can be targeted for measurements during
an experiment
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CVs can be compared based on peak height, the
relative ratio of oxidative and reductive peaks,
as well as peak location and shape
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Although using CV are a powerful tool for
identifying electroactive compounds, some species
such as norepinephrine and dopamine have nearly
identical CV and cannot be differentiated by CV
analysis alone. This is due to the structure
similarity. To Identify dopamine changes in vivo,
electrodes are placed in a known dopaminergic
regions w/ low norepinephrine content
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Reduce ascorbic acid background by coating the
electrode with A blocking membrane which exlcudes
anions
Information about Nafion here
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Drug effect on Dopamine
Represents examples of various drugs on
electrically stimulated dopamine release in
freely moving rates. Haloperidol a dopamine
receptor antagonist, increases dopamine release
by blocking D2 receptor on the dopamine
terminals. Nomifensine a dompamine transporter
antagonist causes an increase in dopaminergic
signal by blocking uptake. Ethanol a general
depressant that affects multiple ligand-gated ion
channels in the brain, decreases electrically
evoked dopamine releases
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Electrically evoked vs naturally occurring
dopamine
No signals were detected when the rats were
sitting quietly in the test cage. However, when a
barrier was lifted and the rats entered a novel
environment, dopamine transiently increased in
the nucleus accumbens. It is clear that dopamine
concentrations increased only at the initial
contact w/ the male and not before or afterward.
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Tools II Voltammetric Detectors

1. Methods Stripping, DPV
2. Metals
3. LC EC and LC EC MS
4. Rat Brains

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