Electrochemical Detection
Amperometric, coulometric and voltammetric detection modes
A sample is introduced in HPLC and separated on the chromatographic
column. The column is connected to an ECD cell, an electrochemical
sensor where a reaction takes place at an electrode. Electrochemically
active substances that elute from the column undergo an
electrochemical reaction, electrons are transferred resulting in an
electrical current.
Electrochemical Detection
Evaluate the noise by measuring single 10x the width(time) of an analyte peak
C. E. Lunte et al. Electrochemical Detection in Liquid Chromatography and Capillary Electrophoresis,
in Laboratory Techniques in Electroanalytical Chemistry, Marcel Dekker, Chapter 27.
Electrochemical Detection
Hydrodynamic Voltammogram – Detection Potential
More positive the detection potential is the more species that can be
oxidatively detection , BUT also S/B and S/N ratios decrease.
BASi
Electrochemical Detection
Thin-Layer, Cross-Flow Design
Linear dynamic range often on the order of 6 orders of magnitude.
Detection can range from as low as 50 pmole/L and as high as 100 µmol/L or more
BASi
Electrochemical Detection
Lower noise
 DA 
il  1.47nFC 

 b 
2/3
U 1/ 3
U = avg. volumetric flow rate (cm3/min)
D = diffusion coefficient (cm2/s)
A = electrode area (cm2)
b = channel height (cm)
Electrochemical Detection
Most detection schemes use a single electrode configuration for detection.
Electrochemical Detection
Dual Electrode Detection – Parallel Configuration
Simultaneous measurement in the DUAL-PARALLEL amperometric detection mode can be
used to determine more than one compound in a chromatogram. For example, the
detector potential at one electrode may be set sufficiently positive to oxidize all
compounds of interest and the second electrode may be set at a substantially lower
oxidizing potential to only react with those compounds that are electrochemically active at
these lower potentials.
Electrochemical Detection
Dual Electrode Detection – Series Configuration
DUAL-SERIES electrochemical detection can improve selectivity and detection limits.
Compounds with higher collection efficiencies will dominate the response at the
downstream electrode and can be measured with improved selectivity. Not all
compounds have a reversible redox couple; these will not react at the downstream
electrode. The upstream electrode functions as a derivatizing (GENERATOR) electrode,
while the downstream electrode detects the product(s) created upstream.
Electrochemical Detection
Tubular Flow Cells
Blaedel et al.,
1970s and 80s
D. C. Johnson et al.
Problems: surface of the electrode is rather difficult to polish and modify, some
electrode materials cannot be fabridated in tubular form and iR drop can be an
issue with the large electrode area and the down stream placement of the
counter and reference electrodes.
Electrochemical Detection
In thin layer cells with a downstream counter electrode, the current between the working
and counter electrodes may not be uniform across the face of the electrode. The potential
between the working electrode and the solution will not be uniform across the face of
the electrode. Potential of the downstream edge will be closest to the value of the
potentiostat. Best to orient counter electrode normal to the working electrode.
Electrochemical Detection
Coulometric Detection (ESA)
Flow-through porous electrode - carbon
Could be a single or dual electrode design. Nearly 100% conversion
efficiency of the injected analyte molecules are detected by an oxidation
or reduction reaction. Lower detection limits achieved. Issues with larger
background currents, slow stabilization time and fouling. Not easy to
clean the electrode by conventional methods.
Electrochemical Detection
Useful for the detection of alcohols, amines, carbohydrates and sulfur compounds.
Au electrode
In pulsed amperometric detection (PAD), a working potential is applied for a short time
(usually a few hundred milliseconds), followed by higher or lower potentials that are used
for cleaning the electrode. The current is measured only while the working potential is
applied, then sequential current measurements are processed by the detector to produce
a smooth output.
D.C. Johnson, W.R. LaCourse, Anal. Chem., 62 (1990) 589A-597A.
Electrochemical Detection
 Linear dynamic range = 6 orders of magnitude is typical for many analytes
 Response variability = <5% RSD is common
 Limits of detection ranging from the femtomol (10-15 mol) upwards to micromol (10-6
mol)
 Selectivity controlled by the applied potential
 Stability, depends on the electrode material