Electrochemical Detection

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.

Amperometric, coulometric and voltammetric detection modes

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

BASi

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

Electrochemical Detection

3/1

3/2

47.1 Ub

DAnFCil

U = avg. volumetric flow rate (cm3/min) D = diffusion coefficient (cm2/s) A = electrode area (cm2) b = channel height (cm)

Lower noise

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

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.

Blaedel et al., 1970s and 80s

D. C. Johnson et al.

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)

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.

Flow-through porous electrode - carbon

Electrochemical Detection

D.C. Johnson, W.R. LaCourse, Anal. Chem., 62 (1990) 589A-597A.

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.

Useful for the detection of alcohols, amines, carbohydrates and sulfur compounds.

Au electrode

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