choice. The SenCell is currently available only with a glassy carbon electrode of 2-mm diameter.
Under certain circumstances, other materials are favorable.
For example, for the analysis of iodide, a silver WE can be used. At the silver WE, the following
oxidation reaction occurs for iodide: Ag + I
-
→ AgI + e
-
This reaction already takes place at a very low working potential (1 mV), which results in
extremely high selectivity. This allows the identification of iodide in urine samples with almost no
sample pretreatment.
Table 15–1: Working potential limits and application area for different WE materials
WE
material
Potential limits (V)
Major application
Alkaline
Acidic
Glassy
carbon
-1.50
+0.60
-0.80
+1.30
Catecholamines
Gold
-1.25
+0.75
-0.35
+1.10
Carbohydrates
Platinum
-0.90
+0.65
-0.20
+1.30
Alcohols, glycols
Silver
-1.20
+0.10
-0.55
+0.40
Halides, cyanide
Copper
-
+0.60
-
-
Amino acids,
carbohydrates
Another consideration in choosing a WE is the oxidation or reduction of mobile phase
constituents or WE material that occurs when the potential exceeds the limits as given in the
preceding table. At high positive working potentials, the water in the mobile phase electrolyzes
and results in a strong increase in background current and noise. Formation of metal oxides,
resulting in an increase in background current, is a limiting factor for metal electrodes. Glassy
carbon and platinum have the highest positive potential limits and are therefore often used in
oxidative ECD. For negative potentials, the use of platinum electrodes is limited by the ease of
reducing hydrogen ions to hydrogen gas.
15.1.4 Detection limit
One of the most important parameters used to characterize the performance of a detection
system is the signal-to-noise ratio (S/N ratio), from which the concentration detection limit is
derived. It enables objective comparison not only between different electrochemical detectors but
also between complete analytical methods, irrespective of the detection system used.
Table 15–2: LC-EC conditions for analysis of norepinephrine
Parameter
Description
Column
ODS-2, 3 µm, 100 x 4.6 mm
Flow rate
1.0 mL/min
December 16, 2021, 715007395 Ver. 00
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