Antec Scientific VT-03, User Manual, Page: 7 / 36

CHAPTER 1 The electrochemical flow cell 7
Table I. 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 Table I. At high positive working potentials the
water in the mobile phase electrolyses and results in an strong increase of
the 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.
Detection limit
One of the most important parameters used to characterise 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 what detection system is used.
Table II. LC-EC conditions for analysis of norepinephrine.
column ODS-2, 3 µm, 100 x 4.6 mm
flow rate 1.0 ml/min
mobile phase H
3
PO
4
50 mM, citric acid 50 mM, 20 mg/l EDTA, 100 mg/l
octane sulphonic acid (OSA), pH=3.1 with KOH, 5%
methanol
sample 1.0 µmol/l norepinephrine, 20 µl injection
temperature 30 o C
flow cell VT-03 flow cell with 3 mm GC WE mounted with 50 µm
spacer
E cell 800 mV (vs. Ag/AgCl, filled with saturated KCl)
I
cell
ca. 3 nA