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Encyclopedia of Physical Science and Technology EN008B-382 June 30, 2001 18:58

Liquid Chromatography 681

of the eluting peak gives a stable response with low de-
tection limit. Only reversed phase HPLC using a buffered
mobile phase is compatible with EC detection because the
solution must be electrically conductive. However, this is
not considered to be a practical limitation because about
80% of the HPLC applications are of the reversed-phase
type. Because nanoamp current levels can be measured,
the EC detector is very sensitive, on par with the fluores-
cence detector. HPLC with EC detection is the method of
choice for the determination of catecholamines and neu-
rotransmitters in biological fluids.
The pulsed amperometric detector (PAD) developed by
Johnson and co-workers using an Au or Pt electrode has
permitted the direct detection of aliphatic alcohols includ-
ing carbohydrates, amines, and sulfur compounds. Foul-
ing of these electrodes is prevented by application of both
positive (to eliminate sample adsorption) and negative (to
reduce any metal oxide) reactivation step potentials on
the order of 100 ms before resetting the potential for de-
tection of the analyte. The analytical current is usually
sampled near the end of the detection potential pulse to
permit decay of the charging current. The oxidation of
these aliphatic compounds such as carbohydrates is fa-
cilitated in basic solution at about pH 12, so postcolumn
addition of 0.1 M NaOH or the use of a polymeric column
with a basic mobile phase is required. Detection limits of
alcohols and carbohydrates are at the 10 ppb level. Alka-
nolamines, amino acids, and sulfur compounds other than
sulfonic acids and sulfones can also be detected.
The conductivity detector is based on the ability of ions
FIGURE 8 Electrochemical detector cell (Bioanalytical Sys-
to conduct electricity across two electrodes in a flow cell
tems, Inc.). (a) Diagram of the flow cell. A = auxiliary electrode,
between which an electric field is applied. The current
W = working electrode. R = reference electrode. (b) Dual thin-
layer working electrodes in parallel (1) and series (2) config- measures is proportional to the conductivity of the solu-
urations. [From Bratin K., and Kissinger, P. T. (1981). J. Liq. tion. Usually a sinusoidal wave potential is applied to the
Chromator. 4, 321–57. Reprinted with permission from Marcel electrodes and the only current measured is that in phase
Dekker, Inc.]
with the applied potential. Most conductivity detectors
opposite the carbon working electrode while the Ag/AgCl for HPLC can be set electronically to compensate for the
reference electrode is slightly downstream. Upon applica- background eluent conductivity. In addition, because the
tion of a voltage to the cell, oxidation of the solute of inter- mobility of ions varies with solution temperature, a ther-
est occurs and the resultant current is measured. Oxidation mistor is mounted close to the cell to permit electronic
of organic compounds such as phenols to the correspond- compensation. The conductivity detector has proved to be
ing quinone is the preferred mode of operation. Reduction important for ion-exchange HPLC of simple inorganic and
because of the interference of dissolved oxygen is more organic ions.
difficult but has been shown to be useful for nitro com- The evaporative light-scattering detector is an excellent
pounds. Dual electrode flow cells with either a parallel alternative to the refractive-index detector for nonvolatile
or series arrangement (Fig. 8b) have been developed. The organic compounds with no UV–VIS chromophore be-
parallel arrangement (1) permits oxidation or reduction of cause of the improved detection limits. Operation can be
the separated components at two different electrode poten- described by a three-step process (see Fig. 9) involving
tials providing extended detection capabilities. The series nebulization of the solvent, evaporation of the solvent in
arrangement (2) is useful to remove dissoloved oxygen a heated tube to give particles of pure solute, and finally
by reduction at the first electrode before detection of the measurement of the scattered laser light caused by the
sample solutes at the second electrode. The coulometric solute particles. Detection limits can be improved by gen-
EC detector which can oxidize or reduce a major fraction erating large particles through efficient nebulization and