Page 323 - Instrumentation Reference Book 3E

Polarography and anodic stripping voltammetry 307
of very low volume, and inside diameters of com- 15.3 Polarography and a~~~~~
ponents should be of similar size. Tubing of stripping voltammetry
2-3 nun internal diameter is most often used. and
temperature control is sometimes necessary. This 15.3.1 Polarography
can be done by water-jacketing or by containing
the columns within air-ovens. Polarography is an electrochemical technique, and
a specific polarographic sensor for the onstreain
determination of oxygen in gas streams is
Stcitionary phases A very wide variety of mater- described in Chapter 18. However, there are also
ials can be used as solid stationary phases for many laboratory polarographic instruments; these
HPLC-a summary of materials to use has been are described briefly here together with the related
compiled (R. E. Majors, Ani. Lab., 4(5), 17, May technique of anodic stripping voltammetry.
1972). Particle sizes must be small: e.g., 35-50 pni
and 25-35 pm.
There are various methods of packing the sta- 15.3.1.1 Direct current poltrograpliy
tionary phase into the column. Materials such as In polarography an electrical cell is formed with
ion exchange resins, which swell when they come two electrodes immersed in the solution to be
into contact with a solvent, must be packed wet as analyzed. In the most simple version of the tech-
a slurry. Other materials are packed dry, with the nique (d.c. polarography), the anode is a pool of
column being vibrated to achieve close packing. mercury in the bottom of the cell (although it is
Packed columns should be evaluated before use often preferable to use a large capacity calomel
for efficiency (a theoretical plate height of about electrode in its place), and the cathode consists of
0.1 mm), for permeability (pressure required), and a reservoir of mercury connected to a fine glass
speed. (Theoretical plate height is a measure of the capillary with its tip below the surface of the
separating efficiency of a column analogous to the solution. This arrangement allows successive fine
number of separating plates in a liquid distillation drops of mercury to fall through the solution to
column.) ‘Guidance on column packing materials the anode at the rate of one drop of mercury
can be obtained from manufacturers such as every 3 or 4 seconds. Figure 15.3 shows the
Pechiney-St. Gobain, Waters Associates, E.M. arrangement in practice. The voltage applied
Laboratories; Reeve Angel, Restek, Dupont, and across the two electrodes is slowly increased at a
Separations Group. constant rate and the current flowing is measured
and recorded.
Mobile phase The mobile phase must have the Figure 15.4 shows the step type of record
correct ”polarity” for the desired separation, low obtained; the oscillations in the magnitude of
viscosity, high purity and stability, and com- the current are due to the changing surface area
patibility with the detection system. It must also of the mercury drop during the drop life.
dissolve the sample and wet the stationary phase. The solutions to be analyzed must contain an
“inert“ electrolyte to reduce the electrical resist-
ance of the solution and allow diffusion to be
Detectors Commercially available detectors the major transport mechanism. These electro-
used in HPLC are fluorimetric. conductiometric, lytes can be acids. alkalis: or citrate, tartrate, and
heat of absorption detector. Christiansen effect acetate buffers, as appropriate. The cells are
detector, moving wire detector, ultraviolet designed so that oxygen can be removed from
absorption detector. and the refractive index the solution by means of a stream of nitrogen,
detector. The last two are the most popular. for otherwise the step given by oxygen would
Ultraviolet detection requires a UV-absorbing interfere with other determinations. The voltage
sample and a non-UV-absorbing mobile phase. range can run from +0.1 to -2.2. volts with
Temperature regulation is not usually required respect to the calomel electrode. At the positive
Differeintial refractometers are available for end the mercury electrode itself oxidizes; at the
HPLC, but refractive index measurements are negative end the “inert” electrolyte is reduced.
temperature-sensitive, and good temperature con- The potential at which reduction occurs in a
trol is essential if high sensitivity is required. The given base electrolyte, conventionally the half-
main advantage of the refractive index detector wave potential, is characteristic of the reducible
is wide applicability. species under consideration. and the polaro-
HPLC has been applied successfully to analysis gram (the record obtained during polarography)
of petroleum and oil products, steroids, pesti- thus shows the reducible species present in the
cides, analgesics. alkaloids. inorganic substances, solution. The magnitude of the diffusion current
nucleotides, flavors, pharmaceuticals, and envir- is a linear function of the concentration of the
onmental pollutants. ion in solution. Thus, in Figure 15.4. E112 is