Page 252 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

8 COLUMN AN0 THlN-lAVER LlllUlO CHROMATOCRAPHV

packing techniques are used in which the particles are suspended in a suitable
solvent and the suspension (or slurry) driven into the column under pressure.
The essential features for successful slurry packing of columns have been
~ummarised.~~ Many analysts will, however, prefer to purchase the commercially
available HPLC columns, for which the appropriate manufacturer's catalogues
should be consulted.
Finally, the useful life of an analytical column is increased by introducing a
guard column. This is a short column which is placed between the injector and
the HPLC column to protect the latter from damage or loss of efficiency caused
by particulate matter or strongly adsorbed substances in samples or solvents. It
may also be used to saturate the eluting solvent with soluble stationary phase
[see Section 8.2(2)]. Guard columns may be packed with microparticulate
stationary phases or with porous-layer beads; the latter are cheaper and easier
to pack than the microparticulates, but have lower capacities and therefore
require changing more frequently.
Detectors. The function of the detector in HPLC is to monitor the mobile phase
as it emerges from the column. The detection process in liquid chromatography
has presented more problems than in gas chromatography; there is, for example
no equivalent to the universal flame ionisation detector of gas chromatography
for use in liquid chromatography. Suitable detectors can be broadly divided
into the following two classes:
(a) Bulk property detectors which measure the difference in some physical
property of the solute in the mobile phase compared to the mobile phase
alone, e.g. refractive index and conductivity* detectors. They are generally
universal in application but tend to have poor sensitivity and limited range.
Such detectors are usually affected by even small changes in the mobile-phase
composition which precludes the use of techniques such as gradient elution.
(b) Solute property detectors, e.g. spectrophotometric, fluorescence and electro-
chemical detectors. These respond to a particular physical or chemical
property of the solute, being ideally independent of the mobile phase. In
practice, however, complete independence of the mobile phase is rarely
achieved, but the signal discrimination is usually sufficient to permit
operation with solvent changes, e.g. gradient elution. They generally provide
high sensitivity (about 1 in IO9 being attainable with UV and fluorescence
detectors) and a wide linear response range but, as a consequence of their
more selective natures, more than one detector may be required to meet
the demands of an analytical problem. Some commercially available
detectors have a number of different detection modes built into a single
unit, e.g. the Perkin-Elmer '3D' system which combines UV absorption,
fluorescence and conductimetric detection.
Some of the important characteristics required of a detector are the following.
(a) Sensitiuity, which is often expressed as the noise equivalent concentration,
i.e. the solute concentration, C,, which produces a signal equal to the
detector noise level. The lower the value of C, for a particular solute, the
more sensitive is the detector for that solute.

*The conductance detector is a universal detector for ionic species and is widely used in ion
chromatography (see Section 7.4).