Page 163 - Illustrated Pocket Dictionary of Chromatography

164 REFLUX

reflux A distillation process whereby volatilized portions of the
sample are returned to the sample in order to recycle and produce
higher-purity distillate.

refractive index, n The refractive index of a material is calcu-
lated as the ratio of the speed of light in a vacuum, c, to that in the
substance, v:n = c/v. The refractive index is a function of wavelength,
20°C
temperature, and pressure and therefore is often denoted as n D
(assumed standard pressure), where D represents the sodium D-line
and a temperature of 20°C.
refractive index detector Monitors the change in the refractive
index between a reference solution (or solvent) and the sample. The
-6
output is therefore in refractive index units (often 1 ¥ 10 to 1 ¥ 10 -4
full scale). There are two fundamental types of refractive index detec-
tors: Fresnel and beam deflector. In both cases the chromatograms
can generate both positive and negative peaks (see figure). Both
require significant warm-up times to equilibrate. In both cases the
system temperature needs to be carefully controlled because small
temperature variation leads to significant baseline drift. In order for
this to occur it is best to work at least 5° above ambient temperature
(to allow the temperature control unit to operate effectively). For all
practical purposes gradient elution is incompatible with refractive
index detection; there is no effective way of rapidly compensating for
the reference solution composition change.

refractive index effect In UV-visible detectors, the change of the
refractive index of the effluent passing through the column can gen-
erate a “peak” that is not due to an absorbing species but is due to a
refractive index effect. This effect is most commonly observed near
the void volume and when a sample solvent varies considerably in
composition from the mobile phase into which it is injected. Remem-
ber that the changes in the refractive index of a solution cause the
incident beam to change refraction and thereby alter the intensity of
radiation reaching the detector, hence a “peak.”
regeneration (1) Column regeneration typically has a specific
meaning of restoring column performance back to or near its original
efficiency. For example, ion-exchange columns are regenerated by
flushing with an appropriate counterion and normal- and reversed-
phase LC columns are regenerated by back flushing with appropriate
strong solvents. (2) Column regeneration is also sometimes the term