Page 101 - Illustrated Pocket Dictionary of Chromatography

HOMOLOG AND HOMOLOGOUS SERIES 97

high-pressure gradient pump Utilizes multiple high-pressure
pumps to deliver different solvents into a gradient mixing chamber
that is positioned after the pumps. The gradient itself is therefore gen-
erated under high pressure. The advantage is that there is low dwell
volume for the system. The disadvantage is the fact that the heat of
mixing occurs immediately before the mobile-phase interaction with
the sample and the cost associated with multiple high-pressure deliv-
ery pumps.
Hildebrand solubility parameter, d Derived from the cohesive
energy density of a solvent, C:

C= D H - RT V m
(
)
where DH is the heat of vaporization for the solvent, R is the gas con-
stant, T is the temperature, and V m is the molar volume of the solvent.
The solubility parameter, d, is:

÷
d =C
3
The units for d are ÷cal/ ÷cm. This parameter was derived from
the fact that solubility should be a function of similar cohesive
energies and so it is assumed that solvents with similar d values
would be miscible in each other. See individual solvents for their d
value.

hold-up volume, V M Represents the entire volume of the system
that is accessible to a small molecule. The system hold-up volume,
determined through the use of an unretained/unexcluded compound,
includes the volume in the injector, connecting tubing, column, detec-
tor, etc. The column hold-up volume includes the interstitial and pore
volumes, V i and V p, respectively:
V M = V i + V p

homolog and homologous series A homolog is a molecule that
is related to others through a fixed and constant change. For example,
n-butane, n-pentane, n-hexane, and n-heptane form a homologous
series based on a methylene group (—CH 2—) increment. Kovat’s
retention index is based on the use of homologous series for a wide
range of stationary phases. HPLC is also used to stand for high-
pressure liquid chromatography.