Page 205 - Illustrated Pocket Dictionary of Chromatography

208 VAN DER WAALS INTERACTIONS

H = A + B u + C u

where A, B and C represent the different physical influences con-
tributing to peak spreading.

van der Waals interactions There are three basic types of
van der Waal interactions: dipole-dipole, dipole-induced-dipole, and
induced-dipole-induced-dipole. Other than hydrogen-bond interac-
tions, these are the interactions that effect/generate chromatographic
separations because they describe solvent/solvent, solvent/solute,
solute/solute, solvent/sorbent, and solute/sorbent interactions.
van’t hoff curve Generated through the plot of log V¢ versus 1/T,
where V¢ is the corrected retention volume and T is the system
temperature in °K. From a thermodynamic derivation of the retention
process:
log V¢= -D H o RT + D S o R - D V s ,

where DH o and DS o are the standard enthalpy and entropy changes
associated with the chromatographic equilibrium constant, K.
vaporization The critical phase change from liquid to gas that is
utilized for liquid sample introduction into a GC system. Note that
this fact is a serious limitation to the applicability of GC for samples
because the analyte of interest must be stable when volatized at high
temperature.
variable A measurable (or observable) characteristic of an analyte/
sample/event. Some common operating variables for GC are temper-
ature, flow rate, split ratio, injection volume, column type/length, etc.

variable-wavelength detector Uses a deuterium lamp for UV
work and a tungsten lamp for visible work and a monochrometer
(grating or prism) to select the operating wavelength.
2
variance, s Mathematically the variance is the square of the
deviation, s, of individual result from the mean divided by the total
number of results in the set. For the following set, 6.1, 6.4, 6.2, 6.2,
6.1, the mean is 6.2, the deviations are -0.1, +0.2, 0.0, 0.0, -0.1 and
2
2
2
s =- ( [ 01) + ( . 2 00) + (. . 2 01) ] 5 = 0 012
2
.
.
02) + ( .
.
00) =- (