Page 44 - Illustrated Pocket Dictionary of Chromatography

CONCAVE 39

column volume, V c The volume contained within an unpacked
column tube:
2
V c =prL
where r is the column radius and L is the column length.

component (1) A unique and identifiable constituent in a defined
phase. (2) A unique and well-defined part of an instrument.

compressibility A measure of how much a liquid or gas changes
volume as a result of changing pressure. Liquids typically have a com-
pressibility of 0.005–0.015%/bar, and adjustments for this change are
not necessary. Gases, on the other hand, are strongly affected by pres-
sure changes, and a compressibility factor is used to correct for the
change.

compressibility factor, j The flow of a gas through a GC
system is not constant because the gas stream volume is considerably
compressed at the inlet of the column, where pressure is the greatest
compared with atmospheric pressure at the exit of the column.
This change is compensated for by the use of a compressibility
factor:

3
2
1
j = [ 3 ( p p o ) - ] [ 2 ( p p o ) - ] 1
i
i
where p i is the inlet pressure and p o is the outlet pressure. Therefore,
the average flow rate, F, through a column is :
F =¥ F
j
where F is the measured flow rate.
concave Describes a nonlinear gradient having a positive slope
that tends toward infinity:
A concave gradient is, for the %B or strong solvent, mathematically
described as:
n
(
%B = tT) ¥ 100
where t is the time since the start of the concave gradient, T is
the total time for the gradient, and n defines the steepness of the
gradient.