Page 116 - Illustrated Pocket Dictionary of Chromatography

ISOTHERMAL 113

isopycnic Refers to the condition in which the mobile phase
is held at constant density (typical parameter monitored in SCF
work).

isotachophoresis The separation technique that is based on
constant-speed electrophoresis. In isotachophoresis, unlike CZE, the
buffer is heterogeneous across the support and is composed of a
leading electrolyte (one that moves faster than the analyte) and a ter-
minating electrolyte (one that moves more slowly than the analyte) .

isotherm The relationship between the concentration of a sub-
stance S in the mobile phase, C mp, and its concentration in the sta-
tionary phase, C sp, is defined by the substance’s isotherm under those
operating conditions. Under ideal conditions the distribution of S
between phases can be represented as:

S mp ´ S sp

The equilibrium distribution between phases is mathematically
represented as:

K eq = [ S sp ] [ S mp ] = C sp C mp

in this case a linear relationship and, hence, a linear isotherm. Non-
linear isotherm relationships can also be described mathematically.
Some more common representations are Langmuir and Freundlich
isotherms. See figure next page.

isothermal The operating condition in GC where the column tem-
perature does not change over the course of the separation is termed
an isothermal separation.