Page 10 - Illustrated Pocket Dictionary of Chromatography

ADJUSTED RETENTION TIME, T R ¢ 3

CH 3 CN
Acetonitrile

achirality Condition in which a molecule and its mirror image are
superimposable (i.e., the molecule does not contain a chiral center).
Compare with diastereomer.

activity (1) The activity of an analyte is determined from the con-
centration of the analyte, [A], and its activity coefficient, g A,

Activity of A = g A []
A
(2) Used to describe the extent and strength of surface interactions
caused by residual hydroxyl groups (i.e., silanol and aluminol) in
adsorption support materials.

g
activity coefficient, g A Number experimentally generated to
quantitatively correct an analyte’s concentration for the nonideality of
the system (i.e., it is not at infinite dilution). In an ideal solution an
activity coefficient is 1.0. (See activity.) As a case in point, the activ-
ity coefficient for a solute in a liquid solution is dependent on the
charge density, shape, and size of the solute and the composition of
the ionic species in the solvent (e.g., ionic strength). As the concen-
tration and charge density of the ions increase, the activity coefficient
decreases.

adapter Component used to couple solid-phase extraction
columns or reservoirs.

additive See mobile-phase modifier.

adjusted retention time, t R ¢ The adjusted retention time is cal-
culated from the chromatographic retention time, t R, and the elution
time of a nonretained compound, t M:
t R ¢= t R - t M

The adjusted retention time is used to compensate for system-related
retention time variability and allows for a rapid comparison of results
from instrument to instrument and lab to lab.