Page 114 - Illustrated Pocket Dictionary of Chromatography

ISOELUOTROPIC 111

by milling larger particles down to smaller sizes. Sieving and other
sizing techniques are then used to produce a product with a desired
average particle size. Irregular particles are infrequently used in HPLC
work, having been replaced by spherical supports. Irregular particles
are commonly found in semipreparative and preparative columns,
solid-phase extraction columns and cartridges, and TLC plates
because of the lower cost of production compared with spherical
materials.
irreversible adsorption Occurs when the interactions of the
analyte with any component of the column, usually the packing mate-
rial, are so strong that the mobile phase cannot cause it to elute.

isobaric Refers to the condition in which the mobile phase is held
at a constant pressure.

isoconfertic Refers to the condition in which the mobile phase is
held at a constant density.

isocratic elution An HPLC system generating a separation under
conditions in which composition of the mobile phase does not change
with time. Contrast with gradient.

isoelectric point, pI The pH at which the effective net charge for
an ampholyte (or polyampholyte) is zero. For a compound that has
one base and one acid functional group, for example, an amino acid,
the isoelectric point can be calculated directly from the pK a values of
the carboxylic acid and the protonated amine:

pI = [ pK a,-COOH + pK a,-NH ] 2
+
3
It should be noted that at the pI for a species the solubility is typically
at its lowest.

isoeluotropic Two solvents that generate the same average
elution time for a range of analytes are termed isoeluotropic. These
mobile phases would also be said to have the same solvent strength.
Note that this determination must be done on identical column
packing with all operating parameters identical (except for the
solvent). Also note that although the average retention for the test set
of analytes is the same, the retention of a particular group of analytes
(say phenolics) may be greater in one system over another.