Page 75 - Illustrated Pocket Dictionary of Chromatography

EXTERNAL STANDARD 71

desolvated particles (or partially solvated droplets), and analyzed
through the resultant 90° scattering of a laser source.

exchange capacity A term used for ion exchange supports that
is defined as the number of equivalents of ion that the support can
adsorb per gram of support. Exchange capacity can be expressed on
a dry weight or wet (volume) basis. The latter is done because many
ion exchange supports are organic resins that swell dramatically when
they are solvated (some by as much as 50%). Typical exchange capac-
ities range from 50meq/g to 5meq/g.

excitation wavelength, l ex The wavelength that is designated
for incident radiation on a fluorescent molecule such that emission
will occur. (See fluorescence.) The relationship between the excita-
tion and emission wavelengths is shown in the figure under emission
wavelength. Note that there is overlap between the two manifolds
and that the excitation wavelength is always shorter than the emission
wavelength.

exclusion limit A parameter used in size-exclusion chromatogra-
phy that represents the lowest molecular weight that is excluded from
all pores in the packing material. This means that a column rated as
4
a 10 molecular weight limit will elute all molecular weights at or
4
above 10 at the same volume, that is, the void volume of the system.
See molecular weight calibration curve.
exclusion volume, V e Defined by the volume in a size-exclusion
column exclusive of the pore volume, that is, the interstitial column
volume only. Molecules that are too large to penetrate any pores in
the packing material still have access to the volume external of the
pores, the interstitial volume. In the exclusion volume all excluded
molecules elute at the same time.
external standard Method used when as instrument operation is
stable and reproducible over time. To work successfully, the external
standard material itself must be a high-purity form of the analyte of
interest. This material is used to generate a working curve from which
the analyte concentration is determined. For the external standard
method, standards are run before and after the sample series and for
longer runs between samples. This allows the analyst to assess system
performance continuously.