Page 186 - Illustrated Pocket Dictionary of Chromatography

188 STANDARD, EXTERNAL

Extrapolation of the data through to the x-axis yields the analytical
result.

standard, external Known purity materials that match the identity
of the analyte. These standards are analyzed separately from the sample
(contrast with internal standard). External standards are effective in
systems where the overall stability is extremely high. These standards
are used to make up calibration curves that cover the expected range
of the analyte in the sample. The sample concentration is then calcu-
lated directly from the curve established by the external standards.

standard, internal A compound that is similar to the analyte of
interest, is added to the sample before analysis, and is completely
resolved from all other peaks in the elution profile. The purpose of an
internal standard is to compensate for any variability that may occur
during the preparation, separation, and analysis processes. Although
not mandatory, an internal standard typically (1) is chemically similar,
(2) elutes near the analyte, (3) is commercially available in high purity
form, (4) is stable and unreactive under the conditions of preparation
and analysis, and (5) is not in the sample itself.

standard operating procedure (SOP) A written document that
formally details the manner in which a process, protocol, test, method,
or operation is to be performed so that a peer knowledgeable in the
field can understand and apply these instructions to gain valid results.

static headspace In this form of headspace analysis, a fixed
volume of liquid sample is placed in a septum-sealed vial. This vial is
heated to a predetermined temperature and allowed to equilibrate.
Once equilibrium is reached a sample is taken by syringe from the gas
layer above the liquid in the sample vial and injected into the GC
instrument. See purge & trap. The static headspace method is repro-
ducible and inexpensive but does not have as low a detection limit as
a dynamic headspace method such as Purge & Trap.
stationary phase The retentive component(s) of the packing
material. In adsorption chromatography the packing material itself is
the stationary phase, whereas in partition chromatography the
adsorbed liquid phase or bonded phase is the stationary phase.

steric hindrance Relates to the bulkiness of a molecule, which
limits the accessibility of other molecules to a certain space or area.
In bonded phase work this is particularly important because the size
of a C18 chain ultimately limits the percentage of the surface silanols