Page 48 - Illustrated Pocket Dictionary of Chromatography

CRAIG COUNTERCURRENT DISTRIBUTION 43

correlation coefficient, r A mathematically determined value of
how closely a dependent variable is linearly related to the independ-
ent variable. Perfectly correlated data have an r value of 1.0 (direct
relationship) or -1.0 (inverse relationship). For many methods in
which detector response and concentration are expected to have a
linear correlation, minimum acceptable r values are typically >0.97.
To make a reasonable determination of the correlation between the
variables, it is typical for at least five equally spaced standard con-
centrations that span the anticipated linear range to be analyzed and
the responses plotted against concentration.

coulometric detector Used on LC; similar to an amperometric
detector except that the redox reaction is driven to 100% completion.
The importance here is that under these conditions, the total number
of coulombs transferred (i.e., the integrated peak area) is directly
related to the moles of analyte present in the sample through
Faraday’s law:

moles = Q nF

where Q is the charge transferred, n is the number of moles of
electrons used, and F is Faraday’s constant.

countercurrent chromatography (CCC) A technique that
uses two immiscible liquid phases, one stationary and one moving
through it, to separate, concentrate, or purify analytes.

counter ion (1) Used to compete with the sample ion for charged
surface sites on an ion-exchange column. Retention times and peak
shapes are strongly affected by the identity and concentration of the
counter ion. (2) Any charged species that, when associated with its
opposite charge, produces a neutral complex. For example either a
+
+
sodium (Na ) or a potassium (K ) ion is a counter ion to the monoba-
-
sic phosphate ion (H 2PO 4 ).
coverage The level of stationary phase that is deposited or bonded
to a support material. In LC coverage is expressed as micromoles/
gram or % carbon (w/w), whereas in GC it is weight percent or film
thickness.
Craig countercurrent distribution The Craig countercurrent
distribution apparatus was used to perform numerous distinct sequen-