Page 121 - Illustrated Pocket Dictionary of Chromatography

120 LEADING ELECTROLYTE

leading electrolyte The electrolyte that contains the highest-
mobility analyte ion in an isotachophoretic separation (see also
terminal electrolyte).

leak test Done to make sure that all unions and seals are properly
set. In GC gas leaks are often discovered by using a drop of a soap-
water or IPA-water mixture at the union and looking for air bubbles.
If bubbles appear, then the union is not properly set. In LC a piece of
tissue paper is placed in contact with the union. If there is a leak, then
solvent will be wicked up into the tissue, indicating that the union is
incomplete.

least squares For sets of data that are expected to be linearly
related, the technique used to determine the line that best fits the data
is called the least-squares fit of the line (also called linear regression
of the line). The least-squares technique minimizes the sum of the
squares of the dependent variable value from that of the determined
line. The resulting relationship is expressed as y = mx + b, where y is
the dependent variable, m is the slope of the line, x is the independ-
ent variable and b is the y-intercept value.

ligand A molecule that forms strong chemical interactions with
another atom or molecule. The ligand can be bound to a support
material as in affinity chromatography or free in solution. In affinity
chromatographic separations there are two classes of ligand: high-
specificity ligands that strongly bind to one molecule or one class of
closely related molecules, or general ligands that bind to entire classes
of compounds (e.g., lectins are ligands for sugar separations). In a
more general sense a ligand is an organic molecule (such as ethyl-
enediaminetetraacetic acid, EDTA) that binds to metal ions:

EDTA 4- + Ni 2+ ´ NiEDTA 2-
light-scattering detector cell The light-scattering cell gener-
ates a response based on the amount of laser light scattered 90° from
the beam path by particles flowing through it. The detector responds
x
as A = am , where a is the response factor for each individual analyte,
m is the solute mass, and x is the slope of the response.

linear flow velocity, u The velocity at which a component
moves through a column is called its linear velocity and is mathe-
matically expressed as:

u = L t