Page 66 - Illustrated Pocket Dictionary of Chromatography

62 ELECTRIC FIELD STRENGTH, E

electric field strength, E In capillary electrophoresis techniques
a potential is applied across the length of the column. The resulting
field strength, E, is described as:

E = ( E i - E d ) L

where E i is the potential at the injector end of the column, E d is the
potential at the detector end of the column, and L is the column
length.

electrochemical detector (ECD) These detectors are used in
LC and are of three basic types, all based on different aspects of elec-
trochemically generated “current”: (1) amperometric, which measures
current from the oxidation or reduction of the analyte, (2) conducto-
metric, which measures the change in solution conductivity as the
sample passes through a fixed electric field, and (3) potentiometric,
which measures the potential developed at an electrode as the sample
passes through the detector.

electrokinetic potential See zeta potential, z.

electron-capture detector (ECD) The electron-capture detec-
63
tor is used in GC. It utilizes a b-radiation source (most commonly Ni)
to ionize the carrier gas and thereby produce a constant stream of
electrons that generate a signal current. This current is measured and
is attenuated as analyte molecules pass through, causing a decrease
of current reaching the detector. This attenuation is recorded as the
output signal. The extent of the decrease in current is dependent on
the electron-capture cross section of the analyte and its concentra-
5
tion. The linear range of an ECD is ~10 with a minimum sensitivity
-9
of around 10 g. Very effective electron-capture functional groups
include halogen atoms (e.g., chloride, bromide), sulfur, phosphorus,
and nitro groups.
electron-impact ionization (EI) An EI source causes molecu-
lar ionization through the production of electrons that are accelerated
across the flow path of volatilized sample. The analyte is charged and
accelerated into a mass spectrometer detector:
+
-
A + e Æ A + fragments