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Jun 2001
Detectors
Agilent 6890 Gas Chromatograph Service Manual
340 Electron Capture Detector (ECD)
Theory of operation
The ECD is based on the phenomenon that electronegative species can react
with thermal electrons present to form negatively charged ions. The loss of
such electrons is related to the quantity of analyte in the sample. In order to
produce capturable (low energy) thermal electrons, the carrier gas is ionized
by beta particles from a radioactive source in the cell. This electron flow
produces a current, which is collected and measured. When the sample
molecule is introduced into the cell, electrons which would otherwise be
captured at the electrode are captured by the sample, resulting in decreased
current. This change is what is recorded and measured for the chromatogram.
Figure 340-1 The Electron Capture Detector (ECD)
Vent line
Electrometer
Electrometer ribbon cable
Pneumatic lines
Detector body
(under cover)
Heater/sensor cable
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