Detector Systems
Electron capture detector (ECD)
138
Uncaptured electrons are collected periodically by applying short•term
voltage pulses to cell electrodes. This cell current is measured and
compared to a reference current, and the pulse interval is then adjusted
to maintain constant cell current.
Therefore, pulse rate (frequency) rises when an electron•capturing
compound is passing through the cell. The pulse rate is converted to a
voltage, linearly related to amount of electron•capturingmaterial in the
cell.
Flow Diagram, Electron Capture Detector (ECD)
Figure 7-6
Flow Manifold Block
Capillary
Makeup
Gas
Traps
Final Pressure Regulators
and Gauges
On/Off
On/Off
PURGE FLOW
MAKEUP GAS
Detector
Vent
Anode Purge Flow
Column
Makeup Gas Adapters
Capillary Columns
Only
Internal
Plumbing
External
Plumbing
Thus, the ECD responds to compounds having an affinity for electrons
(for example, such halogenated materials as pesticides and related
compounds). Table 7•2gives general indication of expected sensitivity to
different classes of organic compounds.
