PerkinElmer CLARUS 590 GC, User Manual

Page: 127 / 284

ECD Overview . 127
ECD Overview
The Electron Capture Detector (ECD) is the most sensitive detector available for the analysis of electrophilic
compounds such as chlorinated hydrocarbons and pesticide residues. It is concentration-sensitive and
nondestructive.
The ECD cell contains a foil in a cylinder through which the carrier gas flows. The coating of the foil's inner
surface contains the radioactive isotope Nickel-63 and has a nominal sensitivity of 15 mCi. To optimize
detector response, make-up gas is combined with column effluent.
Beta particles emitted from the isotope ionize the carrier gas. The resultant ions and electrons travel to the
collector anode assembly under the influence of a pulsed polarizing voltage applied between the source and
the collector. The pulse frequency is varied to maintain a constant average current. The presence of an
electron absorbing species in the detector decreases the current as the absorbed electrons form ions, which
travel more slowly than electrons. The pulse frequency of the polarizing supply is automatically controlled
to maintain a constant current and is used to form the detector output signal.
The ECD works with nitrogen carrier or makeup gas or a carrier or makeup gas mixture of 95% argon/5%
methane. When using capillary columns at low flows (5 mL/min or less), helium or hydrogen may be used
as the carrier gas provided nitrogen or argon/methane is used as the make-up gas. A cross section view of
an ECD is shown below.
If you are using the ECD with a packed column, check to see whether the optional Flow Readout was
calibrated with nitrogen or argon/methane carrier gas. If not, select the appropriate carrier gas.
To function properly, the
total
carrier gas flow through the detector should be 30 mL/min or more. Where
required, the total carrier gas flow is adjusted to this value with make-up gas using the make-up gas
control valve located on the pneumatics control panel.
NOTE: If you want to switch between a FID and an ECD, when using the same packed column injector, use
nitrogen as the carrier gas for both detectors. Argon/methane is not recommended for use with an FID
because it produces a high background, more noise, and a wandering baseline
NOTE: To minimize detector contamination, run the ECD hot, at a temperature of at least 375 ºC.