PerkinElmer CLARUS 600 GC, User Manual

Page: 59 / 362

Clarus 600/680 GC User’s Guide
2-37
ECD
The Electron Capture Detector (ECD) is the most sensitive detector available for
the analysis of electrophilic compounds such as chlorinated hydrocarbons found
in pesticide residues. It is concentration-sensitive and nondestructive.
The ECD cell contains a nickel 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.
The 63 Ni radioactive foil coating and requires careful handling to ensure that it
does not create a safety hazard. In order to prevent a Thermal Runaway risk, the
GC shuts down if any heated zone exceeds 470º C. If this occurs, the following
message appears:
Warning
Instrument Shutdown
Thermal Runaway xxxxxx zone
Call your PerkinElmer service representative