Detectors 5
Advanced User Guide 79
About the µECD
The micro-cell detector (µECD) contains a cell plated with
63
Ni,
a radioactive isotope. The
63
Ni releasesparticles that collide
with carrier gas molecules to produce low-energy
electrons—each particle produces approximately 100
electrons. The free electrons produce a small current—called the
reference or standing current—that is collected and measured
in a pulsed circuit.
When a sample component molecule comes into contact with
the free electrons, the electrons may be captured by the sample
molecules to create negatively charged ions. The voltage across
the cell electrodes is pulsed to collect the remaining free
electrons while the heavier ions are relatively unaffected and
swept out the vent with the carrier gas flow.
Cell current is measured and compared to a reference current.
The pulse rate is adjusted to maintain a constant cell current.
The more uncaptured electrons, the lower the pulse frequency
required to match the reference current. When a component
that captures electrons passes through the cell, the pulse rate
rises. This pulse rate is converted to a voltage and recorded.
µECD safety and regulatory information
The
63
Ni isotope
The radioactive isotope used in the cell is
63
Ni. It is plated onto
the inner surface of the cell body and is solid at temperatures
used in chromatography. Some other properties are listed
below.
Table 14 Properties of
63
Ni
Property Value
Half–life: 101.1 years
Emission: 65.87 keV max., beta radiation
Melting point: 1453 °C
Dimensions of the active part of the
µECD:
Inside diameter: 6 mm
Height: 4.2 mm
Total activity (µECD cell): 555 MBq (15 millicuries) maximum
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