Chapter 5. The Electron Capture Detector Part 1. Regulatory And Safety Information - Agilent Technologies 6890 Series Operating Manual

Chapter 5
The Electron Capture Detector
Part 1. Regulatory and Safety Information
Agilent Technologies provides two types of electron capture detectors. The
"regular" detector, abbreviated as ECD, has a larger internal volume
(approximately 10 times) than the micro-cell detector, abbreviated as µ-ECD.
The two types can be distinguished by the top cover of the detector—the ECD
has a solid cover and the µ-ECD has a perforated cover.
This chapter describes the "regular" ECD. The micro-cell detector (µ-ECD) is
described in the next chapter.
The ECD contains a cell plated with
β particles that collide with carrier gas molecules to produce low-energy
electrons—each β 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.
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