ECD detector, Agilent 6890
The electron capture detector (ECD) cell contains 63Ni, a
radioactive isotope emitting high energy electrons (Boparticles).These
undergo repeated collisions with carrier gas molecules, producing
about 100 secondary electrons for each initial Boparticle.
Further collisions reduce energy of these electrons into the
thermal range. These low energy electrons are then captured by
suitable sample molecules, thus reducing total electron population
within the cell.
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 capturing material in
the cell.
Thus, the ECD responds to compounds having an affinity for
electrons (for example, such halogenated materials as pesticides and
related compounds).
General indication of expected sensitivity to different classes of
organic compounds.
|
Chemical Type
|
Sensitivity
|
|
Hydrocarbons
|
1
|
|
Ethers, esters
|
10
|
|
Aliphatic alcohols, ketones, amines; mono-Cl,
mono-F compounds
|
100
|
|
Mono-Br, di-Cl and di-F compounds
|
1000
|
|
Anhydrides and tri-Cl compounds
|
10-4
|
|
Mono-I, di-Br and nitro compounds
|
10-5
|
|
Di-I, tri-Br, poly-Cl and poly-F compounds
|
10-6
|
These are only approximate figures; sensitivity varies widely
within each group, depending upon structure of the material. For
example, DDT (5 chlorine atoms/molecule) can be measured in the 1 to
10 picogram range.
The ECD is designed for use either with N2 or with 5% CH4 in Ar as
carrier gas. A switch selects operation with either carrier gas; the
ECD does not operate properly if the switch is incorrectly set.
Because of its high sensitivity, the ECD should never be used without
traps (moisture, chemical), should be in good condition, and should be
installed in the carrier gas supply line. For capillary systems, this
must also be done for the makeup gas supply.
An ECD is normally used to detect compounds that are reactive
enough to interact with metal columns. Therefore, only ¼" packed
glass, or fused silica, or glass capillary columns are recommended
with this detector.
|
