Chemistry Reference
In-Depth Information
Anode (+)
e
-
Carrier
gas in
Carrier
gas out
β-emitter
Cathode (-)
Figure 2.15
Electron capture detector.
2.6.2 Electron Capture Detector
The electron capture detector (ECD), as its name suggests, works by captur-
ing electrons. The ECD (Figure 2.15) is a selective detector with greater sen-
sitivity for specific elements (i.e., those with high electron affinities, such as
halogens). It has a more limited linear dynamic range (10
4
) compared to the
FID. The typical carrier gas for GC-ECD is nitrogen.
An ECD consists of a small radioactive source,
63
Ni (a β-emitter), that
produces electrons on collision with the carrier gas, producing a standing
current that is measured:
N
2
+ β → N
2
+
+ e
-
(2.2)
The electrons generated then interact with an eluting compound (X),
resulting in a decrease in the standing current. It is this reduction in stand-
ing current as a result of the generation of an anion (X
-
) that the presence of
a compound is measured:
X + e
-
→ X
-
(2.3)
Finally, the generated compound anion (X
-
) then interacts with the
charged carrier gas (N
2
+
), resulting in the generation of two neutral com-
pounds (i.e., the compound X and carrier gas N
2
):
X
-
+ N
2
+
→ X + N
2
(2.4)
The ECD is therefore a nondestructive detector; care is needed with the
venting of toxic gaseous products into the laboratory. The GC-ECD should have
appropriate ventilation via a fume hood. In forensic science, GC-ECD can be
used in the analysis and identification of nitro-organic explosive compounds.
2.6.3 Nitrogen-Phosphorus (or Thermionic) Detector
The nitrogen-phosphorus (or thermionic) detector (Figure 2.16) is both a
destructive and selective detector. It functions in a very similar way to the
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