Environmental Engineering Reference
In-Depth Information
(cathode) above the flame to collect the ions formed at the H
2
-air flame. The number
of ions hitting the collector is measured and a signal is generated. In FID, three gases
are therefore needed; H
2
and air as the auxiliary gases for the flame, and He used as a
carrier gas.
The FID is a universal detector and can be used for all organics with good
sensitivity and linearity. Exceptions include some non-hydrogen containing organics
(e.g., hexachlorobenzene) and compounds with carbonyl, alcohol, halogen, and
amine functionality that yield fewer ions or none at all in a flame. FID does not
respond to H
2
O and gives little or no response to most inorganic gases, such as CO
2
,
SO
2
,NO
x
, and other non-combustible gases. Since FID responds to organics but not
water, aqueous solutions can be directly injected. However, gases used should be
free of any organic impurities.
Electron Capture Detector
The ECD uses a radioactive chemical that can emit beta (b) particles (high-energy
electrons), which will in turn ionize some of the carrier gas (often N
2
) and produce
a burst of electrons. The production of electrons will generate current between a
pair of electrodes (Fig. 10.9). The b particles (electrons) are normally emitted
by radioactive
63
Ni. When organic molecules (M) that contain electronegative
functional groups are introduced from the column, they capture some of the
electrons and reduce the current measured between the electrodes. Reactions are as
follows:
Ni
63
! b
ð10
:
11Þ
b
þN
2
! 2e
þN
2
ð10
:
12Þ
Mþe
!M
ð10
:
13Þ
where M is an electronegative molecule such as those molecules with halogens,
phosphorous, and nitro groups. For the ECD to be detected, the molecule must
have these functional groups such that they are electron affinitive (electron-
capturing).
Anode
+
Gas outlet
Cathode
-
Electron source
from Ni
63
Chemical from
GC column
Figure 10.9
Electron capture detector
(ECD)
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