the bead is not electrically heated the response is the same as a conventional FID. The detector is thus a
three-in-one device which can be very easily switched between the three modes FID, NP, and P only.
Sensitivity for nitrogen and phosphorus compounds exceeds 10–^13 g s–^1 and the linear range is about 10^5.
Electron Capture Detector (ECD)
This is the most widely used of several detectors which employ a β-ray ionizing source. A schematic
diagram is shown in Figure 4.21. Unlike the FID the electron capture detector depends on the
recombination of ions with free electrons and therefore measures a reduction in signal. As the nitrogen
carrier gas flows through the detector a tritium or^63 Ni source ionizes the gas forming 'slow' electrons
which migrate towards the wire anode under an applied potential difference of 20–50 V. The drift of
'slow' electrons constitutes a steady current while only carrier gas is present. If a solute with a high
electron affinity is eluted from the column, some of the electrons are 'captured' thereby reducing the
current in proportion to its concentration. The detector is very sensitive to compounds containing
halogens and sulphur, anhydrides, peroxides, conjugated carbonyls, nitrites, nitrates and
organometallics, but is virtually insensitive to hydrocarbons, alcohols, ketones and amines. Additional
selectivity can be obtained by increasing the applied potential when the response of weakly electron-
capturing compounds is eliminated. Application of a pulsed dc potential rather than a continuous one
improves sensitivity and linear range and reduces contamination of the anode by deposition of
negatively charged solute species. However, this mode of operation is more costly and requires the use
of an argon/methane carrier gas mixture. The electron capture detector is particularly useful in the
analysis of halogen-containing pesticides which can be detected in the sub-picogram range. Although it
is the most sensitive available, its linear range is restricted to only 10^2 or 10^3 and it
Figure 4.21
Electron capture detector (ECD).