Instant Notes: Analytical Chemistry

(i) Thermal conductivity detector(TCD). This is one of the oldest types, and

is known also as a katharometeror hot-wire detector. It is a universal

detector consisting of a heated metal block containing a reference cell

through which pure carrier gas constantly flows, and a sample cellthrough

which carrier gas flows after emerging from the end of the column (Fig. 5).

The cells contain identical, heated platinum filaments whose resistances

depend on their temperatures, which in turn depend on the rates of heat

loss from their surfaces, these being a function of the thermal conductivity

of the surrounding gas. When pure carrier gas flows through both cells, the

resistances of the two filaments are the same, and a Wheatstone bridge

circuitinto which they are both incorporated can be balanced to give a

stable baseline or background signal. When a solute is eluted from the

column and passes through the sample cell, its presence alters the thermal

conductivity of the carrier gas. The temperature of the filament and hence

its resistance changes, and an out-of-balance signal proportional to the

solute concentration is created in the bridge circuit. When the solute has

passed through the cell, the signal returns to the baseline value.

The TCDis robust and reliable, but has only moderate sensitivity and a

limited dynamic range making it more suitable for qualitative than quanti-

tative work.

146 Section D – Separation techniques

Table 5. Characteristics of GC detectors

Detector Sensitivity Linear Characteristics
(g s-^1 ) range

Thermal conductivity (TCD) 10 -^9104 Robust, non-destructive, flow and temperature
sensitive, poor linear dynamic range, insensitive to
inorganic solutes

Flame ionization (FID) 10 -^12107 Excellent sensitivity and linear dynamic range, best
universal GC detector

Nitrogen-phosphorus (NPD) 10 -^14 (N) 105 Similar to FID, but selective for N and P containing
10 -^15 (P) 105 solutes, limited linear dynamic range

Electron capture (ECD) 10 -^13103 Excellent sensitivity for solutes with electronegative
elements, temperature sensitive, easily contaminated,
limited linear dynamic range

To waste To waste

Analytical

filament

Reference

filament

Analytical

column

effluent, in

Reference

gas stream, in

Fig. 5. Schematic diagram of a thermal

conductivity detector, TCD. Reproduced

from I.A. Fowlis, Gas Chromatography:

Analytical Chemistry by Open Learning,

2nd edn, 1995, with permission from Her

Majesty’s Stationery Office. Crown

Copyright.