Instant Notes: Analytical Chemistry

(ii) Flame ionization detector (FID). This is the most important of a group of

detectors where the signal is related to the ionizationof eluting solutes.

Carrier gas emerging from the column is mixed with airand hydrogenand

burnt at a small metal jet (Fig. 6). A 150–200 V DC potential is applied

between the burner jet (positive) and a collector electrode (negative)

positioned just above the micro-flame. The electrodes are connected to an

external circuit where the signal can be amplified and recorded. Eluted

solutes are combustedto yield ionswhich increase the electrical conduc-

tivity of the flame and are collected by the negative electrode, thus allowing

a current proportional to the concentration of ions derived from the solute

to flow around the external circuit.

The FIDis the most widely used universaldetector, being extremely

sensitive and responding to all organic solutes except formaldehyde, formic

acid and fully halogenated compounds. It has the widest linear dynamic

range of all GC detectors, making it ideal for quantitative analysis, and its

only disadvantage is the lack of response to inorganic solutes.

(iii)Nitrogen-phosphorus detector(NPD). This is a selectivedetector that is

basically a flame ionization detector, modified by positioning a ceramic

beadcontaining a rubidiumor caesiumsalt, electrically heated to 800∞C,

between the burner jet and the collector electrode. The response to

nitrogen-containing compounds is enhanced by a factor of about 50 over

that of an unmodified FID, and the response to phosphorus-containing

compounds is enhanced by a factor of about 500. The linear dynamic range

is intermediate between those of the FIDand TCD.

(iv)Electron capture detector (ECD). This is another form of ionization

detector, and shows a selective response to solutes containing halogens,

sulfur and unsaturated structures, all of which have high electron affinities.

D4 – Gas chromatography: principles and instrumentation 147

Flame ignition coil

Insulated jet

Heated detector base

Collector electrode

Detector body

Polarizing voltage

(150–300 V)

Air H2

Column

+

Fig. 6. Schematic diagram of a flame

ionization detector, FID. Reproduced from

I.A. Fowlis, Gas Chromatography: Analytical

Chemistry by Open Learning, 1995, 2nd

edn, with permission from Her Majesty’s

Stationery Office. Crown Copyright.