Gas Chromatography - GC Detectors > The Flame Ionization Detector > Page 38


Figure 22. The Flame Ionization Detector

The ionization process is not very efficient, only 0.0018% of the solute molecules produce ions, (about two ions or electrons per 105 molecules). Nevertheless, because the noise level is very small, the minimum detectable mass of n-heptane is only 2 x 10-12 g/sec. At a column flow rate of 20 ml/min. this is equivalent to a minimum detectable concentration of about 3 x 10-12 g/ml. The detector responds to mass per unit time entering the detector, not mass per unit volume consequently the response is almost independent of flow rate. This is particularly advantageous and allows it to be used very effectively with capillary columns. Although the column eluent is mixed with the hydrogen prior to entering the detector, as it is mass sensitive and not concentration sensitive, the diluting effect has no impact on the sensitivity. The FID detects virtually all carbon containing solutes, with the exception of a small number of small molecular compounds such as carbon disulfide, carbon monoxide, etc. In fact, due to its diverse and comprehensive response, it is considered a universal detector.


An example of the use of the FID in a paraffin, isoparaffin, aromatic, naphthene and olefin analysis of a hydrocarbon mixture (frequently called the PIANO analysis) is shown in figure 23. The column was the Petrocol DH 50.2, 50 m long and 0.5 mm I.D. and made from fused silica. The column temperature was held a 35oC for 5 minutes and then programmed up to 200˚C at 2˚/min. The carrier gas was helium and the mobile phase velocity of 20 cm/sec. Many standard tests carried out in the hydrocarbon and pharmaceutical industries and for environmental testing have been designed to utilize the FID as the detector