Gas Chromatography Detectors - The Response Mechanism of the FID > Page 42


Desty et al. also showed that the air flow should be at least 6 times that of the hydrogen flow for stable conditions and completecombustion. They also demonstrated that thebasecurrentfromthehydrogenflowdepends strongly onthepurityofthehydrogen. As would be expected, tracesof hydrocarbons significantly increase the base current. Consequently, very pure hydrogen should be employed with the FID if maximum sensitivity is required. Employing purified hydrogen Desty et al. reported a base current of 1.45 x 10-12 amp for a hydrogen flow of 20 ml/min. This would be equivalent to 1 x 10-7 coulomb per mole. The sensitivity reported, for n-heptane, assuming a noise level equivalent to the base current from hydrogen of ca 2 x 10-14 amp (a fairly generous assumption), was 5 x 10-12 g/ml at a flow rate of 20 ml/min. It follows that although the sensitivity is amazing high, the ionization efficiency is still very small ca. 0.0015%. The general response of the FID to substances of different type varies very significantly. For a given homologous series the response appears to increase linearly with carbon number but there is a large difference in response between a homologous series of hydrocarbons and a series of alcohols. An example of the response of the FID to a number of different hydrocarbons is shown in the lower curves in figure 19.


The linear dynamic range of the FID covers at east four to five orders of magnitude for 0.98<r<1.02. This is a remarkably wide range that also accounts for the popularity of the detector. Commercially available detectors shows considerable difference in electrode geometry and operating electrode voltages, yet they all appear to have very similar performance specifications. This supports the claim of Desty et al. that the FID is surprisingly forgiving with respect to specific detector geometry.