Nitrogen Carrier Gas Research Articles

Linearity and response characteristics of the flame ionisation detector

Non-linearity of the flame ionisation detector at high concentrations has been shown to be caused by inefficient ion collection, due to ion recombination and space charge effects, rather than a basic non-linearity of ion formation. This can be overcome by using a flat probe electrode which subtends a large angle at the jet or a hat-shaped collector in conjunction with an adequate probe-jet voltage. With a detector unit similar to that described previously5,6 a linear response up to 10 % sample in the nitrogen carrier gas stream (5 % at the jet) has been obtained (50,μg/sec at 25 ml/min nitrogen). A non-linear response system to avoid sensitivity changes with wide range samples is described. A possible explanation of the abnormally high degree of ionisation found in hydrocarbon flames is suggested.