We know that ions are present in hydrogen-air flame and when the burning of an organic compound takes place in this flame more ions are produced in the flame.
Thus when we apply a voltage across this flame, the ion collector plate attracts the all the ions in the flame.
The presence of organic compounds increases the voltage across the hydrogen ion flame produced at the ion collector increases and as the voltage increases, the detection of the organic compound can be made in turn.
Thus flame ionization detector clearly responds to the variation in the collection of ions or electrons in a flame.
