A flame ionization detector is used for carrying out a quantitative analysis of chromatographically-separated substances in thin-layer chromatography. The detector comprises a gas burner (1) form forming a hydrogen flame (4). A thin-layer chromatographic element (9) or a rod-like element which carries the chromatographically0separated substances is passed through the hydrogen flame (4) so that the separated substances are burned and ionized. The detector further comprises an electrode (5) which is disposed above the hydrogen flame (4) and exposed to the ionized gas generated by the burning of the separated substances. A voltage is applied between the gas burner (1) and the electrode (5) so that the former is given a positive polarity and the latter a negative polarity. An electric current which is caused by the exposure of the negative electrode (5) to the ionized gas is detected and amplified. A third electrode (8) disposed to surround the negative electrode (5) so that an amount of each of the separated substances can be proportioned to the amplified output value thereof.
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