Adalbert Wollrab and Reinhard Wandrey Justus Liebig University 63 Giessen Federal Republic of Germany
A Flame Ionization Detector with a Transistor Amplifier
Flame ionization detectors are often used for gas chromatography because of their high sensitivity and widespread application possibilities.' In this paper a simple flame ionization detector with a voltage supply of 4.5 V or 9 V will he described. As an indicator device one can use an ammeter with 1 mA measuring range, which is usually available at schools and universities. A recorder can also be used if a 1 Mohm resistance is inserted between its input socket and the amplifier. This flame ionization detector can easily and effectively be incorporated into a demonstration chromatograph for teaching purpose^.^.^
Figure 2. Detector circuit variant with voltage supply of 9 V.
&MPLIFIER
Description
OUTPUT +
,M
77
rDER
INPUT
--C Figure 3. Circuit variant incorporatingrecorder.
The D e t e c t o r
Brahson's detector3 has been slightly modified and is shown in Figure 1. It consists of two knitting needles for electrodes that have been inserted through a cork. They should not he allowed to touch one another and should have a minimal distance of about 0.5 mm a t the point where they cross. Hydrogen, which is used as carrier gas, is passed from the chromatograph through a capillary that perforates the middle of the cork. After the hydrogen has
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been lighted, a funnel is turned upside down over the detector providing a still air environment for the flame. The end of the knitting needles are connected to wires that lead to the amplifier. The Amplifier
Two circuit variants are here descrihed. The one in Figure 2 uses two 4.5-V batteries to produce the power supply. The series connection of hoth batteries amounts to an electric potential of 9 V, that can he regulated by means of a potentiometer from &9 V. Choice of sensitivity can he regulated by adjusting the voltage. The ionization current (several nanoamperes) is amplified by means of a Darlington switch and is conducted from here to a measuring device. In this order the total amplification corresponds to the product of the amplification of the individual transistors. Because of the high current gain at a simultaneously high inductive resistance this circuit is well suited as an FID amplifier. Type 2 N 3117 can he used as transistors, although all npn transistors with a 200-500 times current amplification range and a small residual current can he used. The simpler of the two variants has only one, instead of two 4.5 V batteries, and no potentiometer. This equipment guarantees adequate sensitivity for a measuring apparatus with a measuring range of 1 mA. A recorder can also be used for hoth variants; an adequate circuit is shown in Figure 3. ... '.Jent~seh. D.. and Ottt,. E.. 'Derrcturs "3rd in Ca5-Chrornnlograyhy." Akadrrnirrhe Verlngrge~eli,.rhafr. Frankfwr hlam.
Figure 1. Brabson's Detector. A , column: 8. capillary with carrier gas (hydrogen) and outlet: C, knitting needles; D, cork: E, funnel: and F. wires leading to detector circuit.
818 / Journal of ChemiollEducation
,o-" .,a". 3
Wollrab A,, J. CHEM. EDUC., i n print. Brabson G. D., J. CHEM. EDUC., 49,71 (1972).
