D. E. P. Hughes
The Schools Shrewsbury, Shropshire England
Flame-ionization Detector for Gas Chromatography
A
hydrogen flame detector1and a thermal conductivityz device for gas chromatography were The flamerecently described in THIS JOURNAL. ionization detector presented here is very easy to construct, extremely sensitive, and readily visible for demonstration purposes. To work satisfactorily, a flame-ionization delector must have a good signal-to-noise ratio and the device must be coupled to an amplifier capable of responding to currents as low as 10-l2 amp. The usual method of achieving a high signal-to-noise ratio is to add purified hydrogen and oxygen to the carrier gas and burn the resulting mixture a t a platinum tipped cathode. The flame then contains very few ions and the background current is correspondingly small. I n the simple system described here, hydrogen is used as the carrier gas and is burnt a t a No. 23 stainless steel hypodermic needle, 1 in. long, which has been cut square a t its end. This jet works well as long as the hydrogen pressure is adjusted to give a nearly invisible flame about 0.6 cm high, which does not make the tip of the needle red hot. Most of the background current then appears as "kicks" caused by ionization from dust impurities in the air, and unless these are first filtered out from the flame,s there is little to be gained by using a more expensive platinum needle. The needle is supported on an insulated base plate (a volyethvlene s t o ~ ~from e r a bottle serves admirably)
' MCLEAN. P. I,.. . J.,. AND PAUSON.
J. CREM.EDUC..40, 539
(1963).
HERBENEE. R. E.. J. CREM.EDUC..41. 162 (1964). VIJBNELL, H., '*G&s~hromatagraphy,';~uhn'~ile; and Sons, Inc., New York, 1962, p. 305.
and the steel anode is adjusted so that it is just above the tip of the flame. The position of the anode is not very critical, but the tip should not be so close to the flame that it becomes incandescent. The flame should be shielded from drafts, and if a glass chimney is used for this purpose the whole of the detector remains clearly visible. The chimney can be secured to the base plate with an epoxy resin, making sure that the joiht does not completely exclude air from the flame. Ideally a commercial recording electrometer should be used with this detector, but satisfactory results can be obtained with an ordinary pH meter as a high impedance amplifier. The detector works well with a polarizing voltage as low as 6 v, so that the battery and high resistance can be supported under the detector and the output led by coaxial cable to the pH meter (Pig. 1 ) . A limitation of using a pH meter as at1 amplifier is that it responds to every flicker caused by dust impurities. A simple electrometer valve amplifier (Fig. 2)&attached to a dead beat meter eliminates this trouble, and after an initial warm-up period, the zero reading is fairly stable atid steady. The amplifier gives a linear output, arid lhe area of the peak obtained is roughly proportional to the sample size. At present the detector is being used with a celitesilicone oil column, 6 ft. long, using hydrogen at 8 psi as a carrier gas. This system efficiently separates a mixture of n-pentane and isopentane and the detector works quite adequately if the sample size is less than 1 ,'I. "A Low Voltage Electrometer," Educational Electronic Experiments Na 8, The Mullard Edr~catior~al Service, London, 1964.
1 1 11
Cathode +
-
Carrier -6v Gas
M Icm
Figure 1.
450
/
Constructional details of Ihe flame ionirotion detecmr.
Journal o f Chemical Education
Figure 2. A simple electmmeter. V l is o Mullord ME1404 electrometer valve and TRl and TR2 are Mullard OC140 tmnrirtors. The meter is o 100 pA instrument; o series reristanca must be added so that Ihe rerirtmnce ocrotr the output of t h e electrometer bridge is not less than 1000 ohm% The manvfacturen recommend that the voltage to the heoter of V1 be applied before the m o d e voltage ir srltched on.
