G. D. Brabsonl US. Air Force Academy
Colorado 80840
Demonstration Chromatograph
with a Flame Ionization Detector
I n essence, the function of a flame ionization detector is to sense changes in the electrical conductivity of a flame as it consumes the effluent from a gas chromatography column. This detector was first described in the literature in 1958.2 The purpose of this paper is to describe how a flame ionization detector can be incorporated easily and effectively into a demonstration chromatograph suitable for use in instrumental analysis, organic, and even freshman chemistry courses. Description
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Figure 1 illustrates a typical instrument.
STOPPER
Figure 1 .
pressure typically available in the laboratory is only 8 oz./im2. Injection Port. The injection port is fitted with a sleeve type serum bottle stopper. A quite satisfactory substitute for the serum bottle stopper can be made using surgical rubber tubing sealed at one end with a clamp; the wall of surgical rubber tubing is readily pierced by a hypodermic needle. For this particular instrument, it is not necessary to heat the injection port since compounds which give reasonable retention times volatilize within a few seconds after injection. A lass wool plug a t the bottom of the injection port facilitates volatilization. On the other hand, it probably is desirable to heat the injection port to demonstrate an im~ o r t a n t feature of commercial instruments. Two schemes for heating the injection port are suggested by Figure 2.L Carrier Gas. Natural gas is a quite satisfactory carrier gas, and is safer to use than hydrogen. The gas is burned in air at the detector; the flame should be about 1 cm high. It is important that the flame he nonluminous since luminous flames deposit soot on the electrodes; the soot, in turn, forms whiskers which tend to short the electrodes, thus overloading the detector circuitry. Light hydrocarbons are equally satisfactory carrier gases; a cylinder of propane can be used to operate the instrument in a location in which natural gas is not readily available. Detector. The detector consists of two copper wires
Demonshalion chromatograph. SERUM BOTTLE STOPPER
Column. The column consists of a 2 4 - f t length of 7-mm Pyrex tubing packed with a household detergent such as T i d e - X ~ . 3 . ~The dimensions of the column are dictated by consideration of the desired number of theoretical ~ l a t e sand the accentable Dressure dron along the column; a relatively short column is required when natural gas is used as the carrier gas, since the gas
RESISTANCE
TO AUTO TRANSFORMER
WIRE-
-TO
COLUMN
GAS-
Presented at the 41st Annual Meeting of the Colorado-Wyoming Academy of Science. 'Present address: Frank J. Seiler Research Laboratory (AFSC), U S . Air Force Academy, Colorado 80840. J., NEL, W., AND PRETORIUS, V., Nature, 181, 177 %HARLEY, I. G.. AND DEWAR.R. A.. Nature., 181. (1958). MCWILLIAM. i s 0 (i958). "egigistered trademark of the Procter and Gamble Co. COWAN, P. J., AND SUQIHARA, J. M., J. CAEM.EDUC.,36.246
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BRENNAN, D., AND KEMBALL, C., J. CAEM.EDUC.,33, 490 E., "Laboratory (1956). NELSON,K. LEROI,AND THOMPGON, Projects in Orr~nieChemistry.'' .. Allvn . and Bacon.. Inc... Boston. 1966, p. 105. -
HOT PLATE
F ~ ~ Y 2. W
Methods of heating the injection port.
Volume 49, Number 1, January 1972
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71
a. Circuit for Galvanometer
b
SENSITIVITY R1 10K
SAMPLE: SIZE: ONE MICROLITER COMPOSITION: ONE PART C 2 H p
R2 5 0 M
THREE PARTS CClq CARRIER: NATURAL GAS COLUMN: 80 CM TIDE-XK
GALVANOMETER 0.0lpamplmm Resistance: 5K
b. Circuit for Recorder TIME
h DETECTOR
RECORDER 10-mv
Figure 4.
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Typical c h d recorder trocing.
baseline; while this circuit is not essential, it has been found to be quite useful. Discussion
Figure 3. 0 , Circuit for use with a galmnometer. o recorder.
b, Circuit for we with
crossing at right angles -1 cm above the tip of the column and separated from one another by -0.5 mm. Designed specifically for the halogen-containing compounds, the detector provides both a visual indication and an electrical signal when the sample leaves the column. The visual indication is, of course, the blue green flame characteristic of the Beilstein test for halogens.& A glass chimney is necessary to provide a still air environment for the flame. Detector Circuitry. With a sensitive galvanometer, the current through the flame can he measured directly as illustrated by Figure 3a. For optimum results, the galvanometer should have a sensitivity of a t least 100 mm/FA. Resistor Rz protects the galvanometer from accidental overloads; the value of this resistor is selected so that in the event of a malfunction, such as a short in the detector, the galvanometer will not he overloaded. The key is depressed momentarily when it is desired to take a reading of the flame current. Potentiometer R, is incorporated to adjust the sensitivity of the galvanometer circuit. A circuit designed for use with a ten millivolt strip chart recorder is shown in Figure 3b. The recorder must have a reasonably high input impedance; a value greater than or equal to 20,000 o h m is satisfactory. Resistors Rz and Ra permit the operator to adjust the
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lournol of C h e m i m l E d u c a t i o n
In a typical demonstration, a 1-dsample consisting of approximately one part of ethyl bromide and three parts of carbon tetrachloride is injected. As the two components are eluted, the color changes of the flame are observed by the students and the changes in conductivity of the flame are determined either point by point with a galvanometer or continuously with a strip chart recorder. A typical chart recorder tracing is shown in Figure 4. Within about five minutes, both components have been eluted. The instructor can elaborate on the demonstration if he wishes, for example, by introducing each component of the mixture individually. It is interesting to ohsenre that bromine-containing compounds yield the largest response with this detector. For chlorine-containing hydrocarbons, the signal is smaller by a factor of about five. The detector is effectively insensitive to hydrocarbons not containing halogens. The copper electrodes react chemically with the halogen in the flame, not only providing the visual evidence of the presence of the halogen, but also markedly increasing the conductivity of the flame. This fact was demonstrated by replacing the copper with platinum electrodes; when this was done, the conductivity of the flame decreased by an order of magnitude. There is incomplete evidence to suggest that both positively and negativelycharged species are important charge carriers in the flame. The author is indebted to Dr. D. W. Seegmiller for his helpful suggestions and comments during the development of this demonstration.
