A. V. Nowak and H. V. Malmstadt University of Illinois Urbana, 61801
A Versatile Ionization Detector System for Gas Chromatography
T h e development of flame ionization, electron capture, sodium thermionic, and photometric detector systems has tremendously increased t,he applicability of gas chromatographic methods. To make full use of these detectors adequate instruction in their performance capabilities and limitations is important. Commercial gas chromatographic instrumentation possessing all of the detector modes is, unfortunately, very expensive and out of reach of many educational budgets especially where instruments are required for individual student experimentation. Therefore, a definite need exists for inexpensive and versatile instrumentation which, although designed primarily for instructional purposes, possesses the performance capabilities of research instrumentation, a t least in the most important modes. The introduction of a low-cost gas chromatograph' has been a step in this direction. This system, however, provides only for thermal-conductivity detection and is also limited t o isothermal operation over a narrow temperature range. A versatile and inexpensive ionization detector system is described here that can greatly increase the utility of any gas chromatograph to which it is adapted. The detector module can be connected to the outlet of a thermal-conductivity detector. Therefore, it should be readily adapted to most commercial gas chromatographs originally purchased with only a thermal-conductivity detector. This series connection also allows nearly simultaneous recording of the two detector responses, thus enabling direct comparison of thermal conductivity and ionization detector response. A simple electrode-holder arrangement allows quick conversion from flame ionization to sodium thermionic to electron capture modes simply by insertion of the proper electrode in a spring clip. Another modification, presently being developed, will enable operation as a photometric detector. The detector system is quite easy to construct and requires no machining other than the drilling of a few holes. The detector module contains flow-control valves and polarizing voltage supplies and can he quickly attached or removed from the chromatograph.
brought outside the cabinet and reconnected by means of gum-rubber surgical tubing. With this system it is possible to disconnect gas connections quickly and det,ermine gas flow rates by means of a bubble flow meter. The necessary polarizing voltages are provided by two batteries, a 300-v battery for flame ionization and a 67-v battery for the electron capture mode. A D P D T switch is used to select the appropriate voltage. 1/4" HOLE
TEFLON RING
I
n(b1ZI ;GE
' PASS-THROUGH
EFFLUENT
\ 1/16" SWAGELOK SCALE
4,
'
INCH
"TEE"
-1,
Defector Design
The ionization detector shown in Figure 1 is made from readily available parts. A Swagelok "Tee" in. od tube, Crawford Fitting Co.) is used for hydrogen gas and column effluent inlets and for connection of t,he hydrogen burner t,ip (b). The tip is a 1-em length of 21gauge stainless steel syringe capillary tubing t,hat is silver-soldered t,o a Swalrelok nut. The nut also serves to fasten the detector base, which is the ton from a 35mm film can, to the wag el& "Tde." The top of the film can is drilled with a diameter hole a t the center of the top to fit the "Tee" and it hole in the side to accommodate a 1/8-in. diameter copper tubing for the air inlet. The air tube is soft-soldered in the side hole. The air stream flows through the center hole of a Teflon ring so that the air surrounds the burner tip. %.
Instrumentation ~
~
~~~
Detecfor Module
The detector is mounted in a 4-in. X 5-in. X 6-in. aluminum cabinet along with two needle valves, batteries, switches, and the necessary connecting tubing. The gas connect,ionn t,o and from the detector are
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holes in the Teflon ring are used to mount the electrode holders and the fourth hole is used as a pass-through for the #20 plastic-coated wires from the electrodes. A 3/,&1. hole in the film-can top allows a pass-through for the electrode wires, and the hole is sealed with epoxy. Since the top of the film can is used as the detect,or base the film can itself can be quickly twisted on for prot,ect,ion of the hydrogen flame during operation, and easily removed for electrode substitution or cleaning.
ELECTRODES
-
OPDT
F I D B STD
+
HEATH EUW-301 PHOTOMETER ATTACHMENT
Electrode Substitution
The electrodes (d) are held in place by spring-brass shim stoclc bent so as to hold the electrodes by spring tension. The clectrodes and electrode holder arc seen in more detail in Figure 2. For operation a s an ordi-
-
AIR INLET
(e)
TEFLON CAP
NazS04 COATED Pt
NEEDLE VALVES COLUMN EFFLUENT
V
FOIL CONTAINING OCCLUDED TRITIUM
GAUZE
ALL OTHER TUBING 1/8" COPPER Figure 3.
Electrical wiring d i a g r a m a n d gas Row connections.
rent readout connected to the Carle chromatograph is shown in Figure 5 . Flame Ionization Mode
SCALF
-4 I
INCH
/+-
Figure 2. D i o g r m of electrode holder and electrodes for flame-ionimtion, sodium-therrnionic, and electron-capture modes.
nary flan~c-ionization detector two parallel stainless steel plates (a) are placed in the electrode holders ( O ) . For conversion to the sodium thermionic mode one of the electrodes is removed and replaced mith a similar electrode (c) coated with NazSOa. A length of platinum gauze is wrapped around the top half of this electrode and acts as a medium to hold the salt. Conversion to the electron-capture mode involves replacing the proper polarity electrode mith another electrode (d) to which has been attached a foil containing occluded tritium. I n this mode a Teflon cap (e) is also placed over the electrodes and forms an enclosed ionization chamber. The interelectrode distance for all detector modes was set a t 6 mm.
One of the most important gas chromatographic detectors is the flame ionization detector (2, 3). To obtain maximum performance from the flame-ionization detector the gas flow rates must be optimized. The hydrogen flow rate is ordinarily the most important of these variables (4). The optimum hydrogen flow rate for this p:~rticulardetector is 50 ml/min, and was determined by measuring the response to identical samples a t various flow rates. However, the hydrogen flow rate is not extremely critical and could vary within + 10% without significantly affecting the response. Air flow rates are not critical if sufficient air is present. This
Readout
Recording of ionization currents and integration of peak areas is accomplished by means of two Heath EUW-301 recording pH Electrometers equipped mith photometer modules (1). The electrical miring diagram and gas flow pattern are shown in Figure 3. The interior details of the detector module are shown in Figure 4. A picture of a complete gas-chromatographic setup consisting of the versatile detector system and cur520
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lournol of Chemical Education
Figure 4.
Photograph of ionirotion-detector module
Comparison of Flame-Ionization and ThermolConducfivify Response
An experiment which demonstrates another feature of the flame-ionization detector involves the simultaneous recording of both thc flame-ionization and thermalconductivity responses. The flame-ionization det,ector does not respond to water vapor, thus allowing the direct analysis of orgauic materials in aqueous solutions. This is demoustrated by the analysis of a sample of Scotch whiskey which contains water and ethanol as major constituents. The chromatograms obtained for both flame-ionization detectiou and thermal-conductivity detection are shown in Figure 7. Under the con-
THERMAL CONDUCTIVITY RESPONSE
Figure 5. Photograph of ionization-detector ryrtern connected to the C o r k chromotogroph.
FLAME iONIZATION RESPONSE
amounts to a n air flow rate of about 400 ml/min. Carrier gas flow rates affect response but also in a noncritical way. The main advantages of the flame-ionization detector are high sensitivity and wide linear dynamic range. These are demonstrated by measuring the response for increasingly dilute solutions of a test substance, as illustrated in Figure 6 for a sample of pentane in ethanol. H2 He
50ml/m8n 75mllmin
Figure 7.
Air 400 m l l m i n Sample Irl SCOTCH WHISKEY
Dual recording of Rmwionizotlon a n d thermal-conductivity
response.
ditions used t,he ethanol and water are not separated and t,liermal-conductivity detection shows the unresolved peak. The flame-ionization detector shows only the ethanol peak. For the chromatogram shown in Figure 7b, the ionization detector is operated a t a low readout sensitivity and the minor peaks due to impurities in the sample do not appear. By increasing the current readout sensitivity by lOOOX somc impurity peaks in the vicinity of the ethanol peak are recorded with the flameionization detector as shown in Figure 8, but the thermal-conductivity detector sensitivity is not sufficient to show these impurity peaks when using standard laboratory recorders. PENTANE Hz
50 ml/min
He 75 ml/min Figure 6.
CONCENTRATION 400 ml/min
Alr
Sample 5yl C 5 H t 2
in E ~ O H
m z
Dynomic range and renritirity of the flame-ionizotlon detector.
The detector is capable of detecting nanogram quantities and the response is quite linear over a wide concentrat,ion range. Reproducibility of response was also good as shown by the fact that duplicate data points are plotted in Figure G for each concentration of sample. There is only a slight deviation from unity slope a t the highest concent,mtion. This simple system is thus cnpablc of sensit,ivit,yand reproducibility comparable to some of the most sophisticated and expensive commercial flame-ionization detectors.
I
- --
ELUTION
Hz
5Omi/m>n
Ha
25rnVmrn
Figure 8.
TlME
A , ! 400 ml/mln S a m ~ l e I r l SCOTCH
WHISKEY
Flame-ionization response at high sensitivity.
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Sodium Thermionic Mode
One of the more selective detectors is the sodium themionic detector (5, 6). Addition of an alkali metal salt-coated probe to the flame of the hyrogen flameionization detector produces selective ionization-current enhancements in the presence of halogen and phosphorus-containing compounds. Response enhancements for halogen (or phosphorus)-containing compounds can be easily observed by measuring the response in the ordinary flame ionization mode, then converting to the sodium-themionic mode, and again measuring the response. This can be done for a sample of CHJ and n-CH3CHzCH20Hin n-CHZCHGHzCHz0H. The 7%-propanolis addcd to demonstrate that the detector response to a nonhalogen (or phosphorus)-containing compound is not affected by the presence of the NazSO.,. The results of these experiments are shown in Figure 9. In the normal flame-ionization mode a peak-
I
FLAME IONIZATION MODE
H z 113 mllmin ~e 58mllmin Figure 9. sponse.
1
Compari.on
of Rome-ionization
I
in n-BuOH
ond sodium-thermionic
re-
height current of 5.75 X amps is obtained for the CHJ. Upon conversion to the sodium-thermionic mode and injection of a duplicate sample the response is increased to 1.54 X lo-' amps, which amounts to a rcsponse enhancement of 26X. The n-propanol response is not significantly affected. Electron-Capture Mode
One of the most sensitive detectors is the electroll-capture detector (7,s). Due to the nature of the electroncapture process and geometrical factors affecting response the electron-capture mode does not lend itself well to this simple electrode-substitution process. Therefore, the high sensitivities usually associated with electron-capture detection are not realized with the simple system, hut the modification is so simple that it can serve as a means of demonstrating some of the principles of operation and characteristics of the electron-capture detector. The ordinary flame-ionization detector response is first measured for a sample of 10% CH1C12in C8HIR. The system is then converted to the electron-capture mode and the rcsponse is again measured. The results of these experiments are shown in Figure 10. The flame-ionization detector gives the expected ionization currents for both components of the sample. The electron-capture detector, however, operates on an entirely different principle, and the electron522 / Journal of Chernicol Education
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capture response to the same sample is quite different. Ionization of the carrier gas by the tritium source results in the production of a relatively large (ti X amps) standing current. Substances such as halogens which have a great affinity for free electrons capture electrons and are converted to negative molecular ions. Recombination effects involving these negative molecular ions then produce a decrease in the standing current. The curves obtained resemble ordinary chromatographic curves except that the peaks appear inverted. This is shown in Figure 10 where the halogenated compound produces a decrease in the standing current whereas the hydrocarbon gives a slight increase in the standing current. This demonstrates that
SODIUM THERMlONlC MWE
~ i r4 0 0 mllmin Sample 5 ~ 10.1% CH+, n-PIOH,
ELECTRON CAPTURE MODE
tained from simple electrou-capture increase in standing current results processes which can compete with the process and cause a decrease in the response.
detectors. The from ionization electron-capture electron-capture
Summary
The versatility and perfonnance capabilities of :I simple ionization-detector system arc demonstrated. Except for the electron-capture mbde t,he system is capable of performance comparable to the more sophisticated commercial ionization detectors. Ahrc careful attent,ion to geometrical considerations would result in a more sensitive electmn-capture detector, hut since the general operation of the electmn-capture detector could be demonstrated using this system it was felt that further modification would reduce the basic simplicity of the syst,em. The relative ease with which the various detector modes can be obtained makes it possible to study the characteristics of these detcctor systems with a minimum of time and manipulative effortand t,o apply them for chemical experiments. Literature Cited (1) MALMST.\DT, H. V., B.\RNNES. 13. ill., . A N D I t o ~ n m u ~ P. z , A,, J. CHEM.EDUC.,41, 263 (1964). I. G., A N D 11~\1.111,R . A,, "Gas Chromatog(2) MCWILLI.\M, raphy," (Editor: DESTY,1). TI.). Biltterworths. London, 1958, p. 142. (3) LOVELOCK, J. E., Anal. Chem., 33, 168 (1961). (4) STERNTIERG, J. C., GALLAWAY, W. S., .AND JONES,11. T. I,.,
"Gaa Chromatography," Third International Symposium, Instrument Society of America, (Editor: Brenner, N.), AcademicPress, New York, 1962, Vol. 3, p. 243. (5) K A ~ M E A., N , AND G I U F F ~ IL., D ~N a t u ~ e201, , 1204 (1964).
(6) GIUFFRIDA, L., .lour. of the A.O.A.C., 47, 293 (1964). (7) L O V E ~ CJ.KE., , AND LIPSKY,S. R., J . A m . Chem. Soc., 82, 431 (1960). (8) LOVELOCK, J. E., A n d . Chem., 3 3 , 171 (1961).
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