A Modular Apparatus for Demonstrating Gas Chromatography Using Serial Coupling of a Thermal Conductivity and Flame Ionization Detector Erwin Wiederholt, Volker Fahrney, and Dorothea Behrens Bergische Universitat-Gesamthochschule.Wuppertal, West Germany Gas-solid chromatoeraohv " . . .(GSC) is es~eciallvuseful for separating gas mixtures. An aidmethane gas mixture may be senarated usine zeolite a s the stationary phase and h .v d r o.~ e n as'the mobile phase. All of the eluted &mponents may be detected by a thermalconductivitvdetector (TCDJ,whereas a flame iohization detector (FID) will only respond to the combustible organic species. Since zeolites are known to tightly bind not only inorganic compounds such a s C02 and H 2 0 but also gaseous hydrocarbons other than methane, recourse must he made t o gas-liquid chromatography (GLC) in order to separate the gaseous hydrocarbons. Here the substrate is coated with. for examole. . . hexadecane, which s e r w s a s the liquid phasein which thegaseous hydrocarbons are soluble to different extents. T h e identification of the individual components is farilitated hy serial coupling of different detectors such a s a TCD and a FII) a s described hy Nowak and Malmstadt.'
Figwe 1. Circuit for me mwml conductivitydetector amplifier. Power supply: Dual supply +9 to 515 V. OP = 112 CA 358, TCD = lamp filament.
Experimental We constructed a simple gas chromatograph for demonstrating the separation of gas mixtures a t room temperature; therefore, heating or thermostating of the column is unuecessarv. Glass tubes are used a s columns, and the two detectors are constructed using inexpensive components. Hydroeen is used a s mobile phase for the GSC and GLC. It serves a good thermal conductor for the I'CD and simultaneousIs as a com1)ustible pas fur the FID. - T o register the detector signals, a two-channel x l t recorder is used. Alternatively, the detector voltage, which can be measured with a voltmeter, may be plotted vs. time on graph paper. Apparatus Characteristics The carrier gas follows the typical route through the instrument, but passes through the FID after passage through the TCD. The pressure-reduction valve is adjusted so that the carrier gas flow rate is about 50-70 mL1min when measured by a soap bubble flow meter. The samole is inserted throueh a seotum that is held in a treaded tube by cap having a b o r e d k t tip. The stationary phase of the GLC was prepared by stirring finely ground sodium chloride2in a solution of hexadecane in chloroform. After allowing the solvent to evaporate in a hood over a ~ e r i o dof several hours, the residue was poured into a glass tube that was plugged with glass wool. The TCD is a low-weight thermal sensor. It consists of the filament of a miniature signal lamp (3 Vl150 mW), the glass bulb of which has been removed, and is operated directly in the flowing gas with a nominal current of 50 mA. The filament temperature and thus the voltaee .. droo. across it deoends on the eas comwsition and the rare of flou aince the rrsistanreot'rh~tilament is prupmional to its remperaturt. Comparison oi rhia voltage drop tu that uf thr purr carrier gas rnalrles the derrctian of the variuuu comp.>nrnrsof the
a
' Nowak, A. V.; Malmstadt, H. V . J. Chem. Educ. 1968, 45, 519-
523.
Wollrab, A. J. Chem. Educ. 1975, 52,200.
Figure 2. Circuittalheflame ionization detector amplifier. Power supply: Dual supply +9 to 15 V, OP = 112 CA 358
+
gas mixture. This device is very sensitive, and is affected by drafts at low operating power (25 mW). The TCD amplifier is shown in Fim,re 1. .".. .. . -
The FID detects a thermally induced ionlplasma current as low as 2 X 10-"A. Organic substances present in the hydrogen carrier gas are burned in an excess of air between the source electrode (combustion nozzle: 3 mm stainless steel tube) and capture electrode (sodiurngrid). The resistance experiencedby theion current is high (lo610W). A circuit design for the FID amplifier is shown in Figure 2. Examples and parameters: Separation of air-methane Column: 80 cm X 5 mm i.d. Stationary phase: synthetic zeolite 0.5 nm, per1 form 2 mm Carrier gas: hydrogen, 54 mllmin Sample: 1mL air-naturalgas mixture (1:l) (natural gas >90% methane) Separation of air-gaseous hydrocarbons Column: 80 cm X 5 mm i.d. Stationary phase: 2% hexadecane on NaCl Volume 67
Number 2
February 1990
181
Figure 3. a, b: Oas dlmma(qlrams of air: c, d: 80 cm X 5 mm
mixture. Column: zeolite 0.5 nm.
meume: e,
f: air-methane
i.d.. hydrogen 54 mllmin.
Carrier gas: hydrogen, 72 mLlmin Sample: 1mL air-gaseous hydrocarbons mixture (13)(Esso gas for cigarette lighters)
Figwe 4. a, b: Gas chromamgramsofair: c, d: gaseous hydrocarbons: e, I: airgaseous hydlocarbons midure. Column: hexadecane2% on NaCI. 80cm X 5 mm i.d.. hydrogen 72 mllmin.
On the other hand, three components are detected by the TCD (Fig. 4c) and FID (Fig. 4d) when the gaseous hydrocarbons mixture is injected into the GLC column. These comResults and Dlscusslon ponents may be identified as propane, isohutane, and buAir can be separated into argonloxygen and nitrogen on a t a ~ ~The e . ~mixture of air and gaseous hydrocarbons thus zeolite column. We even succeeded on a 20-cm-lone, 5-mm gives chromatograms containing four TCD peaks (Fig. 4e) i.d. zeolite column with a hydrogen flow rate of 6 m X / ~ n i n . ~ and three FID peaks (Fig. 40. The fixed gases can be detected by the TCD (Fig. 3a) but not The above two examples demonstrate that it is possible to by the FID (Fig. 3b). Both detectors respond to methane, distinguish inorganic and organic components even with an which is the major component of our natural gas (Fig. 3c and inexpensive self-constructed gas chromatograph with two 3d). whereas the minor com~onentslike ethane and Drooane serial detectors. are absorbed hy the zeo1ite.l and therefore cannot 6e r&istered. The mixture ot' airlmethane shows three oeaks wlth the TCD and only one with the FID (Fig. 3e and '30. Wiederholt. E.: Fahrney. V. Naturwiss. Unterricht-PC 1987, 35. Air is not separated on hexadecane; thus only one signal is 130-133. obtained from the TCD (Fig. 4aJ while nu signal comes from Breck, D. W. J. Cham. Educ. 1964, 41.678-689. the FID (Fig. 4b) because air is not cumhustible. Wollrab, A. Praxis Natunviss.-Chem. 1983, 32,170-186.
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Journal of Chemical Education
