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edited by JOE RICH Blackhawk Christian School Fort Wayne, IN
A Simple Computerized Gas Chromatograph John N. Fox
Department of Physics Ind ana University of Pennsylvania Ind ana. PA 15705
Robed A. Shaner south side Area nigh Schwl
Hwkstown, PA 15050 The apparatus described below was developed for the National Science Foundation s~onsoredProied EXCELS ducati ion in earning the Sciences). ( ~ x ~ a n dComputer in~ Althoueh other simple em chromatoera~hshave been described-in the literature? the simplici~yand large signal-tonoise ratio clearlv makes this apparatus easily adaptable to the secondary school lahoratorir ~ o shigh t ichooiteachers do not have the luxury of possessing the equipment necessary for gas chromatography and must h i satisfied with simply describing the apparatus to their students. Now, if theieachers can-capturethe Apple computer from the library or from the typing class, helshe can make it into a versatile eas chromatoera~hfor the chemistrv lab. In our k m e r workihips we used the sunset Laboratory Advanced Interface Board (AIB) alone with Voltage Plotter 1112 to convert the Apple II+, IIe, o r 3 W into aieal-time plotter for the lahoratorv. To acauire the data discussed in ;his paper, the real-timeplotting mode is used. The apparatus is illustrated in Figure 1. The chromatograph column consists of a glass tube of 8 mm outside diameterand 4 f t in length. Approximately three-foudhs of the tuhe is filled with Tide detergent, which is kept in place with glass wool at either end of the packed column. The detergent was first sifted (100-200 mesh) to remove the fine powder that might otherwise clog the glass tuhe. A good alternative to a calibrated sieve is a nvlon stockine of coarse mesh. The carrier gas is natural gas.- he gas supply is connected to the glass tubine bv a piece of sweical tubine. The Drooer safety cautions shckldhe observei when co&ecting the apparatus to the natural gas line. The gas flows through the glass tuhe and is ignited at the outlet as shown in Figure 1. A 4-in. diameter CVC pipe, available from a plumbing store, is placed around the flame so as to inhibit flickering of the flame by air currents. A small hole is drilled through the CVC pipe in order to receive an infrared detector. The simple circuit for the IR detector is shown in Figure 1.As the flame burns hotter, the IR detector .passes more electrical current and the voltage drop across the 1-mi2resistor increases. The computer, by means of the The work was supported in part by the Natlonal Science Foundation Project No. 87-51295. RAS was suppwted by a Chrism McCullffe Fellowship Grant. Project No. 190AH70156. ' Tayl0r.M. A.;Koib, K. E. J. Chem. Educ. 41; 1981, 58, Wollrab,A. J. Chem. Educ. 21975, 52,2000;another common technique is to use a differential thermocouple. The Sunset Laboratory Advanced Interface Board ($225)and the software ~ackaaeVoitaae Plotter 111 ($40)are available throuoh VerniOR 97225: Project er Softw&e lnc, 2920 5.w. 89th ~t;eet.'~wtland. EXCELSemployed the Advanced Interfacing Board (AIB) which is a 1Bbit analog-todigital interface.However, If a teacher IS interested in just this one application of computer interfacing,the less expensive Voltage Input Unit ($35) can be used ~nplace of the Al8. The Voltage input unit will still require the Voltage Plotter Ill software. 894
Journal of Chemical Education
Figure 1. Apparam fwa gas chromatograph. The inhared sensor, costlng $1. Is a TRW OP598A. R Is avallable through Newark Electronics. 1202 Butler R W , Freeport. PA 16229. The +5 V. wound and lnpvt connectlonsare clearly labeled on h Advanced Interface Board.
POTENTIAL US TIME (HEXANE/PENT&NE M I X ) 5.00
Flgve 2. IR s s ~ wresponse as a tundm of time. Hexane and pentane Injected Into me gas streem. The pentane anlw approximately 82 s befweme hexane.
were
Advanced Interface Board, records the increased voltage drop across the resistor. With the natural gas flowing, a hypodermic syringe is prepared. T h e syringe contains 0.25 cc of hexane [CH3(CH2)&Hd and 0.25 cc of pentane [CH3(CH2)3CHs]. The syringe is then drawn hack an additional 2.5 cc to add air to the mixture. The svrinee needle is introduced throueh the surgical tubing into the $owing carrier gas, data recording is started, and the fluidlair mixture is injected into the gas. Samples partition themselves between gas carrier and the thin layer of the stationary phase associated with the packing material. The relative retention of the sample by the stationary phase depends upon the attraction of the sample for that phase. The data recorded is illustrated in Figure 2. First, the one or two data points that occur at approximately 12 s correspond to the time when the sample is injected. All time measurements are then made from that zero of time. Later,
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