A Sonar Detector for Gas Chromatographs Chaim N. Yarnitzky Technion-Israel Institute of Technology, Haifa 32000, Israel
Gas chromatographs are frequently used in students' laboratories of various kinds. The thermal conductivity detector is, undoubtedly, the most popular detector in gas chromatoma~hv.I t is simole and cheap. Its sensitivitv and lineariti,adiittedly limked,areade&ate for routinework. All this combined with its negligible maintenance cost has made i t the preferred detector in gas chromatographic experiments carried out in student labs. The detector is based on a physical property of the gaseous phase that can be derived from the kinetic theory of the gases, namely, the thermal conductivity. We have designed and constructed a sonar detector that is based on measuring the velocity of sound in the analyzed gas passing through the detector ( I ) . The detector is mounted a t the outlet of the thermal detector, enabling simultaneous recording of both thermal and sonar signals. A better understanding of the gaseous phase is thus achieved. The student also becomes familiar with latest gas mixture analyzers ( 2 )that have recent]" been introducedin industry. The prediction of the sonar signal is much easier, since aicording t o Newton's modified equation (3), sound velocity is equal to V M =(all terms have the usual meanings). The thermal conductivity, however, is equal to the viscosity of the gas multiplied by its heat capacity (at constant volume). The experimental value differs considerably from the theoretical one. This also demonstrates the difference between translation energy (connected to sound transfer) and vibration-rotation energy (additional contribution to the thermal conductivitv). As exwected, the carrier gas of best choice is identical for-thermal'and sound detectors. A list of various cases, their thermal conductivity, and in the table (4). I t can be seen that sound velocity is
712
Journal of Chemical Education
hydrogen and helium are expected to be the Best carriers, and signals for different s a m ~ l ewill s be similar when usine t h e r m z or sonar detector. indeed, the sonar,, or acoustic, detectors was used in the past (5,6).Unfortunately, a t the time, the total price of the detector was extremely high (compared with the thermal detector): therefore, manufacturing a commercial unit was not worthwhile. The following considerations were taken into account in the design of thk sonar detector: (1)volume-not exceeding 1mL; (2) differential measurement-a reference and a sample cell are operated in the differential mode; (3) low pricebased on available cheap components.
Veloclty of Sound and Thermal Conductance of Varlour Gases at 20 "C Oas
Velocity of Sound
Air, dry Ammonia Argon Carban dioxide Emane Elhylene Helium Hydrogen Methane Nitrogen Oxygen
331 415 319 259 308 (10 "C) 317 965 1284 430 334 316
Thermal COndunance
SONAR DETECTOR
OSCILLOSCOPE
TRIGGER IN
A
TIME
-v
Figure 1. Basic system and elecbonlc diagram of the sonar detector. Potentlometer P, is adjusted for maximum square wave ouipui at channel 8. IC,-LM 308, IC2-ICL 401 1. Sensor is made of plexiglas. All resistors are 5 % l/pW. All capacitors are b pF. R-microphone Radio Shack Cat. No. 270090. Tcrystal earphone. S.W.G.-Square wave generator. AMP-Amplifier.
Figure 2. Differentialsonar detector for gas chmmatographicanalyzer. Sensor 1and 2are as in Figure 1. P, is adjustedfor zem ouipui before injection. IC3PLL SCL 4M6A. D,EXOR SCL 4030.
Consiruciion Figure 1shows the schematic diagram of a system used for the demonstration of the phase shift caused by the change of the sound velocity in the sample cell. A square wave generator drives a small crystal earphone t o produce a sound having a frequency of 4 KHz. A small microphone transduces the sound to an electric signal, which is fed into a high-gain amolifier (X1000). The output of the amplifier is fed along with the generator signal to a dual-be& oscilloscope. Any phase shift hetween the signals will he easily detected. A gas
Figure 3. A chromatogram recaded with the sound detector. The chromatp graphic system oonslsts of a Oaw-Mac Oas Chromatograph Series 150.
burst coming out of a smail cigarette lighter (which does not have an automatic electronic ignition system!) and directed toward the detector inlet will cause a distinct phase shift. The construction of a complete differential detector is more complicated. It consists of two earphone-microphone sets identical with the one shown in Figure 1. The inlet of each is connected t o the outlet of the thermal detector (the reference and the samole outlets) or directlv connected to the carrier gas and theLoutletof the column- he output of the reference amolifier is fed to a ~ h a s locked e loop causing an electronic phase shift of d 2 . The output of the sample am~lifieris fed to an EXOR gate with the phase-shifted signal.The EXOR signal,asquhe wave that hasaduty cycle of 50% (after adjusting the \TO of the P1.L with the trimpot) is filtered and compared with half of the supply potential; the difference will remain zero as long as the carrier is not contaminated with other gases. Any change in sound velocity will cause a change in the phase shift and result in a similar chanee in the dutv cvcle and the filter outout. Fieure 3 shows a cfromatogr& recorded with the acoustic dekctor. Finally, the differential system can he packed in a small box to form a gas leak detector. Such a unit was constructed and tested. The svstem also includes a small alarm svstem (a piezoelectric transducer) that is activated when
