Page:Concepts for detection of extraterrestrial life.djvu/27

 CHAPTER IV

Gas chromatography has been proposed as an excellent method for detecting the gases of planetary atmospheres and for identifying organic chemical compounds which are of biological interest.

The essential parts of the gas chromatograph are a long tube, or column, containing a powdered material which will adsorb, or bind, different gases with different degrees of strength and a detector that is placed at one end of the tube. During an analysis, the unknown sample, which usually consists of a mixture of gases, is forced through the column by an inert gas, or carrier gas, such as helium or argon. The gases of the sample that are more strongly bound to the material in the column pass through more slowly than the gases that are weakly bound. In this way different gases pass out of the column at different times and are indicated by the detector. (See figs. 5 and 6.)

A basic gas chromatograph for detecting and measuring atmospheric gases or for analyzing organic compounds of biological interest is shown in figure 6. In the case of an atmospheric analysis, a sample of the atmospheric mixture is transported to the sample injector. A constant flow of carrier gas from the carrier-gas storage tank is delivered to the column by the flow regulator and is permitted to flow continuously before injection of the sample. The sample is then put into the carrier-gas stream by the sample injector. The different gases in the mixture separate as they pass through the column, with each gas finally passing through the detector, and causing it to produce an electrical signal. The signal is fed into an electronics system where it is amplified and transmitted back to Earth.

Under suitable conditions, the strength of the signal will indicate the amount of each gas (see fig. 7). The kind of gas is determined by the length of time it takes to pass through the column. The detectors used in gas chromatography usually detect changes in the physical properties of the carrier gas; for example, electrical or thermal conductivity. 17