Concepts for detection of extraterrestrial life/Chapter 4
The Gas Chromatograph
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.
Figure 5.—Gas chromatograph.
Figure 6.—Block diagram of the gas chromatograph.
Biological substances do not normally occur as vapors and therefore cannot be directly detected by gas chromatography. In the analysis of these compounds it is necessary to convert nongaseous materials to vapor form before they can be analyzed by gas chromatography. This can be done in two ways. One way is to heat the sample at relatively low temperatures; for example, 100° C to 150° C. With this treatment, some biological compounds can be converted to vapors which can be injected into the carrier gas and analyzed in the usual way. Other biochemical compounds cannot be vaporized so easily and must be heated to higher temperatures. When these substances are strongly heated their molecules break up into smaller molecules, some of which are gaseous. By analyzing these smaller molecules on the gas chromatograph it is possible to tell what the original biological substances were. This type of analysis requires a great deal of research in order to learn how large molecules break down when they are heated at high temperatures. The oven in figure 6 is used to heat biochemical compounds in order to convert them to gases.
Through the use of gas chromatographs that are designed to perform an analysis automatically on space probes landed on the surface of Mars it will
Figure 7.—Typical gas chromatogram. Zero is the time of injection. The time on the minutes scale, for each peak, is its retention time, and identifies the gas passing through the detector. The area under each peak is proportional to the amount of gas.
be possible to determine what gases in the Martian atmosphere may be important to living organisms. For example, tests for water vapor, oxygen, carbon dioxide, and nitrogen can be made as well as for many other gases, and samples of Martian soil can be collected and heated. If these samples contain organic matter, it will be possible to tell whether substances which are known to be part of living organisms are present. If proteins, nucleic acids, sugars or fatty substances are found, this would be strong evidence that life is present, although it would not be conclusive. The information obtained through gas chromatography, combined with information obtained through other experiments, would not only establish the presence of life, but it would tell whether or not this life was chemically the same as terrestrial life.
One of the outstanding advantages of the gas chromatograph is its versatility. It can be made very complex or relatively simple depending upon the kind of analysis desired and the constraints of the space probe mission. By using a system having several columns and detectors, an instrument can be designed which will analyze a wide variety of biochemical substances. This would be desirable if there is no prior clue to the possible kinds of organic chemicals in an unknown mixture, such as might be the case for a sample of Martian soil.
A second important advantage is that the analysis results in the separation of complex chemical mixtures. This feature permits confirming the analysis of each constituent by other methods; for example, mass spectroscopy.
Finally, the instrumentation is readily adaptable to miniaturization and ruggedness of construction, which is an essential feature for instruments intended to perform remote automatic analysis on unmanned space probes.
Several model instruments have been studied for application to the biological exploration of the solar system. These instruments range from 5 to 14 pounds in weight and are of various degrees of complexity. Gas chromatographs which can analyze planetary atmospheres in 10 seconds are being studied, as well as instruments which can detect tens or hundreds of gaseous compounds in a single analysis. Because of this extreme versatility, the scientists working with gas chromatography believe that it is one of the most useful methods for the detection of biologically relevant chemical compounds and the constituents of planetary atmospheres.