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.
