Concepts for detection of extraterrestrial life/Chapter 5

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CHAPTER V

The Mass Spectrometer

Although mass spectrometry, like gas chromatography, cannot prove the existence of life, it is an experimental tool which would enable us to learn much about the organic chemistry of Mars.

The mass spectrometer approach to exobiological studies is being carried out under the supervision of Dr. Klaus Biemann at the Massachusetts Institute of Technology. Dr. Biemann has concentrated much of his experimental work on amino acids and peptides. This method accomplishes identification through the mass spectra (i.e., the distribution of the masses) of the pyrolysis products of the introduced samples. In one type of instrument an amino acid is heated near the ion source. The molecular fragments so produced vaporize off the sample and are accelerated according to their masses onto an electron multiplier. The identification of the original amino acid is based on the characterestic masses of these fragments.

The mass spectrometer is perhaps unique for the specific identification of small amounts of compounds which have been roughly classified by other methods. While not as sensitive as color reactions, ultraviolet absorption and fluorometry, mass spectrometry is an extremely versatile and powerful method for identifying organic compounds. The ability to recognize organic structures, regardless of whether they do or do not show any resemblance to the molecules with which we are familiar in terrestrial biology, could be crucially important for Martian exploration.

The sample size for mass spectroscopy ranges from a few tenths to a few millionths of a milligram. Spectral interpretation is simplified if this small sample is not too complex. Therefore, some sample preparation is required, with gas chromatography being the favored method for accomplishing this.

Mass spectrometry could also provide data on the composition of the atmosphere and the abundance of ratios of stable isotopes of the elements of low atomic number. Both of these areas are of obvious relevance to the biological exploration of celestial bodies within the solar system.

 

Figure 8.—Origin and appearance of the mass spectra of two amino acids.

Figure 8 shows the mass spectra of two amino acids, phenylalanine and tyrosine. Upon electron bombardment, certain bonds in the molecules of phenylalanine (top) and tyrosine (bottom) are broken to form various positively-charged fragments which the mass spectrometer separates and records according to mass (see curves). Both give mass 74, characteristic of many amino acids, but the rest of the peaks differ because tyrosine contains one more oxygen atom than phenylalanine.

Any mass spectrometer built for use in space would, of course, incorporate a collection apparatus for sampling, sensors to note the results, and telemetry equipment to communicate these results back to Earth. Such information would be obtained quickly with this instrument. The entire mass spectrum of a biological molecule can be scanned in a few seconds. The instrument should be designed for the determination of spectra up to a molecular weight of 250.