Studies in chemical evolution: Gas chromatographic-mass spectrometric determination of organic and organogenic matter in lunar samples, carbonaceous chondrites and terrestrial samples



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Extraterrestrial samples, such as the lunar samples from Apollo 11 and 12 missions, the Allende and Murchison carbonaceous chondrites; products of abiotic experiments of synthesis, such as alkanes produced in closed Fischer-Tropsch processes; terrestrial samples, such as the Precambrian Gunflint chert (Canada), recent shales from Irati (Brazil), along with certain microorganisms related to geochemical samples such as bacteria (Desulfovibrio vulgaris) and alga (Chlorococcus); environmental hydrocarbons in air (Sierra Nevada), water (Houston Ship Channel) and sand beaches (Galveston), as well as controls directly or indirectly related with the analytical procedures employed and the origin of the samples studied, have all been investigated using gas chromatography and/or combined gas chromatography-mass spectrometry. The significance of the results obtained is discussed regarding the processes and mechanisms governing chemical evolution. For the lunar samples, it has been demonstrated that simple compounds of the organogenic elements such as CO, CO[lowered 2], CH[lowered 4], HC=CH, CH[lowered 2]=CH[lowered 2], CH[lowered 3]-CH[lowered 3] and H[lowered 2]S, have been either thermally evolved and/or generated upon hydrolysis. The thermal evolution of these gases was studied in a stepped fashion (intervals of 100-200°C). While the CO and CO[lowered 2] generated above 400-500°C appear to be produced from solid carbon-containing species indigenous to the lunar surface (e.g. carbides) the indigenous state of the gaseous compounds evolved below 400°C, such as CO[lowered 2] and CH[lowered 4], is questioned; CO[lowered 2] mainly on the grounds of terrestrial adsorption and CH[lowered 4] regarding several observations such as the hydrolytic effects of adsorbed atmospheric water upon lunar carbides. On the other hand, methane and the light hydrocarbons are products of acid hydrolysis of carbon containing solid species such as carbides. No extractable organic compounds were detected in the bulk samples at the ng/g level. A single organic compound, diisopropyl disulfide, extracted from 2 aliquots of Apollo 11 fines was also found in the Apollo 12 York meshes. The solid and gaseous compounds investigated and found to be present in the lunar samples would eventually yield, under appropriate conditions, more complex organic compounds of significance in the chemical evolution of planetoidal systems. The meteorites studied, especially the Murchison, have provided conclusive evidence for the presence of indigenous hydrocarbons and amino acids of most probable extraterrestrial origin. It appears that these results can be generalized to well preserved specimens of other Type I and II carbonaceous chondrites. The evidence for extraterrestrial amino acids and hydrocarbons in carbonaceous chondrites adds support to the occurrence of abiotic synthesis, leading to chemical evolution, in other locations of the solar system and similar systems. Abiotic synthesis of hydrocarbons by means of closed Fischer-Tropsch processes, further substantiates the non-formation in such systems of isoprenoid structures with more than 14 carbon atoms. Isoprenoids with more than 14 carbon atoms are commonly found in samples of the terrestrial biota. It appears that these isoprenoids can safely be taken as biological markers. They have been unequivocally identified in the Precambrian Gunflint chert, Irati shale, and other samples exposed to the terrestrial environment (e.g. Houston Ship Channel water, and untreated York mesh). The results on the effects of contamination in terrestrial and extraterrestrial samples exposed to the environment have been discussed for the lunar samples (atmospheric gases and York mesh), meteorites, terrestrial sediments (Desulfovibrio vulgaris), limestone (Chlorococcus), and sand beaches. Other environmental samples such as mountain air (Sierra Nevada) and water (Houston Ship Channel) have been shown to contain hydrocarbons resembling petroleum crudes. Commercial nanograde benzene was found to contain significant levels of impurities detectable by gas chromatography.



Moon, Organic compounds, Exobiology