Methodologies for Analyzing Soluble Organic Compounds in Extraterrestrial Samples: Amino Acids, Amines, Monocarboxylic Acids, Aldehydes, and Ketones

Abstract

Soluble organic compositions of extraterrestrial samples offer valuable insights into the prebiotic organic chemistry of the solar system. This review provides a summary of the techniques commonly used for analyzing amino acids, amines, monocarboxylic acids, aldehydes, and ketones in extraterrestrial samples. Here, we discuss possible effects of various experimental factors (e.g., extraction protocols, derivatization methods, and chromatographic techniques) in order to highlight potential influences on the results obtained from different methodologies. This detailed summary and assessment of current techniques is intended to serve as a basic guide for selecting methodologies for soluble organic analyses and to emphasize some key considerations for future method development.

Extraterrestrial organic compounds and cyanide in the CM2 carbonaceous chondrites Aguas Zarcas and Murchison

“…The total abundances of carboxylic acids in Murchison reported here are similar to those previously reported (Aponte et al 2019); however, the abundance of acetic acid in the Murchison fragment studied here is about half that reported by Aponte et al (2019). Like the differences seen in concentrations for specific amines and carbonyl compounds, the differences in acetic acid concentration in Murchison could be related to derivatization efficiencies and/or sample heterogeneity (Simkus et al 2019a). Similar to the amines, Aguas Zarcas and Murchison show a full suite of C 1 -C 6 monocarboxylic acid isomers which decrease in concentration with increasing molecular weight.…”

“…Similarly, concentrations of benzoic acid in Murchison varying by up to an order of magnitude per gram of meteorite have been measured in this work and by Huang et al (2005), Martins et al (2006), and Aponte et al (2011). This observation suggests that more than sample heterogeneity, the detection of these dicarboxylic acids may be highly dependent on the specific extraction and analytical protocol used (Simkus et al 2019a). Figure 4 shows the LC-ToF-MS chromatograms of representative injections from hot water extracts and the water-extracted residues of Aguas Zarcas, Murchison, a procedural blank, and a cyanide standard.…”

“…The total concentration of amines in Murchison reported here is about 2.5 times higher than those previously reported for Murchison (Aponte et al 2014); this higher total amine abundance is also due to higher amounts of methylamine and ethylamine detected in this sample compared to previous reports. These contrasting amine abundances in Murchison may be related to differences in extraction efficiencies, derivatization yields, and/or sample heterogeneity (Simkus et al 2019a), as well as curation conditions. However, a notable similarity is apparent in the molecular distribution of aliphatic amines in Aguas Zarcas and Murchison, whereby both meteorites show a full suite of C 1 -C 5 amine isomers which decrease in concentration with increasing molecular weight (Table 1).…”

“…This is important, because when different methods are used, it can be difficult to fully understand whether reported differences between meteorites are intrinsic or reflect methodological biases. In cases where evaluation of methods does occur, the differences in the resulting analyses can be substantial (Simkus et al 2019a). Often, different methods have been used in different laboratories (or even in subsequent years in the same research groups), making it difficult to draw conclusions from any comparisons.…”

“…Often, different methods have been used in different laboratories (or even in subsequent years in the same research groups), making it difficult to draw conclusions from any comparisons. This often goes unrealized or undiscussed and there is little published evaluation of how different analytical methodologies and techniques compare to each other (Simkus et al 2019a). Therefore, direct comparison of the composition of various soluble organic compound classes in Aguas Zarcas and Murchison, using identical preparative methods on both meteorites, should be a more robust way to evaluate the organic composition of these two CM2 chondrites.…”