New Applications of High-Resolution Analytical Methods to Study Trace Organic Compounds in Extraterrestrial Materials

Abstract

Organic compounds are present as complex mixtures in extraterrestrial materials including meteorites, which may have played important roles in the origin of life on the primitive Earth. However, the distribution and formation mechanisms of meteoritic organic compounds are not well understood, because conventional analytical methods have limited resolution and sensitivity to resolve their molecular complexity. In this study, advanced instrumental development and analyses are proposed in order to study the trace organic compounds of extraterrestrial materials: (1) a clean room environment to avoid organic contamination during analysis; (2) high-mass-resolution analysis (up to ~150,000 m/m) coupled with high-performance liquid chromatography (HPLC) in order to determine the elemental composition using exact mass for inferring the chemical structure; (3) superior chromatographic separation using a two-dimensional system in order to determine the structural and optical isomers of amino acids; and (4) in situ organic compound analysis and molecular imaging of the sample surface. This approach revealed a higher complexity of organic compounds with a heterogeneous distribution in meteorites. These new methods can be applied to study the chemical evolution of meteoritic organic compounds as well as the molecular occurrence in very-low-mass extraterrestrial materials such as asteroid-returned samples.

Keywords: enantiomeric amino acids; extraterrestrial materials; high mass resolution; high-resolution chromatography; in situ analysis; molecular imaging.

Figure 1

One grain analysis for the methanol extract of the Murchison meteorite by nanoLC-nanoESI/HRMS.

Figure 2

Schematic flow diagram of 2D-HPLC for enantiomeric analysis of trace amino acids. MP, mobile phase; P, pump; AS, auto sampler; CO, column oven; FLD, fluorescence detector; HPV, high-pressure valve; W, waste. C1: Column 1, Capcell Pak C18 ACR S3 (1.5 mm i.d. × 1000 mm, 45 °C); C2: Column 2, KSAACSP-001S (1.5 mm i.d. × 250 mm, 25 °C).

Figure 3

Molecular imaging on the surface of the Murchison meteorite by desorption electrospray ionization/high-resolution mass spectrometry (DESI/HRMS) under ambient conditions in the clean room of the Research Center for Planetary Trace Organic Compounds (PTOC). HV denotes high voltage.

Figure 4

(a) Total ion current (TIC) chromatogram and (b) its mass spectrum obtained from one grain (~50 μg) of the Murchison meteorite by nanoLC/HRMS.

Figure 5

Mass chromatograms of C_nH_2n−1N₂⁺ series (left) and C_nH_2n−6N⁺ series (right) by nanoLC/HRMS of the methanol extract from one grain (~50 μg) of the Murchison meteorite. Numbers in parentheses denote the intensity of the total ion current (TIC) for each ion (m/z). Note that the longer alkyl-chain homologues exhibited longer retention times with increasing hydrophobicity using reverse-phase C18 column chromatography.

Figure 6

2D-HPLC separation of amino acid enantiomers in the Murchison meteorite by high-sensitivity fluorescence detection. (a) Chromatograms of eight enantiomeric amino acids using 2.11 mg of powder sample, and (b) the enantiomeric distribution of 2-amino butyric acid (2AB) from the four fragments (34–128 μg).

Figure 7

Spatial distribution of CHN compounds in the Murchison meteorite revealed by DESI/HRMS. The spatial distribution of C_nH_2n−1N₂⁺, C_nH_2n−4N⁺, and C_nH_2n−6N⁺—corresponding to alkylimidazoles, unsaturated-, and saturated-alkylated pyridines, respectively—is similar. Color bars show ion intensity (note that scales are different for the three CHN series).