INDIANAPOLIS, March 26, 2023 — If you’ve ever seen a shooting star, you might have actually seen a meteor on its way to Earth. Those that land here are called meteorites and can be used to peek back in time, into the far corners of outer space or at the earliest building blocks of life. Today, scientists report some of the most detailed analyses yet of the organic material of two meteorites. They’ve identified tens of thousands of molecular “puzzle pieces,” including a larger amount of oxygen atoms than they had expected.
The researchers will present their results at the spring meeting of the American Chemical Society (ACS). ACS Spring 2023 is a hybrid meeting being held virtually and in-person March 26–30, and features more than 10,000 presentations on a wide range of science topics.
Previously, the team led by Alan Marshall, Ph.D., investigated complex mixtures of organic materials found on Earth, including petroleum. But now, they are turning their attention toward the skies — or the things that have fallen from them. Their ultra-high resolution mass spectrometry (MS) technique is starting to reveal new information about the universe and could ultimately provide a window into the origin of life itself.
“This analysis gives us an idea of what’s out there, what we’re going to run into as we move forward as a ‘spacefaring’ species,” says Joseph Frye-Jones, a graduate student who is presenting the work at the meeting. Both Marshall and Frye-Jones are at Florida State University and the National High Magnetic Field Laboratory.
Thousands of meteorites fall to Earth every year, but only a rare few are “carbonaceous chondrites,” the category of space rock that contains the most organic, or carbon-containing, material. One of the most famous is the “Murchison” meteorite, which fell in Australia in 1969 and has been studied extensively since. A newer entry is the relatively unexplored “Aguas Zarcas,” which fell in Costa Rica in 2019, bursting through back porches and even a doghouse as its pieces fell to the ground. By understanding the organic makeup of these meteorites, researchers can obtain information about where and when the rocks formed, and what they ran into on their journey through space.
To make sense of the complicated jumble of molecules on the meteorites, the scientists turned to MS. This technique blasts a sample apart into tiny particles, then basically reports the mass of each one, represented as a peak. By analyzing the collection of peaks, or the spectrum, scientists can learn what was in the original sample. But in many cases, the resolution of the spectrum is only good enough to confirm the presence of a compound that was already presumed to be there, rather than providing information about unknown components.
This is where Fourier-transform ion cyclotron resonance (FT-ICR) MS comes in, which is also known as “ultra-high resolution” MS. It can analyze incredibly complex mixtures with very high levels of resolution and accuracy. It’s especially well suited for analyzing mixtures, like petroleum, or the complex organic material extracted from a meteorite. “With this instrument, we really have the resolution to look at everything in many kinds of samples,” says Frye-Jones.
The researchers extracted the organic material from samples of both the Murchison and Aguas Zarcas meteorites, then analyzed it with ultra-high resolution MS. Rather than analyzing only one specific class of molecules at a time, such as amino acids, they chose to look at all soluble organic material at once. This provided the team with more than 30,000 peaks for each meteorite to analyze, and over 60% of them could be given a unique molecular formula. Frye-Jones says these results represent the first analysis of this type on the Aguas Zarcas meteorite, and the highest-resolution analysis on the Murchison one. In fact, this team identified nearly twice as many molecular formulas as previously reported for the older meteorite.
Once determined, the data were sorted into unique groups based on various characteristics, such as whether they included oxygen or sulfur, or whether they potentially contained a ring structure or double bonds. They were surprised to find a large amount of oxygen content among the compounds. “You don’t think of oxygen-containing organics as being a big part of meteorites,” explained Marshall.
The researchers will next turn their attention to two far more precious samples: a few grams of lunar dust from the Apollo 12 and 14 missions of 1969 and 1971, respectively. These samples predate Marshall’s invention of FT-ICR MS in the early 1970s. Instrumentation has come a long way in the decades since and is now perfectly poised to analyze these powders. The team will soon compare their results from the meteorite analyses to the data they obtain from the lunar samples, hoping to learn more information about where the moon’s surface came from. “Was it from meteorites? Solar radiation? We should be able to soon shed some light on that,” says Marshall.
The researchers acknowledge funding from the National Science Foundation Division of Chemistry and the State of Florida. They thank the Chicago Field Museum Robert A Pritzker Center for Meteoritics and Polar Studies and the Arizona State University Buseck Center of Meteorite Studies for the meteorite samples.
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Molecular characterization of soluble organic material from meteorites by 21T FT-ICR MS
Stony chondrite meteorites are the most common type of meteorites that fall to Earth. Of those, three percent are carbonaceous chondrites, which contain the highest concentration of organic material of all classifications of meteorites. Previous studies on their composition have primarily focused on bulk elemental composition and targeted analysis of specific compounds such as amino acids, carbohydrates, and other precursors to molecules essential to life. Non-targeted analysis is rarely performed because meteorites are complex mixtures of presumably tens of thousands of molecular formulas and thus require ultrahigh-resolution techniques for characterization. Here, we utilize 21 tesla Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to characterize the soluble organic material from two meteorites, Murchison and Aguas Zarcas. In 1969, the Murchison chondrite fell in Murchison, Australia and has since become one of the most studied meteorites to date. Its rapid collection after impact minimized terrestrial contamination, with over 100 kilograms of material recovered. Aguas Zarcas chondrite is a relatively recent meteorite that hit a doghouse in rural Costa Rica in 2019, and as such, very little is known about it.
A range of solvents with diverse eluent strengths was used to extract the organic material from the two meteorites. Analysis was performed by positive- and negative-ion electrospray ionization (ESI) coupled to 21T FT-ICR MS. Formic acid and tetramethylammonium hydroxide were used as solvent modifiers to aid during the ionization process. Over 30,000 peaks were detected in each mass spectrum, of which >60% could each be assigned a unique molecular formula, and collectively sorted into heteroatom classes, such as CHNOS, CHOMg, and CHO, among others. As previously seen for terrestrial complex mixtures, the more polar, protic solvents extracted more material, and oxygenated compounds were detected in high relative abundances. More specifically, acidic O2 species were observed in high abundance: up to fifteen percent of assigned peaks when the analysis was performed in negative-ion ESI assisted by a modifier. Conversely, positive-ion ESI revealed a higher relative abundance of NOx and SOx species. Overall, detected compounds reveal species with low aromaticity (double bond equivalents of 1-10) with carbon numbers that range from C10 – C60, as well as high oxygen content (up to O16 in some molecular formulas).