1H and 13C Solid state Nuclear Magnetic Resonance (NMR) spectroscopyof an HCl extracted Ryugu C0002 particle

G. D. Cody1, H. Yabuta2, and the Hayabusa2-initial-analysis IOM team1
2Earth and Planets Laboratory, Carnegie Institute for Science, Washington, DC, USA
Hiroshima University, Hiroshima, Japan

Introduction: 1H and 13C solid state NMR studies of extraterrestrial organic solids (Insoluble OrganicMatter, IOM) in carbonaceous chondritic meteorites has the potential to reveal the extent of IOM’s
molecular evolution that occurred due to parent body processing (largely hydrothermal) in the interiorof the original planetesimal from which the meteorite was derived. For example 1H and 13C NMR
studies of IOM from CR, CI, CM, and an ungrouped C meteorites (Tagish Lake, TL) reveal that there
is a wide range of molecular evolution where the least evolved was the CR IOM and the degree ofmolecular evolution increased from CI to CM to TL (Cody and Alexander, 2005). Subsequent NMR
studies of discrete TL clasts with differing degrees of alteration revealed a high degree of molecularevolution across them (Herd et al., 2011). In all previous studies, relatively large (10’s of mg) samplesof pure IOM were available (note for typical CC meteorites, recoverable IOM constitutes ~ 1 wt % of
the meteorite, 10 mg of IOM would require ~ 1gm of meteorite). For studies of Ryugu we were
provided with a single particle that had been treated with aqueous HCL (to remove any carbonate) from
C0002 that was ~ 1 mg where in principle 10 ng of IOM is expected. Notwithstanding the obvious
challenges we endeavored to see what could be seen.
Experimental: All NMR experiments were performed at the W. M. Keck Solid State NMR facility at
the Earth and Planets Laboratory at a static magnetic field of 7.05 Tesla. 1H NMR analysis wasperformed using 2.5 mm rotor diameter Magic Angle Spinning (MAS) probe with the resonant
frequency of 1H being 300 MHz. The samples were spun at 22 KHz to reduce line broadening
associated with H dipolar coupling. In order to reduce 1H background signal arising from hydrogenoutside the rotor we applied the DEPTH pulse sequence. The rotors capable of spinning faster than 20
KHz use endcaps and drive tips made of an aromatic polymer, Torlon, that is inside the rotor and willcontribute some background 1H signal. This background signal is generally negligible with 10-20 mgof pure organic matter. With the very small Ryugu sample it was expected and observed that theTorlon’s 1H back ground signal would be substantial. 13C solid state NMR was attempted using a 5 mmprobe (with MAS = 11.5 KHz) with Teflon (no H) inserts and a background suppression pulse scheme
before Variable Amplitude Cross Polarization to maximize signal. We were unable to obtain any 13C
signal which is not unexpected given how little organic carbon is in a 1 mg Ryugu sample. We arepleased to report that no sample was lost during transfer into and out of both rotors and the sample wasreturned to Japan intact and unperturbed by the analysis. Thus the sample handling protocols adopted
were sufficient.
Results: As expected the 1H NMR signal is dominated by 1H in the aromatic polymer, Torlon (Fig.1),
at 7.1 ppm. We do detect a 1H NMR signal at 1.3 ppm that arises from the Ryugu particle. In previous1H studies of pure IOM, aliphatic H appears at 1.3 ppm, so it may be that we are detecting Ryugu’s
aliphatic H. Note that IOM’s aromatic H peak lies exactly at 7.1 ppm so it coincides with the backgroundsignal. It is now known that Ryugu mineralogy contains considerable amounts of saponitic clays, withtwo 1H peaks at 4 and 0.35 ppm (Levin et al. 2004), thus the H from this clay is expected to be ~ 10 X
that in IOM. However we don’t see any H’s from saponitic clay (Fig. 1). The most likely explanationis that the iron content of clays in the Ryugu sample are high enough that paramagnetic Fe “spoils” thesignal from saponite H’s and as IOM largely exists in the matrix as discrete organic grains, IOM signal
persists. 1H NMR of natural montmorillonite exhibits no signal due to Fe cations in its structure (Levinet al. 2004).
It is tempting to subtract the background signal from the bkgd+Ryugu particle, when one does
this one finds that the line width of the Torlon is slightly greater than that of Torlon+Ryugu signal suchthat obvious spectra subtraction artifacts eventually arise and place a maximum constraint on how muchbackground subtraction can be performed, without this constraint one could range from majority
aromatic-H to nearly 100 % aliphatic-H, neither case having been observed in any previous studies oftype 1 and 2 chondritic IOM. Previous 1H NMR analyses of IOM (Cody and Alexander, 2005; Herd etal. 2011) have been shown that there is a strong linear correlation between the Fraction of either
aliphatic or aromatic H (FARO-H) with atomic H/C (*100) (see Fig.2) and with the fraction of aromaticcarbon (FARO-C). Thus, using the restriction of maximum background subtraction we can place aminimum constraint on FARO-H of 0.5 (there is no restriction on how large FARO-H could be beyond
assuming that there was no Torlon background). This places a maximum constraint of H/C*100 being
58 (there is no constraint on how low H/C*100 could be). Similarly, we can constrain a minimum as
to how low FARO-C could be at 0.64 (the only constraint as to how high it could be is controlled by the1.3 ppm feature). These values would suggest that Ryugu’s IOM has slightly greater molecular
evolution than what has been observed in CM and CI derived IOM (Cody and Alexander 2005), butmuch less than observed for the most molecularly evolved Tagish Lake IOM (Fig. 2).

Figure 1: 1H solid state NMR spectrum of
1 mg Ryugu particle from Chamber C (black).
The background signal from Torlon is in Red

Figure 2: Background subtraction
reveals a minimum FARO-H and amaximum H/C. The red circles arevalues for IOM from the different Tagish
Lake clasts. The red lines highlight anequally possible Ryugu composition

This study demonstrates that it is possible to obtains 1H NMR spectra of Ryugu samples. In thefuture, if it were possible to obtain 100 mg of Ryugu particles, 1 mg of pure IOM could be isolated.
This would increase the 1H signal by 100 X vastly reducing the background signal. We have obtained
fairly high quality 13C NMR spectra on mg quantities of IOM, so additional molecular information
would be obtained directly. Alternatively, if 20 mg of HCL extracted Ryugu particles were available(our 2.5 mm rotors can only handle 20 mg) these could be analyzed directly where the 1H signal wouldbe ~ 20 X that observed in Fig. 1. The advantage to this later approach would be that NMR is nondestructive and the sample would be available for other analyses by other researchers.
References:
Cody, G.D. and Alexander, C.M.O.D. (2005) NMR studies of chemical structural variation of insoluble organic
matter from different carbonaceous chondrite groups. Geochimica et Cosmochimica Acta 69, 1085-1097.
Herd, C.D.K., et al. (2011) Origin and evolution of prebiotic organic matter as inferred from the Tagish Lake
meteorite. Science-SOM 332.
Levin et al. (2004) J. Applied Physics, 96, 5685