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Meteoritic Nanodiamonds


Figure 3 : Plots of N/O vs. the concentrations of two different sets of ions in each of 36 APT reconstructed acid residue samples. Concentrations are normalized to the sum of the counts of all ions of interest, including C, Na, Cl, F, N, and O. The very loose trend of higher concentrations of C+(2×C 2 )+(3×C 3 ) for higher N/O, and the opposite trend of lower concentrations of PtOC+Na+NaO for higher N/O, may be explained as a mixture of two phases in the meteoritic material, such as disordered C and nanodiamonds.


Another analytical method uses the concentrations of each of the four sets of ions that we have reason to believe may distinguish between disordered and diamond forms of carbon. We plot these concentra- tions versus the 12 C/ 13 C ratio for both doubly and singly charged carbon ions ( Figure 5 ). For the 2 + ratios, all of the adjusted R 2 values for the four fi ts are less than 0.1, and the slopes have very large uncertainties (fi t lines not shown). For the 1 + ratios, the adjusted R 2 is slightly negative for N and 0.04 for PtOC+Na+NaO (fi t lines not shown). However, C does increase for the 1 + ratios, with a slope of 0.004±0.001 in units of (C 1 +2×C 2 +3×C 3 concen- tration)/( 12 C/ 13 C), albeit with an adjusted R 2 of only 0.18. T e O species decreases slightly with 12 C + / 13 C + ratios, with a slope of -0.004±0.001 in units of (O concentration)/( 12 C/ 13 C), with an adjusted R 2 of 0.30.


Discussion


Figure 4 : N/O ratio and (C 1 +2×C 2 +3×C 3 )/(PtOC+Na+NaO) ratios plotted vs. the normalized 12 C/ 13 C isotopic ratios. No trends are present, giving no evidence that more than one isotopic reservoir contributed material to the samples.


PtOC+Na+NaO concentration: ~13% (1/8) of data sets with PtOC+Na+NaO concentration > 0.45 had N/O > 0.05, whereas ~32% (9/28) with PtOC+Na+NaO concentration < 0.45 had N/O > 0.05. T ese data suggest that these two ratios record information about a mixture of two phases, such that data sets with extreme values in one of these ratios contain acid residue composed primarily either of disordered C, with O and PtOC+Na+NaO, or of diamond, with N and C 1 +2×C 2 +3×C 3 . Some of the correlation for PtOC+Na+NaO with N/O is due to the fact that O is present in PtOC and NaO, both of which are also included in the ion count used to calculate concentration, but, taken along with the lack of N and higher O for lower C 1 +2×C 2 +3×C 3 concentration; these concentrations can be used as a qualitative proxy for the fraction of the acid residue that is nanodiamond versus disordered carbon. T is trend is not easily discernable when N or O are plotted separately instead of N/O. Instead of using concentration, we could divide the counts of PtOC by the counts of C 1 +2×C 2 +3×C 3 , and the counts of N by O. Since deviation from solar ratios of stable isotopes is the best indicator of whether material is presolar, these values were plotted versus the 12 C/ 13 C ratios of the microtips. T e results of this analytical method are null; no trends are apparent ( Figure 4 ).


2018 March • www.microscopy-today.com


We have detected two weak trends in trace element concentration with increasing 12 C + / 13 C + isotopic ratio: C increases, and O decreases. T ere are several possible interpretations. T e low R 2 fi ts to these trends means that it is possible that this is just the result of random scatter. It is also possible, based on these trends, that nanodiamonds have higher isotopic ratios than the


disordered C and that these two phases formed from two diff erent isotopic reservoirs. However, we note that the trends were only apparent for the 1 + ratios. T is suggests that rather than inherent isotopic anomalies, these trends are due to an instrumental artifact that aff ects 1 + ratios more than 2 + , for example, a 12 CH + hydride interference. T is explanation would imply that contamination of the mass spectrum with hydrides, formed from residual gas in the atom-probe analysis vacuum chamber, goes up with increasing fraction of disordered C (more O) and decreasing fraction of diamond (less C). Such contamination would cause lower measured C + ratios due to misindentifi cation of 12 CH + hydride at 13 amu in the mass spectrum as 13 C + , but not lower C 2+ ratios, since 12 CH 2+ forms much less readily than 12 CH + . Future TEM work will be carried out in order to confi rm


this fi nding by distinguishing disordered carbon from diamonds in the Pt nanotips, using electron energy loss spectrometry (EELS), prior to APT. T e TEM/EELS analysis can also distin- guish between diff erent types of bonding in disordered C, allowing us to comment on how specifi c types of disordered C behave under atom-probe analysis conditions.


Conclusion We have identifi ed an approach that allows us for the fi rst time to distinguish qualitatively between APT reconstructions


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