RESUMEN
OSIRIS-REx will return pristine samples of carbonaceous asteroid Bennu. This article describes how pristine was defined based on expectations of Bennu and on a realistic understanding of what is achievable with a constrained schedule and budget, and how that definition flowed to requirements and implementation. To return a pristine sample, the OSIRIS-REx spacecraft sampling hardware was maintained at level 100 A/2 and <180 ng/cm2 of amino acids and hydrazine on the sampler head through precision cleaning, control of materials, and vigilance. Contamination is further characterized via witness material exposed to the spacecraft assembly and testing environment as well as in space. This characterization provided knowledge of the expected background and will be used in conjunction with archived spacecraft components for comparison with the samples when they are delivered to Earth for analysis. Most of all, the cleanliness of the OSIRIS-REx spacecraft was achieved through communication among scientists, engineers, managers, and technicians.
RESUMEN
A series of near-IR fluorescent dyes were prepared which contained an intramolecular heavy atom for altering the fluorescence lifetimes to produce a set of probes appropriate for base-calling in a single-lane DNA sequencing format. The heavy-atom modification consisted of an intramolecular halogen situated on a remote section of the chromophore in order to minimize the perturbation on the lifetimes and fluorescence quantum yields. In addition, the dye series possessed an isothiocyanate functional group to allow facile attachment to sequencing primers. The unconjugated dyes showed similar absorption and emission maxima (lambda abs = 765-768 nm; lambda em = 794-798 nm) as well as fluorescence quantum yields that were invariant, within experimental error, with the heavy atom. However, the lifetimes of these dyes were found to vary with the identity of the halogen substitution (I, tau f = 947 ps; F, tau f = 843 ps, measured in methanol), with an average variation within the dye series of 35 ps. The spectroscopic properties of the free dyes and the dyes conjugated to sequencing primers on the 5'-end of the oligonucleotide were determined in a DNA-sequencing matrix (denaturing gels containing formamide). The results indicated slight differences in the fluorescence properties of the free dyes compared to those of the dye/ primer conjugates in this particular matrix. Inspection of the ground-state absorption spectra showed significant aggregation for the free dyes in this solution, but the conjugated dyes exhibited no sign of aggregation due to the highly anionic nature of the oligonucleotide. The fluorescence lifetimes of the dye/primer conjugates demonstrated lifetimes which ranged from 735 to 889 ps, with an average variation of 51 ps, an adequate difference to allow facile discrimination of these dyes in DNA-sequencing conditions. In addition, the free solution electrophoretic mobilities of the native heavy-atom-modified dyes were found to be very similar. When the dye/primer conjugates were electrophoresed in a cross-linked polyacrylamide gel electrophoresis capillary column, they comigrated, indicating that, in single-lane sequencing applications, when utilizing these dyes, no postrun corrections would be required to correct for dye-dependent mobility shifts.
