Pretreatment of Nitric Acid with Hydrogen Peroxide Reduces Total Procedural Os Blank to Femtogram Levels.

Determining and correcting for background contributions of Re and Os from chemical reagents is critical for accurate and precise Re-Os dating of materials with parts per billion to parts per trillion Re and Os concentrations. Here we investigate reducing Os content in nitric acid, as it is the main contributor to the Os blank. Pretreating high-purity nitric acid with hydrogen peroxide (H2O2) significantly reduces nitric acid's Os contribution to femtogram levels, greatly reducing Os blank corrections. The improvement in background Os allows analysis of samples with extremely low Os concentrations (into the low ppt level). We present experimental data identifying key factors in reducing Os blank, including nitric acid to hydrogen peroxide volume ratios, wet versus dry glassware, and dark versus lighted reaction environments. These variables affect the reaction time between the two reagents, which in turn correlates inversely with the final Os content. The volume ratio of H2O2/HNO3 is shown to be the fundamental control on reaction time and final Os content, yielding a well-defined exponential relationship; minor variations in reaction time result from wet/dry glassware and light/dark reaction environment. At a H2O2/HNO3 volume ratio of ∼0.24, the total procedural Os blank is reduced from 6.5 pg (no H2O2) to 0.043 pg. The (187)Os/(188)Os of the Os blank ranges from 0.18 to 0.36, consistent with the Os blank compositions obtained by the AIRIE Program and other Re-Os laboratories worldwide, and is uncorrelated with any experimental variables. In contrast to Os, pretreatment with hydrogen peroxide did not improve the Re blank of nitric acid; Re background reduction requires conventional methods such as sub-boiling distillation.