Quantifying Isotopic Heterogeneity of Candidate Reference Materials at the Picogram Sampling Scale

Abstract

We propose a method for quantifying the in situ heterogeneity of solid materials at the picogram test portion scale, illustrating its use by investigating the oxygen isotope ratio (18O−/16O−) of four quartz samples. Using secondary ion mass spectrometry, we could estimate the intrinsic heterogeneity using a large number (~ 100) of closely spaced duplicated measurements. An analysis of variance was then applied to these large data sets to extract the measurement repeatability (typically 0.10–0.15‰, 1s) from the total variability, thereby revealing a variability ranging from 0.18‰ to 2.3‰, which can be attributed to the genuine isotope ratio heterogeneities. A small proportion of outlying values were either rejected manually after inspection, or were accommodated using robust statistics. We also evaluated two distinct approaches for estimating and correcting instrumental drift; the use of a sub‐area of the test material (if shown to have sufficiently low heterogeneity) is judged to be preferable to using a piece of unrelated silicate glass that we believe to be homogeneous. We also compared three approaches for estimating measurement repeatability, from which we show that the ‘duplicate method’ applied to the reference material is preferable to using other methods based either on the drift monitoring material or on assessing residuals of the drift monitoring material after drift correction. Finally, here we propose a strategy for predicting the number of measurements on individual fragments of a material that would be required to achieve a specified target uncertainty.

The Hydrothermal Breccia of Berglia-Glassberget, Trøndelag, Norway: Snapshot of a Triassic Earthquake

“…Combining all of these uncertainty sources, we estimate that the total analytical uncertainty on our individual δ 18 O determinations to be circa ± 0.14‰ (1sd). The repeatability on the N = 31 determinations of the NBS28 quartz sand was ± 0.32‰ (1sd), which is consistent with the level of heterogeneity reported previously for this material when evaluated at the sub-nanogram sampling scale [46].…”

The Schlaining quartz-stibnite deposit, Eastern Alps, Austria: constraints from conventional and infrared microthermometry and isotope and crush-leach analyses of fluid inclusions

“…The 18 O/ 16 O ratios of the quartz samples were measured using the Potsdam large geometry CAMECA 1280-HR SIMS according to the method proposed by Ramsey and Wiedenbeck (2018).…”

Challenges for the estimation of uncertainty of measurements made in situ

“…There have been several publications describing possible approaches to estimating the uncertainty of in situ measurements, usually for the purpose of validation. These include studies of metals in soil [1], bilirubin in infants [18], passive detection of 137 Cs by gamma-ray spectrometry [7] and oxygen isotope ratios in quartz by SIMS [2]. These studies will not be described in detail here, but only those aspects that illustrate general issues and help explain the outstanding challenges that need to be addressed.…”

“…centimetre scale with PXRF [1]) down to the micro (e.g. microns with SIMS [2]), with the mass of the corresponding 'undisturbed sample' ranging from micrograms to picograms.…”