Comprehensive inter‐laboratory calibration of reference materials for δ¹⁸O versus VSMOW using various on‐line high‐temperature conversion techniques

Abstract

Internationally distributed organic and inorganic oxygen isotopic reference materials have been calibrated by six laboratories carrying out more than 5300 measurements using a variety of high-temperature conversion techniques (HTC)a in an evaluation sponsored by the International Union of Pure and Applied Chemistry (IUPAC). To aid in the calibration of these reference materials, which span more than 125 per thousand, an artificially enriched reference water (delta(18)O of +78.91 per thousand) and two barium sulfates (one depleted and one enriched in (18)O) were prepared and calibrated relative to VSMOW2b and SLAP reference waters. These materials were used to calibrate the other isotopic reference materials in this study, which yielded: Reference material delta(18)O and estimated combined uncertainty IAEA-602 benzoic acid+71.28 +/- 0.36 per thousand USGS 35 sodium nitrate+56.81 +/- 0.31 per thousand IAEA-NO-3 potassium nitrate+25.32 +/- 0.29 per thousand IAEA-601 benzoic acid+23.14 +/- 0.19 per thousand IAEA-SO-5 barium sulfate+12.13 +/- 0.33 per thousand NBS 127 barium sulfate+8.59 +/- 0.26 per thousand VSMOW2 water 0 per thousand IAEA-600 caffeine-3.48 +/- 0.53 per thousand IAEA-SO-6 barium sulfate-11.35 +/- 0.31 per thousand USGS 34 potassium nitrate-27.78 +/- 0.37 per thousand SLAP water-55.5 per thousand The seemingly large estimated combined uncertainties arise from differences in instrumentation and methodology and difficulty in accounting for all measurement bias. They are composed of the 3-fold standard errors directly calculated from the measurements and provision for systematic errors discussed in this paper. A primary conclusion of this study is that nitrate samples analyzed for delta(18)O should be analyzed with internationally distributed isotopic nitrates, and likewise for sulfates and organics. Authors reporting relative differences of oxygen-isotope ratios (delta(18)O) of nitrates, sulfates, or organic material should explicitly state in their reports the delta(18)O values of two or more internationally distributed nitrates (USGS 34, IAEA-NO-3, and USGS 35), sulfates (IAEA-SO-5, IAEA-SO-6, and NBS 127), or organic material (IAEA-601 benzoic acid, IAEA-602 benzoic acid, and IAEA-600 caffeine), as appropriate to the material being analyzed, had these reference materials been analyzed with unknowns. This procedure ensures that readers will be able to normalize the delta(18)O values at a later time should it become necessary.The high-temperature reduction technique for analyzing delta(18)O and delta(2)H is not as widely applicable as the well-established combustion technique for carbon and nitrogen stable isotope determination. To obtain the most reliable stable isotope data, materials should be treated in an identical fashion; within the same sequence of analyses, samples should be compared with working reference materials that are as similar in nature and in isotopic composition as feasible.

Current challenges in compound-specific stable isotope analysis of environmental organic contaminants

“…MS-specific sources include ionization differences in ion sources and individual electronic amplifier characteristics resulting in incompatible slopes of isotopic scales among mass spectrometers [10,147]. Sources outside the MS include, for example, conversion effects when using water reduction in high-temperature reactions [148]. Coplen et al [149] reported that isotopic analyses in different laboratories often differ by ten times the reported total uncertainty of their measurements.…”

Isotope Fractionation Processes of Selected Elements

“…2.1.1, an alternative method to mass spectrometry is the direct determination of oxygen isotope ratios by laser absorption spectroscopy (Brand et al 2009a, b and others).…”

δ18O anchoring to VPDB: calcite digestion with 18O‐adjusted ortho‐phosphoric acid

“…This was repeated for all the produced acids. The distillates were then placed into autosampler vials and analysed using either the TCEA setup [50] or the laser absorption Cavity Ring-Down Spectrometer system (Picarro, Sunnyvale, Ca, USA) [53] in our laboratory. The results are given in Table 1.…”

“…Water 1 is Jena tap water; waters II to IV were made by slowly distilling local tap water until the isotopic composition was reached. The oxygen isotopic composition of water (columns 2 & 6) was determined with a CRDS system; [53] the oxygen isotopic composition of the phosphoric acid (column 4) was determined using a high-temperature HTC system coupled to a Thermo Fisher Delta þ XL isotope ratio mass spectrometer [50] $ þ22 calc. * * Due to its high hygroscopicity P 4 O 10 could not be analysed reliably using the HTC system.…”