Inter-laboratory calibration of a Ag3PO4 comparison material for oxygen stable isotope analysis
Authors: Watzinger Andrea , Schott Katharina , Hood-Nowotny Rebecca , Skrzypek Grzegorz , Tamburini Federica , Arppe Laura , Cristini Domiziana , Knöller Kay 
Published: March 2020

&lt;p&gt;A silver phosphate comparison material (Ag&lt;sub&gt;3&lt;/sub&gt;PO&lt;sub&gt;4&lt;/sub&gt;) for measurement of the stable oxygen isotope composition was prepared by the University of Natural Resources and Life Science (BOKU) and distributed to four international isotope laboratories frequently measuring the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O value in Ag&lt;sub&gt;3&lt;/sub&gt;PO&lt;sub&gt;4&lt;/sub&gt;. The contributing laboratories were the University of Natural Resources and Life Science (BOKU), The University of Western Australia (UWA), the ETH Zurich (ETH), the University of Helsinki (UH) and the Helmholtz Centre for Environmental Research (UFZ). Each laboratory analysed the comparison material in a minimum of two independent measuring rounds with a minimum of 10 individual measurements. The instrument used to perform the measurements were high-temperature conversion elemental analyzers coupled with continuous flow isotope ratio mass spectrometers: TC/EA with Thermo Finnigan Delta XP (BOKU), a TC/EA with a Thermo Scientific Delta V Plus (UWA), an Elementar Pyrocube with a Isoprime 100 (ETH), a Flash IRMS EA with a Thermo Scientific Delta V Plus (UH) and a TC/EA with a Finnigan Delta S (UFZ). The working gas &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O was set to 0 &amp;#8240; and the normalization was done by a three-point linear regression calibration (Paul et al., 2007) using the reference material IAEA-601 (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;VSMOW&lt;/sub&gt; = +23.14 &amp;#177; 0.17 &amp;#8240;), IAEA-602 (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;VSMOW&lt;/sub&gt; = +71.28 &amp;#177;0.42 &amp;#8240;) (both benzoic acid) and NBS 127 (barium sulfate) (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;VSMOW&lt;/sub&gt; = +8.59 &amp;#177; 0.20 &amp;#8240;) (Brand et al., 2009). BOKU, UH and ETH had experienced inhomogeneity of the IAEA-602 as already mentioned in Brand et al. (2009). The weighted arithmetic mean and standard deviation (1&amp;#963;) of the new BOKU Ag&lt;sub&gt;3&lt;/sub&gt;PO&lt;sub&gt;4&lt;/sub&gt; comparison material from the single measurements has a &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O value of 13.80 &amp;#177; 0.40 &amp;#8240; on the VSMOW scale (n=131), while the median of the single rounds was 13.76 &amp;#8240; (n=11) and the median of the laboratories was 13.79 &amp;#8240; (n=5). The arithmetic means of two measuring rounds were outside &amp;#177; 1&amp;#963;. When excluding data from these rounds from the statistics the weighted arithmetic mean has a &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O value of 13.80 &amp;#177; 0.32 &amp;#8240; on the VSMOW scale (n = 111) and the median of the single valid rounds (n=9) remained at 13.76 &amp;#8240; and the median of the labs at 13.79 &amp;#8240; (n=5). Excluding NBS127 from the normalization slightly reduced the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O value to 13.74 &amp;#177; 0.31 &amp;#8240; (n = 111). The BOKU Ag&lt;sub&gt;3&lt;/sub&gt;PO&lt;sub&gt;4&lt;/sub&gt; comparison material is available for stable isotope research laboratories to facilitate the calibration of their lab comparison material.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Brand, W.A., Coplen, T.B., Aerts-Bijma, A.T., B&amp;#246;hlke, J.K., Gehre, M., Geilmann, H., Gr&amp;#246;ning, M., Jansen, H.G., Meijer, H.A.J., Mroczkowski, S.J., Qi, H., Soergel, K., Stuart-Williams, H., Weise, S.M., Werner, R.A., 2009. Comprehensive inter-laboratory calibration of reference materials for &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O versus VSMOW using various on-line high-temperature conversion techniques. Rapid Communications in Mass Spectrometry, 999&amp;#8211;1009. doi:10.1002/rcm&lt;/p&gt;&lt;p&gt;Paul, D., Skrzypek, G., F&amp;#243;rizs, I., 2007. Normalization of measured stable isotopic compositions to isotope reference scales - A review. Rapid Communications in Mass Spectrometry 21, 3006&amp;#8211;3014. doi:10.1002/rcm.3185&lt;/p&gt;