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An interesting observation and worth looking into further, especially since it suggests excess O2 to increase temperature in this instance outweighs fundamental principles of chemical reaction equilibrium.
The conversion of carbonate to CO2 can be written as 2 CO3 à 2 CO2 + O2. Since O2 is a product of this conversion, increasing the amount of O2 present in the system and thus on the product side, would shift the equilibrium of the reaction to the side of the educt resulting in an incomplete conversion and thus isotopic fractionation. This is exactly what one observes when converting nitrate into NOx in an EA in the presence of O2, which has been described by Gentile and Siegwolf.
However, be that as it may, my main concern with using inorganic salts (minerals) in an EA-IRMS to scale anchor organic compounds is that it does not meet with the Identical Treatment principle. (A) Obviously, inorganic compounds are not a match for organic compounds be it from a temperature stability or be it from an elemental composition (make-up) point of view. (B) Because of that, one way or another matrix effects may impact on results.
There are plenty of organic reference materials (RMs) available that can be used to scale anchor d13C values of organics measured by EA-IRMS.
It is my personal conviction, the use of inorganic RMs such as carbonates should be confined to scale anchor results of stable isotope analysis of carbonates by acid digest. Analysis of CO2 released by acid digest of these materials is after all how the d13C (and d18O) values of these inorganic RMs were determined in the first place. Speaking of acid digest, CO2 released from carbonate RMs can of course be used as reference gas when analysing off-line combusted organics in Dual-Inlet instruments. ;-)
Dear Wolfram and others,
I know that EA-IRMS analyses of NO3 salts often yields better results if the oxygen pulse is turned off. Wolfram cites Gentile et al., (2013) on this subject in his book. However, when CaCO3 is analysed (e.g. NBS19 or its replacement IAEA-603) at least in my lab we keep the oxygen pulse on. In fact we also add WO3 (some labs add (V2O3) to the CaCO3 to provide the reaction with further oxygen. During the oxygen pulse the tin cup compusts in a flash and that further increases the temperature to well above 1020 degrees. I seem to remember reading that during the flash temperatures of around 1800 degrees are reached for a very short time, and I think that reaching these temperatures is important to convert the CaCO3 quantitatively to CO2 gas. When we have problems with our EA, e.g. the timing of the oxygen pulse is off, we find that our d13C measurements of CaCO3 are also off. Therefore I would argue that the oxygen pulse during EA-IRMS analyses of carbonates is important.
In this regard I am currently looking for publications that have described the flash procedure, and the temperatures that are reached during the flash. Maybe someone can point me towards publications if they exists. In the German language literature I have found "Glühfarben" (glow colour) tables that link the colour of steel in steel producing furnaces to its temperature. Does something similar exist for other metals (e.g. Tin for Tin capsules)?
Have a nice weekend,
Dr. Heiko Moossen, Dipl.-Chem.
Stable Isotope Laboratory
Max-Planck-Institute for Biogeochemistry
P.O.Box 100164, 07701 Jena, Germany
From: Stable Isotope Geochemistry <[log in to unmask]> on behalf of Wolfram Meier-Augenstein (pals) <[log in to unmask]>
Sent: 28 November 2018 08:39
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] preparation of working standards for d13C
The d13C values of the RMs described in Arndt's 2016 paper were scale normalized to the VPDB scale as defined by NBS19 and LSVEC. This does not necessarily mean they were analysed against these RMs although one participating lab did so using optimised EA conditions. Other participating labs used RMs such as USGS40 and USGS41 as scale anchors for scale normalization to VPDB. Internationally accepted RMs such as USGS40, IAEA-CH-6 or IAEA-CH-7 are all anchored on the VPDB/LSVEC scale.
In my book I advised not to use inorganic RMs to scale anchor organic samples as this does not meet with the Identical Treatment principle (due to potential matrix effects).
I also pointed out what the issues are and how to overcome these.
Carbonates do not "burn differently". They do not burn at all. Carbonates represent the highest state of oxidation carbon can assume. It is therefore impossible to burn = oxidise carbonates any further. What happens in an EA is a thermolytic breakdown of carbonate into carbon dioxide and oxygen. For this to happen in a way to yield "true" d13C values the reactor temperature has to be above 1000C and the oxygen pulse must be switched off. While some EA-IRMS systems can apply individualised run parameters for each samples, others may not offer this option (e.g. O2 pulse active for one sample but not active for the next).
Using organic RMs for organic samples avoids these issues.
BTW, Arndt and I have recently published a paper on the subject of arMs and their use in Isotopes Environ. Health Stud.
Hope this helps.
Prof. Dr W Meier-Augenstein, CChem, FRSC
Stable Isotope Forensics & Analytical Sciences
Robert Gordon University
School of Pharmacy and Life Sciences
The Ian Wood Building
E-mail: [log in to unmask]
From: Stable Isotope Geochemistry <[log in to unmask]> on behalf of Matheus Carvalho <[log in to unmask]>
Sent: 28 November 2018 05:23:43
To: [log in to unmask]
Subject: [ISOGEOCHEM] preparation of working standards for d13C
Dear all, I am a little confused about the use of international reference materials for the preparation of working standards. I read the paper by Schimmelmann et al (2016) Anal. Chem. 88, 4294. If I understood it right, they measured the new standards (all organic substances, like caffeine and glycine) in comparison with international standards like LSVEC and NBS19 for d13C. But I also read in the book Stable Isotope Forensics by our estimated contributor Wolfram Meier-Augenstein (excellent book, by the way) that we should not use these materials for organic sample analyzes because they burn differently in the EA reactor, this being the very reason to use the new standards in the Schimmelman paper. But if the standards in that paper were determined against the inorganic substances, why should I trust them more than an organic standard that I determine myself using LSVEC and NBS19?
Matheus C. Carvalho
Centre for Coastal Biogeochemistry Research, Southern Cross University
Lismore, NSW, Australia
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