IsoLab

17O correction

The ion current on the mass 45 Faraday cup is comprised of 13C16O2+ as well as 12C17O16O+. The latter isobaric interference amounts to about 7% of the total ion current. If the oxygen isotopic signature between the sample and the reference gas is different, the 17O contribution to the m/z 45 ion current can be corrected for by measuring the δ18O signature on m/z 46 and assume a constant law for the fractionation of 17O and 18O. Although this does not strictly apply, this is the standard procedure first used by Harmon Craig in 1957. It is usually referred to as 'Craig' correction. A number of improvements or alterations have been proposed in the literature including the set of absolute ratios for the reference materials and the exponent of the fractionation law. We have compiled the different correction techniques and algorithms in an XLS file (17O_corrections.xls) which is available for downloading. The corrections can be tested using different inputs for δ13C and δ18O in the input fields. In 2010, we published a linearized 17O correction, based on the Assonov correction. The new linearized approach shares its simplicity with the Craig correction but takes the isotopic relations in the world oxygen pools into account.

(see: Willi A. Brand, Sergey S. Assonov, Tyler B. Coplen, Correction for the 17O interference in δ(13C) determinations when analyzing CO2 with stable isotope mass spectrometry, (IUPAC Technical Report), Pure Appl. Chem., 2010; 82 (8), 1719–1733 (doi:10.1351/PAC-REP-09-01-05))

Solid Reference Materials - Caffeine, Glutamic acids, Benzoic acids ......

- BGC-IsoLab has engaged generating new international Reference Materials. From a company in Bremen we acquired a large amount of natural caffeine from the decaffeination process. The material was rigorously tested for homogeneity and forwarded to the IAEA in Vienna. The work was conducted under a contract with the IAEA (302-F3-GFR-12020 / B5- GFR-30590). The advantage of caffeine is that it contains both nitrogen and carbon in comparable quantities within the same molecule. Moreover, it has non-exchangeable oxygen that makes the material suited for HTC techniques. Calibration of the material is on the way (with support from IAEA and IUPAC). The material is available as IAEA 600.

A similar effort in collaboration with the USGS in Reston (VA) has been completed. The USGS prepared two isotopically different samples of glutamic acid for high precision determination of δ13C and δ15N. One of the acids was slightly enriched in δ13C and δ15N within the realm of naturally occurring isotope values.

With A. Schimmelmann (Univ. Indiana, Bloomington) we engaged in generating a new (organic) δ18O reference material for on-line oxygen isotope analyis. We purchased a larger amount of high purity benzoic acid from Merck and exchanged the oxygen with 18O enriched water (pH 0, 90°C, 3 weeks reaction under reflux conditions) for 1/3rd of the material. The exchange reaction was made in collaboration with the inorganic chemistry institute of the University of Jena (M. Doerr and W. Weigand). The two materials were then isotopically measured using a high temperature pyrolysis technique (Jena Isolab) and a medium temperature off line decarboxylation reaction (Indiana). Both experiments resulted in similar values for the two materials, one at about +23 per mill and the enriched form at about +71 per mill vs. VSMOW. The material was forwarded to the IAEA in Vienna for further handling. The work done in Bloomington was sponsored by the IAEA. The materials are now available as IAEA 601 and IAEA 602.

Results have been published:

1. Haiping Qi, Tyler B. Coplen, Heike Geilmann, Willi A. Brand, and J.K. Böhlke, Two new organic reference materials for δ13C and δ15N measurements and a new value for the 13C of NBS 22 oil, Rapid Comm. Mass Spectrom. 17 (2003) 2483-2487
2. Coplen TB, Brand, WA, Gehre M, Groening, M, Meijer HAJ, Toman B, and Verkouteren RM, Anal. Chem. 2006; 768: 2439