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.

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