In 2010 the proficiency tests consisted of a cereal mix, paprika powder, and green coffee naturally contaminated with OTA, a toxin produced by the moulds Aspergillus ochraceus and Penicillium verrucosum. The work in the poster details how Exact-Matching Double Isotope Dilution Mass Spectrometry (EMD-IDMS) was utilised by the Institute to assign values for the three materials.