Mass fraction assignment of Amino Acids in acidic aqueous solution (CCQM-K78.a)

The CCQM-K78.a comparison was coordinated by the BIPM on behalf of the CCQM Organic Analysis Working Group for NMIs and DIs which provide measurement services in organic analysis under the CIPM MRA. The key comparison forms part of the OAWG 10-year strategic plan of comparisons. CCQM-K78.a underpins the demonstration of capabilities for value assignment of high polarity calibration solutions. The model system selected was amino acids in aqueous solution. Participants were required to assign the mass fractions, expressed in mu g/g, of phenylalanine (Phe), leucine (Leu), isoleucine (Ile) and proline (Pro) present in solution in 0.01 N hydrochloric acid. The content and analytical challenges of the selected analytes are representative of those for typical calibration solutions for polar organic analytes in aqueous solution. Participation in CCQM-K78, a benchmarked measurement capability for assigning the mass fraction content of polar organic compounds (pK(ow) > -2) present at a mass fraction range between 50 mu g/g and 500 mu g/g in an aqueous solution. It also tested capabilities for the quantitative assignment of isomeric polar compounds of similar chromatographic retention time properties. A satisfactory level of agreement of the results was obtained between participants and with gravimetric values for amino acid content. In the cases where the agreement was not satisfactory, the participants were able to identify a technical cause for the inconsistency. The comparison demonstrated the trueness and precision of double IDMS-based methods as a primary measurement procedure for the quantification of polar analytes in aqueous solution when an isotopically labelled version of the analyte is available as the internal standard. It also demonstrated that amino acid quantification using pre- or post-column derivatization with UV or FLD detection can provide results with comparable levels of performance. In this case, where the purity of the primary calibrators had been assigned with a relative standard uncertainty below 0.2%, results consistent with the KCRV within a relative expanded uncertainty in the range 1% - 2% could be realized and levels of 2%-4% were routinely achieved.