Hwashim Lee

Final report on key comparison CCQM-K55.c (L-(+)-Valine): Characterization of organic substances for chemical purity

Under the auspices of the Organic Analysis Working Group (OAWG) of the Comit? Consultatif pour la Quantit? de Mati?re (CCQM) a key comparison, CCQM K55.c, was coordinated by the Bureau International des Poids et Mesures (BIPM) in 2012. Twenty National Measurement Institutes or Designated Institutes and the BIPM participated. Participants were required to assign the mass fraction of valine present as the main component in the comparison sample for CCQM-K55.c. The comparison samples were prepared from analytical grade L-valine purchased from a commercial supplier and used as provided without further treatment or purification. Valine was selected to be representative of the performance of a laboratorys measurement capability for the purity assignment of organic compounds of low structural complexity [molecular weight range 100?300] and high polarity (pKOW > ?2). The KCRV for the valine content of the material was 992.0 mg/g with a combined standard uncertainty of 0.3 mg/g. The key comparison reference value (KCRV) was assigned by combination of KCRVs assigned from participant results for each orthogonal impurity class. The relative expanded uncertainties reported by laboratories having results consistent with the KCRV ranged from 1 mg/g to 6 mg/g when using mass balance based approaches alone, 2 mg/g to 7 mg/g using quantitative 1H NMR (qNMR) based approaches and from 1 mg/g to 2.5 mg/g when a result obtained by a mass balance method was combined with a separate qNMR result. The material provided several analytical challenges. In addition to the need to identify and quantify various related amino acid impurities including leucine, isoleucine, alanine and ?-amino butyrate, care was required to select appropriate conditions for performing Karl Fischer titration assay for water content to avoid bias due to in situ formation of water by self-condensation under the assay conditions. It also proved to be a challenging compound for purity assignment by qNMR techniques. There was overall excellent agreement between participants in the identification and the quantification of the total and individual related structure impurities, water content, residual solvent and total non-volatile content of the sample. Appropriate technical justifications were developed to rationalise observed discrepancies in the limited cases where methodology differences led to inconsistent results. The comparison demonstrated that to perform a qNMR purity assignment the selection of appropriate parameters and an understanding of their potential influence on the assigned value is critical for reliable implementation of the method, particularly when one or more of the peaks to be quantified consist of complex multiplet signals. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Realization of the Indium Freezing Point

The freezing and melting behaviour of indium samples with nominal purities of 99,9999%, 99,999% and 99,99% has been studied to find the influence of conventional freezing techniques on the freezing temperature and to assess the effect of trace impurities on the quality of a fixed-point sample. Plateaus of constant temperature (± 0,1 mK) are readily obtained from both the induced and outside-chilled induced freezes, while freezing ranges of about 2 mK are observed in both the normal and outside-chilled freezes for all the samples. The maximum freezing temperatures of the conventional freezes are found to coincide very closely for each sample and to within ± 0,1 mK for the purest sample. The purest sample showed a narrow melting range on the melting curve after very fast freezing, in contrast to the other samples which showed the melting ranges expected of alloys. The average melting temperature of the purest sample closely agreed with the maximum freezing temperature. The difference between the average melting temperature after the very fast freeze and the maximum freezing temperature appears to be a useful criterion for assessing the quality of indium as a fixed-point sample.