Quantitative NMR spectroscopy for accurate purity determination of amino acids, and uncertainty evaluation for different signals

摘要

Purity evaluation of amino acids using nuclear magnetic resonance spectroscopy is reported. Three amino acids (aspartic acid, valine, and arginine) and certified reference materials (CRMs), such as acidic, neutral, and basic amino acids, as well as a low pure sample of valine were used as the analytes. DCl solution, D_2O, and NaOD solution were used as the preparation solvents. The quantitative values were obtained from all observed signals and compared with the certified values of the CRMs. When an amino acid was dissolved in water, a strong HOD signal due to proton exchange was observed. When the signal adjoining the HOD signal was considered in the evaluation, the accurate quantitative value could not be obtained. Therefore, under optimized conditions, the analyte signals separated from the HOD signal were chosen for purity determination of amino acids. As a result, the quantitative values were in agreement with the certified values of CRMs. An expanded uncertainty was estimated to be approximately 0.002 kg kg~(-1). We also discuss the effect of impurities on purity determination based on all signals and conclude that agreement of quantitative values determined from different signals in a molecule is a good indication of the accuracy of the results.