Value Assignment and Uncertainty Evaluation for Anion and Single-Element Reference Solutions
Brian Lang, John L. Molloy, Thomas W. Vetter, Shaun Kotoski, Antonio Possolo
The National Institute of Standards and Technology (NIST) assigns certified values to the mass fractions of individual elements in single-element solutions (Standard Reference Materials in series 3100), and to the mass fractions of anions in anion solutions (Standard Reference Materials in series 3180), based on gravimetric preparations and instrumental methods of analysis. The instrumental method currently is high-performance inductively coupled plasma optical emission spectroscopy (HP-ICP-OES) for the single-elements solutions, and ion chromatography (IC) for the anion solutions. The uncertainty associated with each certified value comprises methods specific components, a component reflecting potential long-term instability that may affect the certified mass fraction during the useful lifetime of the solutions, and a component from between-method differences. Lately, the latter has been evaluated based only on the measurement results for the reference material being certified. The new procedure described in this contribution blends historical information about between-method differences for similar solutions produced previously, with the between-method difference observed when a new material is characterized. This blending procedure is justified because, with only rare exceptions, the same preparation and measurement methods have been used historically: almost 40 years for the preparation methods, 20 years for HP-ICP-OES and for IC. Also, the certified values of mass fraction, and the associated uncertainties, have been very similar, and the chemistry of the solutions also is closely comparable within each series of materials. When applied to relevant, existing measurement results (38 sets for single-element solutions and 11 sets for anion solutions), the new procedure achieves a median reduction in uncertainty of about 10% for the single-element solutions, and essentially no change for the anion solutions. However, more consequential than any reduction in uncertainty, is the improvement in the quality of the uncertainty evaluations that derives from incorporating the rich historical information about between-method differences and about the stability of the solutions over their expected lifetimes.
, Molloy, J.
, Vetter, T.
, Kotoski, S.
and Possolo, A.
Value Assignment and Uncertainty Evaluation for Anion and Single-Element Reference Solutions, Analytical and Bioanalytical Chemistry, [online], https://doi.org/10.1007/s00216-022-04410-y, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=934849
(Accessed June 7, 2023)