WP 2: Elemental and isotopic characterisation

The aim of this work package is to develop and validate new and improved methods for determination of the elemental and isotopic composition of primary elemental standards.
Direct determination of the main component of a primary elemental standard is an essential alternative to the impurity assessment techniques developed in WP1. The methodology also serves to establish traceability between those primary standards and different elements. The aim of this task is to evaluate the possibility and conditions for use of main component assay methods to establish SI traceability for elemental solutions. Evaluation of rounded amperometric titration curves in EDTA assay is not yet satisfactorily solved. Assays traceable to EDTA material of known composition will be developed and evaluated using materials of known composition prepared in WP1.
The determination of isotope abundance ratios is urgently needed in establishing traceability in quantitative elemental analysis because they play a fundamental role in two aspects of the traceability chain:

  • Measuring the isotope composition of primary and secondary calibration standards and of the analyte in matrix reference materials or other samples
  • Application of isotope dilution mass spectrometry (IDMS) for transfer of calibrations between primary and secondary standards and reference measurements

The top level of traceability is being realized by primary calibration standards for the determination of element concentrations but these high purity substances may undergo isotope fractionation processes during production. Additionally for many elements there are isotope variations in nature. Therefore the isotope composition between primary calibration standard and secondary calibration standard might differ from each other, but certainly they differ from the isotopic composition of the analyte in many samples. Many measurement principles in chemical analysis are on a molar basis or are isotope selective and therefore a difference in the isotope composition between sample and calibration standard will lead to biased results. Also the conversion from mass fraction (e.g. mg/kg) to amount content (e.g. mol/kg) and vice versa leads to biased results unless the isotope composition is being considered.
Most determinations of isotope abundance ratios are based on mass spectrometry which offers the potential for measuring the ratios with very small uncertainties. However, the instruments are subject to mass discrimination effects which require mass calibration or correction of the spectrometer. A key task for this work package is to evaluate potential approaches for this purpose with sufficiently small uncertainty for measuring the isotope composition of primary calibration standards as well as the other applications mentioned above. The more fundamental approach, with potential for smallest uncertainties, is based on mass bias calibration of the spectrometer for a specific application using isotope mixtures of known composition which are prepared gravimetrically from pure isotopes of the target element. This is time consuming and expensive. A comparison will, therefore, be made with alternative approaches for making mass bias corrections which whilst having larger uncertainties are less time consuming and are potentially feasible for rapid application to a much larger group of elements. Possible approaches to be evaluated include mass bias calibration with mixtures of commercially available “pure” isotopes, mass bias correction using a nearby isotope of another element, and mass bias correction using the “isotopic double spiking” technique.
The elements selected as models for development and optimisation of new isotopic measurement procedures are Mg and Mo. Mg will be used to evaluate procedures for ultra-high purification of single isotopes and gravimetric preparation of isotopic calibration mixtures for mass bias correction of Mg isotope ratio measurements. Mo is an element for which isotopes of reasonable purity are available commercially but for which ultra-high purification would present much greater challenges. Unlike Mg, it also has sufficient isotopes to allow evaluation of the “isotopic double spiking” technique. The WP deliverables include primary isotopic reference materials for each element with uncertainties < 0.01 % for Mg and < 0.05 % for Mo. Both elements show significant isotopic variation in nature. Hence the availability of these reference materials will also improve the uncertainties achievable by IDMS measurements for applications such as materials science, biological/clinical applications, and environmental measurements. The improved methodology for mass bias corrections will make a major contribution to WP3 which uses the MC-ICP-MS technique for certification of elemental calibration solutions.