WP 3: Standard solutions for challenging elements

Elemental solutions with a mass concentration of 1 g/L are used in almost every chemical laboratory for calibration. The solutions are the carrier of traceability and are directly provided by the NMIs to the customers or indirectly via commercial providers which are linked with the primary standards of the NMIs. A relative uncertainty of 0.3 % for the concentrations of the customer solutions turned out to be the present demand. Considering the certification and linking procedures the primary solutions of the NMIs need to have an uncertainty ≤ 0.1 %. At present only calibration solutions of a few of elements meet these requirements whereas solutions for a lot of essential elements are nonexistent. The reasons are the lack of reliable primary standards and of methods for the solid-liquid transfer of the primary elemental materials. To perform this transfer is a main subject of this WP.
For practical use the primary solid elemental materials (e.g. the metals characterized in WP1) have to be converted to a measurable form which is in almost every case a liquid solution. Primary standard solutions are needed for dissemination and for calibration purposes as well. The main challenge of this work is to assure that the primary solid material is completely dissolved and brought into the primary solution (task 3.1). This is in particular problematic for hardly dissolvable solid materials as the refractory elements and for materials which form volatile compounds during the digestion process. The refractory elements considered here are examples for such a challenging solid-liquid transfer and there are no appropriate dissolution methods available so far. The results of the CCQM-pilot study P46 clearly underpin this present situation. Primary Rh solutions prepared by the NMIs showed relative discrepancies of about 0.5 % from the reference value. This is much higher than the requirements mentioned above and could be caused by incompleteness of the digestion process (WP3) and insufficient purity assessment of the primary materials (WP1).
Measurement methods providing highest precision ≤ 0.1 % are needed for dissemination. For that purpose the elemental content of secondary solutions made from any solid elemental material of unknown purity has to be attributed by comparing measurement with primary solutions (task 3.2). Secondary solutions can be provided by the NMI to the customers as transfer standards.
Alternatively instrumental nuclear activation analysis (INAA) should be investigated concerning its suitability for the measurement of the elemental content of the solutions directly against a primary solid material, without the need to dissolve the latter. This technique has the potential to rationalize solid-liquid transfers basically in particular in the case of elements difficult to be digested. Such an approach has not yet been tested for that.

The tasks of this work package are:

  • Development of methods for the complete and loss-free digestion of refractory elements for the preparation of primary elemental solutions
  • Development and validation of precision methods for comparing two elemental solutions of similar composition with small uncertainty (0.05 %) (differential method) by means of ICP-OES, ICP-SFMS, MC-ICP-MS, and INAA
  • Investigations of the feasibility to link secondary solutions with the primary solid material directly by means of INAA