Modern inorganic trace analysis is mainly based on instrumental methods. For these methods, the relation between the signal observed and the concentration, which is intended to be measured, is typically not known exactly. The methods need to be 'calibrated'. For this purpose in practice calibration solutions are used which are prepared gravimetrically from high purity substances either in the own laboratory or in the production line of a commercial producer. The key question is:
Ideal pure substances do not exist. Just looking closely enough, even the purest material available contains impurities of all other elements of the periodic table. To answer the question of how pure a material is requires to measure all these impurities. This takes quite some effort and is therefore hardly done. Typically the nominal purity, which is given by the supplier of a material for often few metallic impurities, is used. Taking the nominal purity to be synonymous for the total purity of the material can result in big errors as shown in Table 1.
Table 1: Comparison of nominal and total purity
| Material 1 | Material 2 | |
|---|---|---|
| 'Nominal' purity | 99,9999 % | 99,99 % |
| Total purity | (99,944 ± 0,017) % | (99,9969 ± 0,0010) % |
At BAM we want to answer the question concerning the total purity of a material with an uncertainty as small as 0,01 %. This uncertainty is about one order of magnitude lower than the typical precision achievable with the best methods for comparison measurements. To measure the total purity of a high purity substance requires a variety of analytical techniques, which typically need to be optimized for the individual measurement problem. Our mostly used analytical method is ICP-SFMS (inductively coupled plasma with sector field mass spectrometry), however, a variety of other methods is also needed.
The fully and with small uncertainty characterised substances form the material's basis of the National Standards for element determination. They are available to the national metrology institutes. In Germany we work very closely together with Physikalisch-Technische Bundesanstalt (PTB), who gravimetrically prepares solutions from these materials.
PTB (Physikalisch-Technische Bundesanstalt)
CCQM (Consultative Committee for Amount of Substance)
