Organic , Vegetation
The IAEA-450 CRM is a unicellular microalga (Scenedesmus obliquus). Two batches of algae were grown in an outdoor bioreactor, one batch on a standard nutrient solution (natural contamination level), the second batch on a nutrient solution containing elevated levels of As, Cd, Cr, Hg, Ni, Pb, and Pt. After harvesting, the bulk material was gamma ray irradiated at the IAEA’s laboratories to avoid a bacterial deterioration, and tested for homogeneity. Thereafter, in order to adjust the Pt mass fraction to the level of environmental pollution in road dust, the two materials were mixed in a ratio of 1:100 (high Pt: low Pt). Using high purity methanol, slurries of the two algae materials were produced and merged under constant stirring in 200 g batches. After 72 hours drying at 60°C, the dry material was homogenized using a shaking device for 8 hours. Then the bulk material was combined/mixed into one batch and homogenized for 1 week in a 50 L drum homogenizer containing 20 ceramic balls of 2 cm diameter. Portions of 10 g were filled into plastic bottles, sealed with security polyethylene caps and labeled with the code IAEA-450.
* The certified values are given as total mass fractions of listed elements.
**The uncertainty is expressed as a combined standard uncertainty with a coverage factor k = 1 estimated in accordance with the JCGM 100:2008 Evaluation of measurement data — Guide to the expression of uncertainty in measurement.
The reference values were assigned as a median of the measurement results of thirty determinations as described above, rounded off to the most significant number of the uncertainty. Three other mean value estimators were also calculated, namely: arithmetic mean, Algorithm A mean and Hampel mean as described in ISO 13528. These estimators were compared with the calculated respective medians and no significant difference was observed, therefore the reference values estimated as a median were adopted.
The uncertainties associated with the reference values were calculated according to the JCGM 100:2008 Evaluation of measurement data — Guide to the expression of uncertainty in measurement and ISO Guide 35. The combined uncertainty of the certified values consists of uncertainty components associated with characterization (uchar) and between bottle heterogeneity (ubb). These different contributions were combined to estimate the standard uncertainty.
The derived certified values reported in this reference sheet were confirmed against the reference values of the Key Comparison CCQM K-75 [attached]. For this purpose, the equivalence statement was calculated for each analyte according to the BIPM guidelines and was found to be less than 0.82 which indicates a very good agreement.