Abstract:
The Joint Research Centre (JRC) was working on a feasibility study to develop biodiesel certified reference materials (CRMs) for parameters from EN 14214, a European standard for requirements and test methods for biodiesel. The candidate material (Rapeseed oil fatty methyl esters) was obtained from a commercial source. During the CRM preparation, care was taken to create a stable and homogeneous material, and extensive homogeneity testing is described in the publication. The ANOVA approach was applied for within and between ampoule standard deviations, and then the maximum heterogeneity u*bb calculated. To ensure the stability of the material, some stabilisation precautions were taken, and short term as well as long term stability studies conducted. To ensure the CRM is fit for purpose, the shelf life of each component was calculated for a target uncertainty similar to the method RSD.
Highlights:
- The candidate material was bought from a commercial source with a certificate of analysis (CoA) indicating the suitability of the material.
- Part of the material was mixed with an anti-oxidant.
- Careful planning was conducted including the number of units needed, the type of material and the quantity per unit.
- The ampouling was done using suitable equipment.
- Upon quantifying the between ampoule heterogeneity, all parameters of interest were tested to detect unexpected problems, e.g. contamination during processing.
- Ampoules were selected by random stratified sampling over the whole batch, for homogeneity measurements under repeatability conditions. The order of measurement did not correspond to the filling sequence.
- The homogeneity testing dataset was screened for outliers using the Grubbs procedure and regression analysis was conducted to check for possible trends in the filling sequence. Also, the distribution of results was checked using normal probability plots and histograms.
- Depending on the behaviour of the different analytes, various approaches were taken to compute sbb/ubb.
- After calculation, ubb was compared with RSDstandard method and RSDRstandard method.
- For stability testing, an isochronous study with a reference temperature of -8°C was applied. Outliers were detected with a Grubbs test and the slope of the data points tested for significance with a t-test.
- The short-term study included a 60°C storage, where samples stored 1, 2, and 4 weeks were afterwards compared to samples from storage at 4°C and 18°C.
- The long-term study used ampoules drawn by stratified random sampling; storage intervals were 4, 8 and 12 months.
- The uncertainty of stability was calculated as uncertainty of the slope multiplied by shelf life (if no trend present).
- It was defined that "Dispatch conditions must be chosen so that uncertainty of stability during transportation is negligible".
Citation:
Ulberth-Buchgraber, M.; Potalivo, M.; Emteborg, H.; Held, A. “Towards the Development of Certified Reference Materials for Effective Biodiesel Testing. Part 1: Processing, Homogeneity, and Stability”, Energy & Fuels (2011) 25 (10), 4622-4629.