In house validation for the determination of total nitrogen content in fertilisers: developing criteria with uncertainty measurement evaluation and compliance assessment

Abstract

The European regulation 2003/2003 stipulates that several methods may be applied for the determination of total nitrogen content in fertilisers for compliance in the European market. For most of the different types of fertilisers with different matrices obliged to comply with this regulation there is a lack of parameters validation data. Thus, these standards must be used beyond scope despite regulation purpose for extended use. This work presents a matrix effect study in the working range carried out using different reference materials of different types of fertilisers. Trueness and repeatability as method performance parameters are practical, understandable and simple tools for the in house validation of two standards, EN 15651 and EN 15750. The gathered new information supports the definition of strategies for evaluation of measurement uncertainty. The comparison of measurement procedures was made also through evaluation of limits of detection and quantification, working range and uncertainty measurement. This comparison led to the conclusion that either method is fit for the purpose with limitations for all the tested materials. The relative uncertainty for the EN 15750 method is half of the one for the EN 15561 method. The final purpose was the choice of the most adequate method for each type of fertiliser regarding compliance assessment.