This paper elucidates the theoretical principles behind the calculation of the size of a representative sample of granular solid waste. The key concept is the number of particles that must be present in a sub-portion of matter to be representative of a larger portion of matter. This depends on the fraction of particles in the waste batch showing the properties of interest, which shall be measured. A representative sample must include a fraction of particles of interest reliably similar to that of the waste batch to be characterized, with a controlled variability. In this context, it is demonstrated that the number of particles of interest that must be collected in a representative sample is 100. From this requirement, the mass of a representative sample can be calculated based on the knowledge of the frequency of particles of interest of the waste lot to be characterized. Data on particles concentrations in different samples of WEEE plastic scraps exemplifies how the presence in the sample of enough rare particles showing the property of interest is key to ensure reliable measurements. Further, the assumptions made on the controlled degree of variability to determine the minimum number of particles are discussed based on data on achievable intra- and inter-laboratory variability of analytical standards for waste characterization. Accordingly, the mass of laboratory samples and test portions recommended in published sampling plans or analytical standards are assessed for the occurring number of particles.

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