# Effect of Particle Size on Composet Analysis by Portable X-ray Fluorescence
## 1 Introduction  
Portable X-ray fluorescence spectrometry (pXRF) has undergone significant technological improvements and has been applied extensively [1, 2], including in archaeology, geology and mining, and environmental sciences as well as in the elemental analysis of numerous materials (alloys, rocks, soils, and sediments, among others). This technique has several advantages over other laboratory analysis procedures for the determination of the content of elements with an atomic number Z > 12, including its speed (results in minutes), unnecessary sample preparation, no generation of polluting laboratory waste, portability, ease of use, and competitive price [3]. Despite these advantages, this technique has been little used in the elemental analysis of composets and other organic amendments, such as manure or sewage sludge [3]. Standardized laboratory procedures for determining plant nutrients and heavy metal contentsand other properties in these materials are generally tedious. They involve fine-grained grinding of the sample (normally less than 0.5 mm), which guarantees the homogeneity and reproducibility of the results. The sample is then digested in strong acids, such as aqua regia (EN 13650:2001 [4], ISO 54321 [5], US EPA methods 3050B and 3051 [6]). The granulometry of compost particles can affect the analysis of trace elements and plant nutrients by pXRF in two ways. On the one hand, composts are nonhomogeneous materials, made up of particles of different sizes and nature. Various authors have shown that the distribution of trace elements and nutrients varies among the different granulometric fractions, with a tendency for heavy metals to accumulate in the finer fractions [7; 8; 9; 10]. On the other hand, the measurement window of the pXRF instruments covers a few square millimeters. The information depth (that is, the thickness of the sample matrix from which the secondary radiation will return to the detector) is of the order of a few millimeters and is specific to each element. [PERSON] and [PERSON] [2] indicate that for a soil matrix, the Ca response is obtained from the first 100 \(\upmu\)m of the sample, while for Ba, the response is obtained from 2 cm. Therefore, in the case of coarse particulate materials, such as compost, the results of a scan can be very dependent on the particles of the materials that have been focused by the instrument. It is a recognized fact that the greater or lesser the homogeneity/heterogeneity of the material greatly influences the dispersion of the results obtained [11]. Applying pXRF to waste samples, [PERSON] et al. [12] concluded that the technique works for qualitative characterization and requires homogeneous, not very wet, and small particle size samples. However, the possible influence of the granulometry of the samples on the pXRF results, in the compost studies carried out to date with the exception of [10], has been scarcely taken into account. Samples of composts or other organic amendments have usually been analyzed without any previous treatment or, alternatively, after a previous grinding process, such as that necessary to carry out analyses by the aqua regia wet method. The particle size is a physical characteristic of the sample that can be manipulated by the operator by grinding the sample. However, the grinding process is tedious, inadequate if rapid, and a field analysis is to be carried out; in addition, as indicated above, it can affect the XRF results. In the present study, the results of an elemental analysis using pXRF in the granulometric fractions of three composts were compared, as well as the effect produced by the milling process on the pXRF readings. Three kinds of composts with different origins and compositions were used: a compost of municipal solid waste (MSW) from a selective collection, a compost of sewage sludge, and a compost from the agrifood industry for the manufacture of olive oil.