# Portable X-ray Fluorescence Analysis of Organic Amendments: A Review
## 3 Results and Discussion
### Determination of Elemental Content with pXRF
[32] compared the concentrations obtained by pXRF with those by ICP-MS (following nitric-hydrochloric acids microwave digestion) in several types of waste: fine fraction reject from solid recovered fuel production, fly ash, biowaste, and compost. The results obtained by the two methods were unacceptably divergent. Regression analysis showed a linear correlation only with Ca and Zn values and, thus, linear correction to transform pXRF results to be comparable to ICP-MS results could be possible for these elements. Nevertheless, pXRF is reported to be best suited for samples such as ash andcompost because of their following physical properties: not too moist, quite small particle size, and not too heterogeneous. The authors concluded that various factors such as sample moisture content, physical and chemical matrix effects (e.g., inter-element effects, particle size, and homogeneity), instrument resolution, and inconsistent sample positioning affected the reliability of the measurement. [PERSON] et al. [34] used pXRF to measure the total elemental content of soils, crops, and organic fertilizer samples (anaerobic digestate, compost, farmyard manure, and straw). The authors measure several nutrients in the amendments but it is not indicated if another comparison method was used. In the case of soils, it is indicated that a CRM sample was used and that the soil calibration does not predict potassium with accuracy, and underpredicts 2-3-fold. [PERSON] et al. [33] used the technique for organic samples from an interlaboratory program that determined certified _aqua regia_ contents. They found good linear correlations between measured and certified values of 16 elements when corrections were applied to pXRF elemental readings by measurements of other main elements, with silicon content being one of the most influential fitting terms in eight of the modelling equations. These results indicate that the low recovery of the _aqua regia_ extraction, due to its inability to dissolve silicates, could be responsible for the poor fit of pXRF results when not compared to total matrix digestion. In the study by [PERSON] et al. [35], the correlations among pXRF and ICP-OES (nitric acid digestion) were strong (R\({}^{2}\) > 0.90) for all elements (P, K, Ca, Mg, Fe) except Mg (R\({}^{2}\) = 0.59) in dried samples (<10% moisture). Analogous to the previous study, [PERSON] et al. [40] compared pXRF and ICP-OES and found R\({}^{2}\) above 0.8 for the elements P, K, Mg, Ca, S, Mn, Fe, Cu, and Zn when analyzed in dry manure samples. The authors found a clear tendency of improving regression models due to drying, but the R\({}^{2}\) was satisfactory (only) for K in fresh and liquid samples. Despite the goodness of the regression coefficients, the slopes of several elements (Ca, Cu, Zn, Fe, K) were greater than unity (greater concentration by pXRF than by ICP-OES). Although the authors indicate that the digestion they applied did not yield total concentrations, they considered this to be a small effect that cannot explain the high slopes and considered the bias to be due to element-specific matrix-dependent effects. Table 3 summarizes the LOQ of various elements in the references mentioned above and compares them with the lowest value established in the legislation for the most demanding type of amendment (extracted from Table 1). The mean LOQ values obtained by the author of this review in organic amendments (unpublished data) and the values indicated by a manufacturer acquired with a reference soil (not specified) [45] are also included in Table 3. As seen in Table 3, the LOQs for Cd and Hg are well above the corresponding lower legal limit, which invalidates the use of pXRF for these measurements. For Cr and Ni, the LOQs were similar to the maximum value of the bibliographic range, which would allow the technique to be applied to a wide variety of types of amendments.