# Portable X-ray Fluorescence Analysis of Organic Amendments: A Review
## 3 Results and Discussion
### Organic Matter Effect
They found in an orthotrophic peat, satisfactory results for Cu (certified content 5.28 \(\pm\) 1.04 mg kg\({}^{-1}\)) and Pb (certified content 174 \(\pm\) 8 mg kg\({}^{-1}\)) but the concentrations of Ca, Ti, Cr, Ni, and Zn were overestimated by 2-3 times, and Fe by 5 times. In the same study, they analyzed the concentrations of K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, As, Sr, and Pb of 183 Scottish toposils that had a wide range of organic carbon (OC) concentrations (1.23 to 48.8%). They concluded that modifications to manufacturer calibrations are needed and that empirical calibration of XRF instruments requires unavailable certified reference peaks and peat soils, certified for total element concentration across a wide range of elements. In a recent study, [PERSON] et al. [38] compared the feasibility of common laboratory methods (microwave plasma-AES and pXRF) to evaluate element concentrations in soil highly influenced by sewage waste. The lowest correlation between both methods was obtained for Ni (R\({}^{2}\) = 0.70, stronger points scattering), although the correlations were satisfactory for the other elements (Pb, Cu, Zn and Cr). The authors also approached another strategy for the analysis of organic samples with pXRF: the elimination by ignition of the organic matter to then carry out the readings in the calcined samples. This same sample calcination strategy was already used by [PERSON] et al. [52] to measure the metals Ba, Sr, V, Cr, Ni, Cu, Zn, Zr, Pb, Y, La, and Ce, although in this case, a nonportable laboratory instrument was used. [PERSON] et al. [38] found that OM removal in the soil sample influences its density and effective volume, which interferes with pXRF elemental analysis. However, if the data were corrected for organic matter, the influence of the matrix on the pXRF measurements was reduced. Their conclusion was that results validate a usage of pXRF as an alternative method for the conventional wet-chemical-based AES analysis for soil samples highly influenced by sewage wastes. However, the authors indicate that the prior calcination invalidates the use of the technique in the field. These research studies show that we still do not know in detail the effect of organic matter on the elemental determination by pXRF. The results indicate that the response is highly dependent on the element considered and that possibly the interference of organic matter is more relevant in light elements such as Si or Al. Similarly, previous studies indicate that there may be an effect of organic matter on the measurement of Ni, and sometimes Cr, which is consistent with the high LOQs found by us in organic matrices (Table 3). Even for the same element, results have been found in both directions (overestimation and underestimation) in different studies (for example, [43] vs. [50]). It may happen that the direction or magnitude of the interference depends on the calibration method or the instrument model used, which could explain the disparity in results. In a recent study using six instruments of the same analyzer model, [PERSON] et al. [37] observed that the photon counts from each instrument varied by a considerable amount between them and the authors thus concluded the need for individual instrument calibrations.