# Portable X-ray Fluorescence Analysis of Organic Amendments: A Review
## 3 Results and Discussion
### Organic Matter Effect  
Surprisingly, there are no studies evaluating the effect that organic matter content may have on pXRF readings in organic amendment matrices. To know about this subject, we must consider articles that refer to pXRF use in soils with a high content of organic matter. This effect in soils has been recently reviewed by [PERSON] et al. [51] when evaluating the application of pXRF in tropical soils. There is common agreement that soil organic matter may cause a scattering and attenuation of signals when a soil sample is irradiated by X-rays and that the influence and interference magnitude of soil organic matter are still poorly understood. Consequently, the result for a target element could be modified. Probably the most detailed study in this regard is that of [PERSON] and [PERSON] [49]. In their study, three organic matter substitutes (cellulose, graphite powder, and confectioner's sugar) were added to a soil standard reference material and the concentrations of 13 elements (As, Cr, Cu, Fe, Mn, Pb, Rb, Sr, Th, Ti, V, Zn, and, Zr) were measured as a function of variable organic matter fractions. Their results demonstrate the attenuation of the pXRF signal with the increase in the organic matter fraction of the sample, although the deviations depend on each element. For example, pXRF-Pb concentrations were not affected, pXRF-As concentrations were underestimated, and Mn showed a more complex behavior, both overestimating and underestimating depending on its concentration. The correction factors for the effect of organic matter were generally less than \(\pm\)8% for organic matter contents up to 30%. [PERSON] et al. [50] verified the effect of organic matter removal on oxides determination by pXRF in soil samples (<2 mm) and their clay fraction. The soil organic matter presence led to underestimation of the pXRF results for the lightest oxides (Si and Al) compared to the heaviest oxides (Fe, Ti, and Mn) which were less affected. Soil organic matter in these samples was below 4.4% (44 g kg\({}^{-1}\)), which is a content one order of magnitude lower than that found in organic amendments. The lightest elements have lower XRF photon energy values and, as a consequence, the effect of organic matter coating the soil particles can be more pronounced. The reduction of the SiO\({}_{2}\) content reached 20-22.5% with respect to the content of the mineral soil (3-5% as SiO\({}_{2}\)). The reduction of contents of the light oxides (SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\)) can be considered relevant. On the contrary, Fe\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and MnO content reductions obtained by pXRF can be considered irrelevant with variation before and after organic matter combustion lesser than 1%. The authors indicate that the possible effect of organic matter may be related to the scattering of primary and secondary X-rays, underestimating the results, in the same way that has been observed for soil with a high moisture content. \begin{table}
\begin{tabular}{l c c c c} \hline  & **Min 1** & **REF 2** & **OA 3** & **Ins 4** \\ \hline As & – & 3-21 & 23 & 21 \\ Cd & 1 & 18–24 & 39 & 36 \\ Cr & 100 & 8–66 & 135 & 90 \\ Cu & 100 & 15–39 & 98 & 45 \\ Hg & 1 & – & 34 & 27 \\ Ni & 50 & 10–60 & 116 & 90 \\ Pb & 100 & 6–45 & 27 & 24 \\ Zn & 300 & 5–30 & 82 & 36 \\ Mn & – & 9–33 & 280 & 195 \\ Fe & – & 15–1500 & 1180 & – \\ K & – & 90–1100 & 1260 & – \\ P & – & 150–2100 & 1390 5 & 900 5 \\ \hline \end{tabular} 1 Minimum value in EU regulations from Table 1. 2 Range of values reported in the bibliographical references [28; 40; 43; 49]. 3 Average values obtained by the author in organic amendments. 4 Optimal detection limits provided by the manufacturer using Soil analysis mode and a certified reference material or 5 Mining mode.  
\end{table} 
Table 3: Limits of quantification (LOQ mg kg\({}^{-1}\)) of elements in organic amendments. [PERSON] and [PERSON]. [43] evaluated pXRF analysis of peat and organic soils.