# Portable X-ray Fluorescence Analysis of Organic Amendments: A Review
## 3 Results and Discussion
### Effect of Moisture Content  
Although pXRF is used in many different field applications involving the measurement of wet and even liquid samples, it is an accepted fact that water causes a decrease in the pXRF signal [7; 8]. In various geological and environmental applications, the effect of moisture content has been shown to be linear, creating a dilution effect on pXRF signals. Under these circumstances, the fit of the pXRF measurements is improved simply by applying a correction factor [8]. Working with compost from several feedstocks, [PERSON] et al. [28] found that pXRF moist scans produced overall lower means than both ICP-OES and pXRF dry data. The R\({}^{2}\) values also worsened in all moist models when compared with the corresponding dry dataset. However, the determination of Cu, Pb, and Zn concentrations of compost samples in situ was possible as sample moisture showed little influence on predictability. [PERSON] et al. [32] studied the results in moist and dried samples of biowaste and compost. The element concentrations of the moist and the dry samples, expressed both on a dry matter basis, were closer to each other for compost samples (\(-\)10 to 10%)than for biowaste samples (\(-\)37 to 33%) due to the greater moisture content of biowaste samples (almost double on average). However, the direction of the difference depends on the element: Zn, Cu, and K presented lower values in the humid samples while Ca and Cl presented higher values with humidity. [PERSON] et al. [35] dedicated a study to the objectives of quantifying the effect of sample moisture content, predicting moisture content, and correcting for moisture effect on elemental concentration determinations in livestock manure. They found that the average intensity of P, K, Ca, Mg, and Fe decreased with increasing sample moisture content. This result in the case of Ca is contrary to that of the previous study by [PERSON] et al. [32]. An interesting result of this study is that the authors were able to determine the moisture content (R\({}^{2}\) = 0.98) using pXRF spectrum data and random forest regression. For this, they used peaks from the spectrum included in regions that were free of the elemental signals and that had great predictive power for the moisture content. In this way, elemental concentrations could be predicted in all wet samples, with R\({}^{2}\) ranging from 0.90 for element Mg (in order to determine this element, the measurements were made under vacuum conditions) to 0.98 for element Fe. A review of the effects of moisture on pXRF soil analysis was recently undertaken by [PERSON] et al. [51]. Although the use of pXRF in soils and sediments has been more widespread than in organic amendments, the authors pointed out discrepancies between different studies and indicated that the general influence of humidity is only known between elements that are less or more affected (such as light elements Z \(<\) 26 or elements with detectable X-ray fluorescence energy less than 5 keV). Although the aforementioned studies made on soils can provide us with certain information, we must consider the specific characteristics of organic amendments that differentiate them from those. The presence of water in the measured sample can affect the result in several ways:  
1. Dilution of the sample by the additional weight of water since the results are normally expressed on a dry matter basis;
2. By absorbing part of the primary incident X-rays;
3. By scattering the secondary X-rays. Regarding dilution, it is a linear effect and we must bear in mind that in organic matrices the water content in situ can be much higher than in mineral matrices such as soil. Water absorption is precisely one of the beneficial effects of organic matter for plant growth. Therefore, the linear factors for the conversion of results from a fresh basis to dry basis could be much higher in the case of organic matrices. Organic amendments are also made up of discrete particles.