Conditions affecting oxide quantification in unknown tropical soils via handheld X-ray fluorescence spectrometer

Soil chemical characterisation has been accurately performed worldwide using portable X-ray fluorescence (pXRF), contributing to fast, low-cost and environmentally-friendly soil analyses. However, many factors can influence the pXRF performance. Thus, this work was carried out to assess the oxides (SiO2, Al-2 O-3, Fe2O3, TiO2 and P2O5) in tropical soils via pXRF, evaluating the effects of soil packing prior to analysis and the soil matrix. The packing of soil samples was performed using X-ray thin-film and common, low-cost plastic bags. Different soil matrices were obtained by grouping of samples according to the Munsell colour system. The obtained results were compared with a conventional method employed for the determination of soil oxides. In addition, the pXRF measurements made on either sieved soil samples or directly on the soil profile were compared. The P2O5 content was below the detection limit of pXRF (0.05 g kg(-1)). The Fe2O3 and TiO2 contents of several tropical soils were accurately determined via pXRF using air-dried and sieved samples. This result was not observed for SiO2 and Al2O3. For Fe2O3, a good correlation (R-2 = 0.91) was obtained between pXRF measurements made on sieved soil samples and directly on the soil profile. The packing of soil samples using plastic bags did not negatively influence the pXRF performance.