Tropical soils characterization at low cost and time using portable X-ray fluorescence spectrometer (pXRF): Effects of different sample preparation methods

ABSTRACT Portable X-ray fluorescence spectrometer (pXRF) has been recently adopted by the Soil Science community for uses in both field and laboratory, obtaining the total content of several chemical elements in a few seconds. Sulfuric acid digestion is an expensive and time-consuming laboratory analysis that provides contents of Fe2O3, Al2O3, SiO2, TiO2 and P2O5, important for soil studies. Due to few pXRF studies in tropical soils, this work aimed to compare contents of Fe2O3, Al2O3, SiO2, TiO2 and P2O5 obtained by pXRF with sulfuric acid digestion results, and to evaluate the effects of varying forms of preparing soil samples and scanning with pXRF on the resulting values in Brazilian soils. Soils were scanned in five conditions in-field (in situ) and in laboratory, evaluating varying sample preparation methods, particle sizes and soil moisture. Four pXRF scanning operational modes were tested. Linear regressions were adjusted between results of pXRF and sulfuric acid digestion. Equations were validated with an independent set of samples. Statistical analyses compared the methods of preparing the samples. Adequate linear models reached R2 of 0.99 and 0.89 for Fe2O3 and TiO2, respectively. Validation promoted R2 greater than 0.97 and RMSE and ME close to zero for both oxides. Statistical differences of pXRF results were found among the methods of preparing samples. pXRF spectrometer has great potential to obtain Fe2O3 and TiO2 content rapidly and economically with high correspondence with laboratory results of sulfuric acid digestion analysis. Varying methods of preparing the samples promote differences in the results of pXRF.

便携式X射线荧光光谱仪（portable X-ray fluorescence spectrometer，pXRF）近年来被土壤科学领域广泛应用于野外与实验室场景，可在数秒内获取多种化学元素的总含量。硫酸消解是一种成本高昂且耗时的实验室分析方法，可测定Fe₂O₃、Al₂O₃、SiO₂、TiO₂及P₂O₅的含量，上述组分对于土壤研究具有重要意义。鉴于当前热带土壤相关pXRF研究较为匮乏，本研究旨在对比pXRF与硫酸消解法测定的Fe₂O₃、Al₂O₃、SiO₂、TiO₂及P₂O₅含量，并评估巴西土壤中不同土壤样品制备方式与pXRF扫描模式对最终测定结果的影响。本研究针对5种条件下的土壤样品进行扫描测试，涵盖原位（in situ）野外与实验室两种场景，同时考察了不同样品制备方法、粒径规格与土壤湿度对测定结果的影响，共测试了4种pXRF扫描操作模式。以pXRF与硫酸消解的测定结果为基础构建线性回归模型，并通过独立样本集对回归方程进行验证；采用统计分析方法对比不同样品制备方法间的差异。研究结果显示，针对Fe₂O₃与TiO₂，所构建的适配线性模型的决定系数（coefficient of determination，R²）分别可达0.99与0.89；验证集分析结果表明，两种氧化物的决定系数均大于0.97，均方根误差（root mean square error，RMSE）与平均误差（mean error，ME）均接近零。统计分析证实，不同样品制备方法对pXRF测定结果存在显著影响。本研究表明，便携式X射线荧光光谱仪具备快速、经济地测定Fe₂O₃与TiO₂含量的巨大潜力，其测定结果与硫酸消解实验室分析结果具有高度一致性；同时，不同的样品制备方法会对pXRF的测定结果产生显著影响。