Portable X-ray fluorescence for environmental assessment of soils: Not just a point and shoot method

Portable XRF is a rapid, mobile, high throughput, and potentially cost effective instrumental analytical technique capable of elemental assessment. It is widely used for environmental assessment of soils in a variety of contexts such as agriculture and pollution both in-situ and ex-situ, to varying levels of success. Portable XRF performance for soil analysis is often validated against wet chemistry techniques but a range of factors may give rise to elementally dependent disparities affecting accuracy and precision assessments. These include heterogeneity, analysis times, instrument stability during analyses, protective thin films, incident X-rays, sample thickness, sample width, analyte interferences, detector resolution, power source fluctuations and instrumental drift. Light elements comprising water and organic matter (i.e. carbon, oxygen) also negatively affect measurements due to X-ray scattering and attenuation. The often-overlooked phenomenon of variability in both soil organic matter and water can also affect soil density (e.g. shrink-swell clays) and thus sample critical thickness which in turn affects the effective volume of sample analyzed. Compounding this, for elements having lower characteristic fluorescence energy, effective volumes of analyses are lower and thus measurements may not be representative of the whole sample. Understanding the effects and interplay between determined elemental concentrations and soil organic matter, water, and critical thickness together with the subtlety of theoretical effective volumes of analyses will help analysts mitigate potential problems and assess the applicability, advantages and limitations of PXRF for a given site. We demonstrate that with careful consideration of these factors and a systematic approach to analysis which we summarize and present, PXRF can provide highly accurate results.