29.07.2021 •

Impact of Sample Preparation Methods for Characterizing the Geochemistry of Soils and Sediments by Portable X‐Ray Fluorescence

We examined the impact of three different sample preparation methods on bulk soil geochemistry data obtained from a portable X‐ray fluorescence (pXRF) spectrometer. We generated data from a soil core recovered from the surface, downward into unaltered loess, and into a buried soil at a site in eastern Iowa. Samples were scanned (i) directly from field‐moist soil cores; (ii) after drying, grinding, and being loosely massed in plastic cups; and (iii) as pressed powder pellets. Data derived using these methods were compared with data obtained from a standard benchtop X‐ray fluorescence (XRF) unit. Generally, the results indicated that data from pressed powder pellets provide the best correlation to benchtop XRF data, although the results were sometimes element- or compound-specific. CaO, Fe2O3, and K2O generally provided the strongest correlations between pXRF and XRF data; SiO2 data were more problematic. Field‐moist pXRF scans generally underestimated element concentrations, but the correlations between pXRF and benchtop XRF measurements were greatly improved after applying pXRF‐derived calibration standards. In summary, although element/compound data provided by pXRF showed significant relationships to benchtop XRF data, the results are improved with proper sample preparation and usually by calibrating the pXRF data against known standards.

Introduction

X‐ray fluorescence (XRF) spectroscopy is an analytical technique used to determine the elemental composition of a sample using high‐energy X‐rays. When bombarded with X‐rays, different elements can be identified by the characteristic fluorescent energy they emit (X‐ray fluorescence). Thus, X‐ray fluorescence offers a rapid and cost‐efficient way to generate multielement analytical data.

Researchers are increasingly using portable X‐ray fluorescence (pXRF) instruments in the field and laboratory [1]. Many studies have demonstrated that pXRF measurements correlate well with data obtained using conventional methods, such as benchtop XRF [2]. Portable XRF instruments can generate robust, accurate, and repeatable data and are applicable to various environmental applications [3]. 

As with any new method, researchers actively attempt to determine its overall accuracy and main error sources. Unfortunately, no universally agreed‐upon protocol for pXRF sample preparation exists, specifically for analyses of soils or finely ground geological samples. Nonetheless, the Soil Survey Staff [4] has observed that the results from soil analyses are more reproducible if the sample has been air dried, homogenized, and finely ground (<75 µm). By comparison, the Soil Science Society of America method for pXRF analysis of soils advocates drying and grinding to pass a 2 mm sieve [5]. The present study addresses this issue by evaluating the effects of different sample preparation techniques on pXRF data.


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