Application of a Portable X-Ray Fluorescence Analyzer for Unconsolidated Sediments at Archaeological Sites: Capabilities and Limitations

Portable X-ray fluorescence (pXRF) is increasingly used in geoarchaeology because it enables rapid elemental measurements directly in the field or with minimal sample preparation. However, quantitative interpretation of pXRF data from unconsolidated sediments is complicated by matrix effects, moisture, grain-size heterogeneity, spectral interference, and the weak detection of light elements. We investigated these limitations using sediment samples from the Orkhon River Valley and Tsagaan Agui Cave in Mongolia. Samples were analyzed by an Olympus Vanta M-series pXRF analyzer and by an ARL 9900 XP laboratory wavelength-dispersive XRF spectrometer after fusion-bead preparation. The laboratory results were used as a reference dataset, and empirical regression equations were established for selected elements. The agreement between the two methods was element- and site-dependent. In the Orkhon River Valley, the strongest relationships were obtained for Fe and Ca, whereas Ti showed the weakest relationship. In the Tsagaan Agui Cave samples, Ca, Fe, and Mn showed the highest explanatory power, while Al showed the weakest relationship. These results indicate that pXRF data can rapidly reproduce major stratigraphic compositional trends under field-oriented conditions, but universal conversion equations should not be applied to different sedimentary or archaeological settings. In particular, Mg, Al, and Si may be underestimated in moist or heterogeneous sediments, and Na cannot generally be quantified reliably under standard air-path conditions. We recommend using pXRF primarily as a rapid field-screening tool for identifying compositional anomalies and stratigraphic intervals that require closer inspection, targeted sampling, or subsequent laboratory analysis. Where quantitative interpretation is required, pXRF data should be supported by matrix-matched calibration and laboratory validation.