Optimization of a ligand-based simultaneous extraction protocol for 57 soil elements using response surface methodology: applications in mineral exploration and environmental geochemistry

Ionic geochemistry is a selective extraction approach developed to target weakly adsorbed or exchangeable elements in soils using a mixed-ligand solution. The method enables the simultaneous extraction of 57 elements, subsequently quantified by inductively coupled plasma mass spectrometry. While reliable and accurate, the initial protocol required large sample masses (50 g) and extended leaching times (6 hours), which limited its practicality for large-scale surveys and environmental assessments. Moreover, the high sample mass prevented the possibility of producing multiple replicates, splitting samples for inter-laboratory or cross-method QA/QC testing, and was not suitable in field contexts where soil collection yields insufficient material. In this study, we optimized the method using a Box–Behnken experimental design applied to soil samples enriched with copper as a tracer. Sample characterization was conducted by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy prior to leaching. The optimization significantly reduced the sample mass (to 10 g) and leaching time (to 5 hours) while maintaining high analytical accuracy and precision: R = 0.999; half absolute relative deviation < 10%; half deviation relative between −10 and +10% for all 57 elements before and after optimization. Structural analyses confirmed that the extraction process did not alter the soil matrix despite an 80% reduction in sample mass. The optimized protocol improves the applicability of ionic geochemistry for both geochemical exploration and environmental monitoring by lowering resource requirements, enabling replicates and QA/QC cross-checks, and ensuring reliable detection of ultra-trace elements. It thus provides a more efficient and sustainable framework for assessing mineralization-related anomalies and soil contamination.