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 ICP-MS. 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, HARD < 10%, HDR 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.