Oregano authenticity by multi-element ICP-OES profiling: detection of olive leaf adulteration supported by complementary fingerprinting

Oregano (Origanum vulgare) is a high-value culinary herb that is vulnerable to economically motivated adulteration, particularly after drying and grinding when botanical identification becomes challenging. This study investigated oregano authenticity, and the quantitative detection of its adulteration with olive leaf (Olea europaea), using multi-element elemental profiling supported by complementary chemical fingerprinting. Elemental data obtained by ICP-OES (Ca, K, Fe, Mn, Mg, Na, and Zn) were first characterised across authentic oregano and olive leaf lots, followed by the preparation of gravimetric adulteration series (1-20%, w/w). Multivariate calibration models were developed to relate elemental profiles to adulteration level; cross-validation indicated an optimal model complexity of LV = 2 (R2(CV) = 0.939; RMSECV = 1.666%). Variable importance analysis identified Zn, Ca, Fe, Mn, and Mg as the most influential predictors (VIP > 1). Model robustness was subsequently challenged using three composite commercial oregano mixtures, prepared from nine independently purchased retail products and spiked at 2, 7, 11, and 16% (w/w). The external set yielded an overall R2pred of 0.67 and RMSEP of 2.96%, indicating that elemental profiling can provide approximate prediction of olive leaf addition in commercial oregano matrices within the studied sampling domain. Complementary chemical fingerprinting further supported matrix differentiation: ATR-FTIR provided qualitative spectral discrimination between oregano and olive leaf, while LC-MS/MS phenolic profiling and GC-MS volatile/essential-oil characterisation supplied orthogonal compositional evidence. Overall, the findings support the use of elemental profiling, complemented by chemical fingerprinting, as a promising analytical framework for oregano authenticity assessment and olive leaf adulteration screening.