Deep eutectic solvent extraction as a route to a ready-to-use natural reagent for automated in-syringe aluminium analysis

Plant extracts provide renewable complexing ligands, but their analytical use is constrained by variable composition and poor compatibility with automated systems. This study investigated whether deep eutectic solvent (DES) extraction can function as a reagent-generation step, yielding a stable extract for direct automated analysis without ligand isolation or solvent exchange.

A hydrated choline chloride:propylene glycol DES (1:3, mol/mol; 30 wt% water) maximized both recovery of Rubia tinctorum anthraquinones and the Al(III)-dependent absorbance. HPLC-UV and HPLC-MS supported the tentative identification of lucidin primeveroside and ruberythric acid, while molecular dynamics simulations indicated stabilization by propylene glycol and close-range Al(III) association with oxygen donor atoms. The extract was integrated into an automated in-syringe procedure using 100 μL of extract and measurement at 550 nm in a 50 mm cell. The response was linear at 0.10-0.80 mg L^-1 (R² = 0.997); detection and quantification limits were 0.03 and 0.10 mg L^-1. Repeatability and intermediate precision were ≤4% and ≤5%, while batch-to-batch error and 30-day response variation remained within ±5%. Results for cocoa products and powdered milk agreed with ICP-OES; certified reference material bias was 3.7%.

DES extraction becomes an analytical reagent-production step directly compatible with automation. The design links ligand recovery, molecular characterization, storage, and low-volume measurement, providing a transferable strategy for automated assays based on multicomponent renewable reagents.