Ultrasonic-enhanced preconcentration of trace Pb(II) using hydrophobic, lighter-than-water ionic liquid microextraction combined with solidification of the aqueous solution prior to detection by graphite furnace atomic absorption spectrometry in human fluids

Highlights: • Extraction using hydrophobic, lighter-than-water IL and aqueous phase solidification. • 5-min ultrasound equals 100-min mechanical shaking for enhancing dispersion of ILs. • HLILM-SAP yields convienent and complete collection of IL and effiencient extraction. • High recovery for Pb(II) by solidification from ice-layer compaction in body fluids. • Pyrolysis and atomization temperatures of Pb(II) are 800 and 1800 °C in fluid samples. -- Abstract: Herein, we adopted a hydrophobic, lighter-than-water phosphonium-based ionic liquid (LTWP-IL) for preconcentration of trace Pb(II)-dithizone complexes prior to detection by graphite furnace atomic absorption spectrometry (GFAAS) in human fluids. Because the solvent injected into GFAAS contained LTWP-IL phase, the routine pyrolysis temperature such as 300–400 °C was not appropriate for LTWP-IL, and the optimized pyrolysis and atomization temperatures of Pb(II) were 800 °C and 1800 °C, respectively. The hydrophobic LTWP-IL floats on the surface of the aqueous phase after centrifugation, which allows for convenient and complete collection after solidification of the aqueous phase (HLILM-SAP), and thus overcomes the prominent disadvantage of conventional heavier-than-water ionic liquids-based microextraction. Ultrasound was used to disperse hydrophobic LTWP-IL in the absence of a dispersive solvent. Several operational parameters were optimized using a one-factor-at-a-time approach: LTWP-IL extractant, 30 μL; chelating agent, 0.3% dithizone; ultrasonic time, 5 min; solution pH = 6.0; solidification at −20 °C, 60 min and back extractant, 200 μL of 0.5 mol L⁻¹ HNO₃. Under optimized conditions, limits of detection were as low as 0.014–0.018 μg L⁻¹ and enrichment factors reached >43-fold in urine and blood samples. Inter- and intra-day precisions had relative standard deviations of 3.77–6.79%. Trueness was evaluated by determining four certified reference materials of Pb(II) resulting in recovery values of 95–103%. Overall, the HLILM-SAP-GFAAS method is convenient, efficient and robust, making it suitable for rapid and accurate analysis of trace Pb() in complex biological fluids.