Switchable hydrophilicity solvent based and solidification-assisted liquid-phase microextraction combined with GFAAS...

A novel microextraction method was developed by combining CO₂-controlled switchable hydrophilicity solvent (SHS) with solidification of the aqueous phase (SAP), referred to as CSHS-SAP. It was applied for pre-concentration/extraction of the complexes of Pb-ammonium pyrrolidine dithiocarbamate (APDC) prior to GFAAS detection in raw bovine milk and milk products (whole-fat, half-skimmed and skimmed milks). In the CSHS-SAP microextraction, a clear interface was easily formed, and convenient and complete collection was achieved by directly transferring the SHS phase into a vial, which overcame the shortcomings of blurred interface and difficult collection of SHS phase using CSHS-based microextraction. SAP led to the increase of extraction recoveries for Pb²⁺ by 8-11%. Some important factors were optimised using a one-factor-at-a-time approach: 800 µL of N,N-dimethylbutylamine and water at a ratio of 1:1 as SHS, 1.0 g L^-1 APDC as chelating agent, sample pH = 6.0, 0.6 mL of 1.0 mol L^-1 NaOH, solidification time of 70 min and 300 µL of 0.5 mol L^-1 HNO₃ for back extraction. Under optimised conditions, limits of detection were as low as 0.01-0.02 µg kg^-1 and enrichment factors reached more than 23-fold. Inter- and intra-day precisions had relative standard deviations of 3.6-6.4%. With the CSHS-SAP/GFAAS method Pb²⁺ was detected at 0.5-1.8 µg kg^-1 in four bovine raw milk samples, which were collected from Hubei and Henan Provincial oil-producing area, China. However, Pb²⁺ was below the LOD in all of the local milk products. Overall, the newly developed CSHS-SAP/GFAAS method is convenient, efficient and robust, giving it great potential for rapid and accurate analysis of trace Pb²⁺ in liquid milks.