Interference-free Determination of Pb in Complicated Matrices by Displacement-Cloud Point Extraction Coupled With Thermospray Flame Furnace Atomic Absorption Spectrometry

In this study, cloud point extraction (CPE) was coupled with micro-sampling thermospray flame furnace atomic absorption spectrometry for trace lead determination. To improve the selectivity of CPE, a displacement strategy was employed based on the stability difference of metal complexes. First, Zn2+ reacts with diethyldithiocarbamate (DDTC) to form Zn-DDTC before it is extracted; then, after removing the aqueous phase, the sample or standard solution containing Pb2+ is added and another CPE procedure is carried out. Because the stability of Pb-DDTC is larger than that of Zn-DDTC, Pb2+ can displace Zn2+ from the pre-extracted Zn-DDTC, and thus the preconcentration and separation of Pb2+ from the complicated sample matrix is achieved. A potential interference from co-existing transition metal ions with lower DDTC complex stability was largely eliminated since they cannot displace Zn2+ from the Zn-DDTC complex. Compared with conventional CPE, the tolerance limit for the co-existing transition metal ions was increased by at least one order of magnitude. Under the optimal chemical and instrumental conditions, the limit of detection was 0.5 mu g/L for lead with a sample volume of 10 mL, and a sensitivity enhancement factor of 31 was achieved. The proposed method was successfully applied to the interference-free determination of trace lead in tea and soil samples with a high content of co-existing heavy metals.