A dual solid and liquid solvent bar microextraction coupled with portable X-ray fluorescence for lead determination...

Trace metals play a crucial role in many biogeochemical processes in water. Due to their low concentrations and the complexity of the matrix, analysing these metals in seawater is a complex task that usually requires a previous preconcentration step. We present a simple two-step method for determining lead concentration in seawater. In the first step, we use a two-phase solvent bar microextraction system (2-SBME), previously prepared with a polypropylene hollow fiber. The extracted metal is then measured directly within the solid fiber using a portable X-ray fluorescence spectrometer. The 2-SBME contains two different extraction reagents: one liquid (Cyanex 272) and one solid (carbon nanotubes functionalized with carboxylic groups (CNT-COOH)). Two different microextraction processes are then carried out simultaneously: a LPME and a SPME, achieving a synergistic effect in the separation process. Efficiency was evaluated in terms of the percentage of lead extracted, with optimum conditions obtained at 0.06 mol L^-1 of Cyanex 272, 10 mg mL^-1 of CNT-COOH, a stirring rate of 50 rpm and an extraction time of 90 min. The influence of X-ray exposure time was also studied. Under optimal conditions, the LOD and LOQ were 0.061 mg L^-1 and 0.203 mg L^-1, respectively. Finally, the greenness of the new method was evaluated using the AGREEprep and SPMS methods. The method was successfully used to determine the lead concentration in fortified real seawater samples from three different locations, thus demonstrating its applicability. Using portable X-ray fluorescence equipment avoids the re-extraction step, and opens the possibility of in situ analysis.