Simultaneous determination of polycyclic aromatic hydrocarbons and phthalate esters in surface water by dispersive liquid-liquid microextraction based on solidification of floating organic drop followed by high performance liquid chromatography

Abstract

Polycyclic aromatic hydrocarbons (PAHs) and phthalate esters (PAEs) are internationally recognized as priority pollutants; hence, it is important to monitor their concentrations in the environment. However, the low concentrations of PAHs and PAEs in surface water make the direct and sensitive determination of these compounds by instrumental methods difficult. Therefore, the development of an accurate and rapid sample pretreating method for the determination of PAHs and PAEs in water has always been the goal of environmental scientists. Dispersive liquid-liquid microextraction based on solidification of floating organic droplet (DLLME-SFO) is a simple, rapid, low-cost, sensitive, and environmentally friendly method. Methods based on DLLME-SFO for the simultaneous determination of PAHs and PAEs in surface water have rarely been reported. In this study, a novel DLLME-SFO method was developed for the simultaneous determination of 16 PAHs and 6 PAEs in surface water samples. To optimize the extraction efficiency for the target compounds, various parameters, including the types and volumes of extractants and dispersants, ionic strength, and extraction time, were investigated. First, 1-undecanol (melting point:19℃) and 1-dodecanol (melting point:24℃) were selected as extractive solvents, and their extraction efficiency was investigated. The results showed that 1-dodecanol had better extraction efficiency. The melting point of 1-undecanol was relatively low, and the droplets that solidified during the experiment were easy to melt and break, which led to the low recovery rate of extraction. Then, the effect of the volume (10, 20, 30, 40, 50 μL) of 1-dodecanol was investigated, and the extraction efficiency of the target compounds was found to decrease with increasing volume of 1-dodecanol. Second, the effect of four dispersive solvents (methanol, ethanol, acetonitrile, and acetone) on the extraction efficiencies was studied. The extraction efficiencies of the target compounds were the highest when methanol was used as the dispersant; hence, the effect of different volumes of methanol on the extraction efficiency was further examined. When the volume of methanol was less than 500 μL, the contact area between the extraction solvent and the water phase increased with increasing methanol volume, and the extraction efficiency increased. However, when the volume of methanol was more than 500 μL, the excessive dispersant increased the solubility of the target compound in the water phase, which led to a decrease in the extraction efficiency. Finally, the effects of salt addition and vortex oscillation time on the extraction efficiency were probed. The experimental results indicated that the extraction efficiency increased with an increase in the quantity of NaCl. When the NaCl quantity was greater than 0.2 g, there was no notable change in the extraction efficiency. Vortex oscillation could accelerate the establishment of the extraction equilibrium, and the extraction efficiency reached a stable state when the vortex oscillation time was more than 2 min. According to the abovementioned results, the optimized DLLME-SFO conditions were established as follows:for 5.0 mL water samples, 10 μL of 1-dodecanol was chosen as the extraction solvent, 500 μL of methanol was used as the dispersive solvent, the vortex oscillation extraction time was 2 min, and the NaCl quantity was 0.2 g. The target compounds were analyzed by high-performance liquid chromatography. Separation of the PAHs and PAEs was achieved on a SUPELCOSILTM LC-PAH column (150 mm×4.6 mm, 5 μm) with acetonitrile-water as the mobile phase using a gradient elution program. Fifteen PAHs were detected using a fluorescence detector, and six PAEs and acenaphthylene were detected by an ultraviolet detector. Quantitative determination was achieved by the external standard method. This method was successfully validated for the analyses of the 16 PAHs and 6 PAEs in two types of water samples (tap water and river water). The average recoveries of the target compounds were 60.2%-113.5%, and the corresponding relative standard deviations (RSDs, n=3) were 1.9%-14.3%. The limits of detection (LODs, S/N=3) ranged from 0.002 μg/L to 0.07 μg/L for the PAHs and from 0.2 μg/L to 2.2 μg/L for the PAEs. The limits of quantification (LOQs, S/N=10) ranged from 0.006 μg/L to 0.23 μg/L for the PAHs and from 0.8 μg/L to 7.4 μg/L for PAEs. The proposed method is simple, fast, low-cost, and environmentally friendly, and it is suitable for the rapid determination of trace PAHs and PAEs in surface water samples.