Determination of lead in environmental waters with dispersive liquid–liquid microextraction prior to atomic fluorescence spectrometry
Introduction
Trace levels of heavy metals are widely distributed in environment due to soil erosion, industrial and agricultural processes [1]. Nowadays the pollution by heavy metals from various environmental sources has absorbed much more attention [2]. Lead, cadmium, nickel, and mercury are heavy metals of unquestionable toxicity [3], [4], [5]. These metals are main sources of contamination for human being found in water and foods. Lead is one of the most hazardous elements to human health, because it can cause adverse effect on metabolic processes of human beings [6], and lead has been proved to be a carcinogenic agent. So many nations and international organizations have set an allowable level for lead. For example, the World Health Organization (WHO) has made a maximum allowable limit of 10 μg L−1 for lead in drinking water [7].
The analytical methods of lead usually involves in using flame atomic absorption spectrometry (FAAS), electrothermal atomic absorption spectrometry (ETAAS), graphite furnace atomic absorption spectrometry (GFAAS), inductively coupled plasma atomic emission spectrometry (ICP-AES), and inductively coupled plasma mass spectrometry (ICP-MS) [8], [9], [10], [11], [12]. These instruments have sufficient sensitivity, however some of them are very expensive, and not available for a common analytical laboratory. Atomic fluorescence spectrometry is a relatively new and sensitive method for elemental analysis, and has been successfully used for the determination of cadmium, zinc, the speciation of mercury and arsenic [13], [14], [15], [16], [17]. Atomic fluorescence spectrometry has also reported for the determination of lead [18], [19]. In this experiment, it is utilized because of its merits such as simplicity, easy to operate, low cost and high sensitivity. Moreover, it is often difficult to determine lead directly at very low concentrations because of the matrix interferences occurring in real samples. Hence a preliminary separation and preconcentration step are often needed.
Up to now, many sample preconcentration methods have been developed for the enrichment of lead in water samples, for example, knotted reactor [20], combination of flow injection and cloud point extraction [21], solid phase extraction [22], [23], cloud point extraction [24], capillary microextraction [25], solid phase microextraction [26], and liquid phase microextraction [27]. Generally, solid phase extraction is the most widely used one because of its merits such as simplicity, rapidness, low cost, low consumption of organic solvents, and high enrichment factor. Solid phase microextraction is also an important technique with many merits, however a derivatization step is needed when it is used for the determination of lead [24]. Recently, liquid phase microextraction has become another popular enrichment method in analytical and environmental fields. Some valuable liquid phase microextraction methods have been developed and applied for the analysis of many analytes. One of them is dispersive liquid–liquid microextraction (DLLME), which has attracted increasing attention for its simplicity, easy to operate, rapidness, and high extraction efficiency. In this extraction procedure, two organic solvents having different characteristics are involved in use, one is hydrophilicity which performs as dispersive solvent such as methanol and acetone [28], [29] and the other is hydrophobicity which performs as extraction solvent such as chlorobenzene and carbon tetrachloride [30], [31]. The extraction solvent would form innumerable infinite small droplets, when it was rapidly injected into the sample solution. These droplets limitlessly enlarge the contact area between extraction solvent and analytes. Until now, this method has been successfully applied for the determination of heavy metals, polychorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), polyphenol-like compounds, polybrominated diphenyl ethers (PBDEs), pesticides and herbicides [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. There are some reports on the application of DLLME for the determination of lead in water samples, but GFAAS and ETAAS are often used as the detection techniques. The extraction solvents were xylene and carbon tetrachloride, and the chelating agents were 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP) and ammonium pyrrolidine dithiocarbamate, and usually methanol and acetone were used as the dispersive solvents [12], [42]. However, it has not found related reports on the combination of atomic fluorescence spectrometry and DLLME.
The goal of present study is to develop a new method for the sensitive determination of lead in water samples utilizing the merits of atomic fluorescence spectrometry and DLLME. Dithizone, an often-used chelating agent, is employed for forming complex with lead. Factors that would influence the efficiency of DLLME extraction and the determination of atomic fluorescence spectrometry were investigated.
Section snippets
Reagents
Stock standard solution of lead with a concentration of 1000 mg L−1 was obtained by dissolving appropriate amount of lead nitrate obtained from Beijing Chemical Engineering technique Co. Ltd. (Beijing, China) in double-distilled water. Working standard solution was obtained by appropriate dilution of the stock standard solution. The solution of dithizone was prepared by dissolving an appropriate amount of dithizone purchased from Shanghai Reagent Factory (Shanghai, China) in ethanol. The
Results and discussion
DLLME is a new mode of liquid phase microextraction, its principle is as the same of conventional liquid–liquid extraction. Compared with a conventional liquid phase microextraction such as a single drop microextraction, DLLME infinitely enlarges the surface area of the extraction solvent and the contact area between extraction solvent and analytes, which significantly enhance recoveries of analytes. In this enrichment procedure, some important parameters that would influence the enrichment
Conclusions
A simple and sensitive method was developed for the pre-concentration and determination of lead in water samples in combination of DLLME and atomic fluorescence spectrometry. The experimental results demonstrated that proposed method had many merits such as excellent enrichment performance, simplicity, sensitivity, easy to operate, cost-effective and low consumption of organic solvents. The results indicated that proposed method had high tolerance to coexisting ions and perfect analytical
Acknowledgements
This work was supported by Natural Science Foundation of China (20877022), the Natural Science Foundation of Henan Province (082102350022), the Personal Innovation Foundation of Universities in Henan Province ([2005]126), and the funds from the Henan Key Laboratory for environmental pollution control.
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