Determination of copper in environmental samples by air-assisted liquid-liquid microextraction coupled digital image colorimetric analysis

Metal ions currently represent an important threat to both the environment and human health. In particular Cu(II), a key trace element for many biological mechanisms, is listed as a priority pollutant by the Environmental Protection Agency (EPA) and World Health Organization (WHO) which recommends that its concentration in drinking water not exceed 2 mg L−1 [1]. For this reason, a wide range of methods has been developed for Cu(II) determination ranging from spectrophotometric methods and atomic absorption spectrometry to inductively coupled plasma mass spectrometry [2]. However, several limitations have been associated with these methods including complicated processing, high-cost instruments, and time-consuming operations. Consequently, the detection of metal ions through simple, low-cost methods is highly attractive as they do not involve either the use of costly instruments or complicated operations. In this study, a rapid, simple and cheep method has been developed based on air-assisted liquid-liquid microextraction (AALLME) coupled digital colorimetry for determination of Cu(II) in different samples. AALLME is similar to dispersive liquid-liquid microextraction but in this method there is no need to use a disperser solvent. In this alternative procedure, an extraction solvent (denser or lighter than water) is transferred into the aqueous sample solution in a conical tube, and the mixture is then repeatedly withdrawn into a glass syringe and pushed out into the tube. By this action, fine organic droplets are formed, and the extraction solvent is entirely dispersed in the sample solution. After centrifugation of the formed cloudy solution, the extraction solvent is settled down at the bottom of the centrifuge tube and used for further analysis [3]. For AALLME, 10 ml of a standard solution or sample containing Cu (II) was placed into a 15 ml centrifuge tube. Next, 1 ml of diethyldithiocarbamate solution (as a chelating agent) and 1 ml of borate buffer (pH = 9.2) were added to the tube in order to form a hydrophobic complex. Then, extraction solvent (1,1,2,2- tetrachloroethane) was transferred into the tube and the mixture was repeatedly sucked into a glass syringe and then injected into the tube. After performing predetermined cycles, the mixture was centrifuged to settle the dispersed droplets of organic phase at the bottom of the tube. Finally, the sedimented phase was analyzed by digital colorimeter. The obtained results showed that the proposed method is powerful in analysis of trace amount of Cu (II) in different samples.