Micelle-based restricted access ion-pair microextraction of phosphate at trace levels in water samples for separation, preconcentration and determination

Measurements were made with UV-Vis spectrophotometer (Hitachi, USA) with micro quartz cuvettes. Sartorius PT-10 pH meter (Germany) with glass electrode was used for pH adjustment of the solutions. To provide the supramolecular solvent, Sonorex brand (Germany) DT-255 model ultrasonic water bath was utilized. Centrifuge (Hettich Rotofix 32 A, Buckinghamshire, England) was used to separate the cloudy solution formed at the microextraction stages. Deionised water had 18.2 MΩ cm resistivity was used during the experimental studies. Milipore MiliQ-Direct 18 model pure water device was used in obtaining deionised water.

Sodium dihydrogen phosphate (NaH₂PO₄) salt obtained from Merck (Darmstadt, Germany) was used in the preparation of the stock solution containing 10^-3 M phosphate anion. This solution was prepared by dissolving the appropriate amount of NaH-₂PO₄ salt in purified water. Decanoic acid was obtained from Sigma-Aldrich. Ion pair formation between ammonium heptamolybdate tetrahydrate and malachite green was carried out by using same method given in literature (19) as follows; 8.6 g of ammonium heptamolybdate tetrahydrate (Merck, Darmstadt, Germany) were taken up in a beaker and dissolved in deionised water (solution-I). Then, 8.6 mL of 98 % H₂SO₄ solution (Merck, Darmstadt, Germany) with a density of 1.83 g·mL^-1, 23 mg malachite green and 2.0 g of tartaric acid were added in the solution-I and last volume was completed to 100 mL with deionised water. The resulting mixture was dissolved in an ultrasonic bath and filtered through blue band filter paper. This solution named as auxiliary reagent was used in experimental studies.

Preparation of the restricted access supramolecular solvent was carried out as follows: 2.5 g of decanoic acid, 0.5 mL of THF and 10 mL of deionised water were placed in a 50 mL centrifuge tube, the resulting mixture was left in an ultrasonic bath for 5 min. In the meantime, nano and micro size cloudy micelles with restricted access properties were obtained. After centrifugation, the micelles-rich restricted access supramolecular solvent phase on the upper side and the water phase on the lower side were obtained. The micelles-rich restricted access supramolecular solvent phase was separated from the aqueous phase by a pipette and taken into another tube.

The developed RA-SUPRASs-LPME method is shown schematically on Figure 1. In this method, 600 μL of 4.5 M H₂SO₄ and 1.0 mL of the auxiliary reagent forming the previously prepared were added to 15 mL of model solution medium (standard solution) containing known concentration of phosphate ions. 5 minutes were allowed in order to allow completing the complex formation. Then, 200 μL of the micelles-rich restricted access supramolecular solvent was added into this solution and the resulting mixture was sonicated 5 min. Formation of a cloudy solution was observed which indicated formation of the nano and micro-sized extraction droplets (17). This solution was centrifuged at 4000 rpm for 10 min. The upper extraction solvent phase was transferred to another centrifuge tube by using a micropipette and the volume was made up to 1.0 mL with ethanol. The final phosphate concentration was measured by UV-Vis spectrophotometer at 625 nm. The same procedures were carried out in blank and standard solutions to obtain calibration curve.

Figure 1

In order to evaluate the applicability of the developed RA-SUPRASs-LPME method to environmental water samples, lake water in biological wastewater treatment plant exit area (Kayseri city, Turkey), natural spring water from village (Kayseri city, Turkey) and mountain-wrapped lake water sample (Kayseri city, Turkey) were collected by glass bottles and stored at 4 °C. Water samples collected were filtered through a PTFE syringe filter (0.45 μm, Osmonics) and subjected to the RA-SUPRASs-LPME method.

Ion-pair complex formation of phosphate ions with ammonium heptamolybdate and malachite green occurs in acidic medium solution. In our method, H₂SO₄ was used to form acidic sample medium. Hence, the influences of the concentration of H₂SO₄ in the range of 0.06-0.24 M on the extraction efficiencies of the presented RA-SUPRASs-LPME system were investigated. The results are depicted on Figure 2. The quantitative recoveries for phosphate ions were obtained in the concentration of H₂SO₄ in the range of 0.18-0.21 M. Therefore, 0.18 M H₂SO₄ was selected as optimal value for the further step of the presented study.

Figure 2

Molybdophosphate complex formation related with the phosphate ion concentration and ion pair forming auxiliary solution concentration. If the concentration of ion pair forming auxiliary solution is lower than the minimum concentration that can form complex with phosphate ion, free phosphate ions will be present in the medium and quantitative extraction efficiency will not be achieved. To develop an accurate analytical method for the analysis of phosphate ions, it is necessary to find the optimum concentration of ion pair forming auxiliary solution. The effects of the volume of ion pair forming auxiliary solution including ammonium heptamolibdate and malachite green which prepared according to literature (19) were investigated in the volume range of 250-1250 μL. The results are given on Figure 3. The quantitative recoveries of phosphate ions were obtained by addition of ion pair forming auxiliary solution in the volume range of 1000-1250 μL. Hence, 1250 μL was selected as an optimal value and was used for further studies.

Figure 3

Since the main component of the micelles-rich restricted access supramolecular solvent phase is decanoic acid, the increase in the amount of decanoic acid leads to an increase in the nano and/or micro-sized micelles and hence the extraction efficiency. The influences of amounts of decanoic acid on the formation of the micelles-rich restricted access supramolecular solvent phase and the extraction efficiencies of phosphate ions in the presented microextraction system were investigated in the range of 50-250 mg. The results are depicted on Figure 4. The quantitative recovery values for phosphate ions were obtained with the addition of decanoic acid in the range of 175-225 mg. For the further studies, 200 mg of decanoic acid was selected as optimal value.

Figure 4

Tetrahydrofuran is one of the most commonly used solvents in the formation of supramolecular solvent systems, since it facilitates the formation of micelles and nanoscale micelles and consequently increases extraction yields (17). Hence we used THF to obtain micelles-rich restricted access supramolecular solvent phase. The effect of THF volume on recovery efficiencies was also investigated. The developed method was applied to the model solutions prepared for this purpose. The results are given on Figure 5. The quantitative recovery efficiency was obtained when 250 μL of THF was used. 250 μL of THF was determined as the optimum working value and the experiments were performed on this value in the following studies.

Figure 5

The application of the method to the highest possible sample volume is an important and critical factor in the separation and preconcentration studies, in enabling the determination of the lowest possible analyte concentration and obtaining a high enrichment factor (20, 21, 22, 23, 24). Therefore, the developed RASUPRASs-LPME method was applied to the model solutions ranging from 7.5-20 mL. Quantitative recovery efficiencies were obtained up to 15 mL sample volume for phosphate ions. Hence 15 mL of the samples studied was subjected to the developed RA-SUPRASs-LPME method.

In order to form nano or micro- sized extraction droplets, RASUPRASs-LPME experiments were carried out an ultrasonic bath. Model solutions prepared were subjected to ultrasonic irritation between 1 and 10 min. Quantitative extraction efficiency was obtained with effective cloudy phases occurring at periods of 5 min and above exposure to ultrasonic vibration. Therefore, for next experimental stages, samples were subjected to ultrasonic irritation for 5 min.

The effects of matrix components of the real samples are an important problem in the instrumental detection of analytes (25, 26, 27, 28, 29, 30, 31, 32). The effect of some alkaline earth, alkali metals and some anions which could have a matrix effect on the RA-SUPRASs-LPME method was also investigated under optimum conditions. These ions were studied because they are the most common ions found in environmental water samples. The results are given in Table 1. It is determined that the matrix ions do not have a disruptive effect on the recovery values of phosphate at the studied levels in Table 1. At the matrix ion concentrations above these levels, the recovery values of phosphate are not quantitative. Hence, real samples for the applicability of the RA-SUPRASs-LPME method were selected by considering these results.