A novel separation/preconcentration procedure using in situ sorbent formation microextraction for the determination of cobalt (II) in water and food samples by flame atomic absorption spectrometry

2.1 Instrumentation

A PG-990 (PG instrument Ltd., United Kingdom) atomic absorption spectrometer equipped with deuterium background correction and cobalt hollow cathode lamp was used for the determination of cobalt at a wavelength of 240.7 nm. The instrumental parameters were adjusted according to the manufacturer’s recommendations and the absorbance signal was measured according to the peak height in continuous aspiration mode. A Hettich Centrifuge (Model Universal 320R, Germany) was used for centrifugation. pH values were measured with a Metrohm pH-meter (model: 827) supplied with a glass-combination electrode.

2.2 Reagents and solutions

All reagents used were of analytical reagent grade. Ultrapure distilled water was used throughout the experiment. A 1000.0 mg L⁻¹ stock standard solution of cobalt (II) was prepared from pure Co(NO₃)₂·6H₂O (Merck, Darmstadt, Germany). Before the investigations, working standard solutions were obtained by appropriate stepwise dilution of the stock standard solutions. A 0.010 mol L⁻¹ solution of 2-nitroso-1-naphthol (Sigma–Aldrich, St. Louis, MO, USA) was prepared in pure ethanol. n-Cetyltrimethylammonium bromide (CTAB), tetra-n-butylammonium bromide (TBAB), n-dodecyltrimethylammonium bromide (DTAB), tetradecyldimethylbenzylammonium chloride (TBAC), sodium perchlorate, ethanol and sodium chloride were purchased from Merck. A buffer solution (pH 6.0, 1.0 mol L⁻¹) was prepared by mixing an appropriate amount of acetic acid with sodium acetate solution. Pipettes and glass vessels used for trace analysis were stored in 10% nitric acid for at least 24 h and washed four times with ultrapure distilled water before use.

2.3 Preparation of the real samples

Spinach and black tea samples were purchased at a local supermarket. Spinach sample was washed with ultrapure distilled water, cut and oven-dried at 100 °C for 24 h. Next, it was ground in a household grinder. 500 mg of the sample was placed in a 100 mL beaker, and 10 mL of concentrated HNO₃ (65% w/w) was added to the beaker. The mixture was evaporated to near dryness on a hot plate at about 130 °C for 4 h. After cooling to room temperature, 3 mL of concentrated hydrogen peroxide (30%, w/w) was added. The mixture was again evaporated to near dryness. The resulting solution was diluted to 25 mL with ultrapure distilled water. The result was filtered and the pH of the solution was adjusted at 6.0 by adding NaOH and buffer solution. Finally, the solution was diluted to 100 mL by ultrapure distilled water. The same preparation procedure was used for black tea sample. One blank test was performed for each sample. The samples were analyzed immediately after preparation.

All of the collected water samples (tap, mineral, river, and sea water) were filtered through a cellulose membrane filter (Millipore) of pore size 0.45 μm and after acidification to 1% with concentrated nitric acid, were stored in polyethylene bottles.

2.4 ISSFME procedure

50 mL sample or standard solution containing cobalt (II) in the concentration range of 2–100 μg L⁻¹ and 1.0 × 10⁻⁴ mol L⁻¹ 2-nitroso-1-naphthol was adjusted to pH 6.0 (acetate/acetic acid buffer solution) and was poured in a screw cap conical-bottom glass centrifuge tube. 1.0 mL of CTAB solution (0.05 mol L⁻¹) was added into the sample solution and the tube was manually shaken to ensure complete homogenization of the CTAB in the aqueous sample. Then, 1.0 mL of NaClO₄ solution (1.0 mol L⁻¹) was quickly added by using a syringe. A cloudy solution (dispersed very fine particles of the n-cetyltrimethylammonium perchlorate salt) was formed. In order to accelerate phase separation, the cloudy solution was centrifuged for 4 min at 4000 rpm. As a result, fine particles of the sorbent were sedimented at the bottom of the centrifuge tube. The aqueous phase was then separated completely by a syringe. Afterward, the sorbent was dissolved in 1.0 mL ethanol. The resultant solution was introduced into the flame by conventional aspiration. The extraction steps are illustrated in Fig. 1.

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Figure 1 Photography of different steps in ISSFME: (a) sample solution (water + CTAB), (b) injection of NaClO₄ solution, (c) end of injection, (d) enlarged view of sedimented phase after centrifuging.

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3.1 Selection of the surfactant

In ISSFME, formation of the fine solid particles (sorbent) depends on the interaction between the cationic surfactant and the ion-pairing agent. The alkyl group in the sorbent interacts with the hydrophobic parts in the extracted analyte and acts as an extraction medium. Several cationic surfactants such as n-cetyltrimethylammonium bromide (CTAB), tetra-n-butylammonium bromide (TBAB), n-dodecyltrimethylammonium bromide (DTAB), and tetradecyldimethylbenzylammonium chloride (TBAC) were examined. Experiments were performed using 1.0 × 10⁻³ mol L⁻¹ of surfactants and 0.02 mol L⁻¹ of NaClO₄. In all cases, the sorbent formation occurred and cloudy solutions were observed. Fig. 2 shows the effect of surfactant type on the extraction recovery of cobalt (II). As can be seen, the best extraction recovery was obtained by the use of CTAB. Therefore, CTAB was chosen as the proper surfactant for subsequent experiments.

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Figure 2 Effect of type of surfactant on the extraction recovery of cobalt (II). Conditions: sample volume: 50 mL, NaClO₄ concentration: 2.0 × 10⁻² mol L⁻¹, 2-nitroso-1-naphthol concentration: 1.0 × 10⁻⁴ mol L⁻¹, pH: 6.0, surfactants concentration: 1.0 × 10⁻³ mol L⁻¹, cobalt (II) concentration: 50 μg L⁻¹.

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3.2 The effect of pH

Separation of metal ions by ISSFME involves prior complex formation with sufficient hydrophobicity to be extracted by the fine particles of the sorbent. The pH of the aqueous phase is one of the most important factors in the extraction of metal ions from various media for the formation of metal complex. The influences of the pH on the quantitative recovery values of cobalt (II) at the presented microextraction system were investigated at the pH range of 1.0–12.0 using HCl and NaOH while other parameters were kept constant. The results are depicted in Fig. 3. Recovery of cobalt (II) increases with the increase of solution pH and is effectively recovered within the pH range of 5.0–10.0. Competition between protons and metal species could thus explain the weak extraction recovery in acid medium. On further increase of the pH, extraction recovery decreases probably due to the formation of hydroxide of cobalt. Therefore, later works were performed at pH 6.0 and in order to control the pH during the analytical procedure, it was adjusted to 6.0 with acetate/acetic acid buffer solution.

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Figure 3 Effect of pH on the extraction recovery of cobalt (II). Conditions: sample volume: 50 mL, NaClO₄ concentration: 2.0 × 10⁻² mol L⁻¹, 2-nitroso-1-naphthol concentration: 1.0 × 10⁻⁴ mol L⁻¹, CTAB concentration: 1.0 × 10⁻³ mol L⁻¹, cobalt (II) concentration: 50 μg L⁻¹.

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3.3 The effect of chelating reagent concentration

In this work, 2-nitroso-1-naphthol was used as the chelating agent due to the highly hydrophobic nature of its metal chelates. Concentration of the chelating agent is a critical variable to be optimized in preconcentration methods. Thus, it is highly important to establish the minimal reagent concentration that leads to total complex formation while achieving the highest extraction. In the light of these points, the effects of the amounts of 2-nitroso-1-naphthol on the quantitative recovery of cobalt (II) in ISSFME system were examined within the range of 1.0 × 10⁻⁵–3.0 × 10⁻⁴ mol L⁻¹. As is shown in Fig. 4, by the addition of an increased amount of 2-nitroso-1-naphthol, the extraction recovery was increased before 5.0 × 10⁻⁵ mol L⁻¹, and then remained constant. At a low concentration of 2-nitroso-1-naphthol, the complexation was not complete and the extraction efficiency was low; hence, the recovery was decreased. Thus, 1.0 × 10⁻⁴ mol L⁻¹ of 2-nitroso-1-naphthol was chosen as the optimum for the elimination of probable interference effects of some coexisting ions in real sample analysis.

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Figure 4 Effect of chelating reagent concentration on the extraction recovery of cobalt (II). Conditions: sample volume: 50 mL, NaClO₄ concentration: 2.0 × 10⁻² mol L⁻¹, pH: 6.0, CTAB concentration: 1.0 × 10⁻³ mol L⁻¹, cobalt (II) concentration: 50 μg L⁻¹.

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3.4 The effect of CTAB concentration

The amount of sorbent has influence on the extraction efficiency of SPE methods. In ISSFME, the amount of sorbent that forms into the sample solution depends on the surfactant concentration. In order to investigate the effect of CTAB concentration, several experiments were carried out using different CTAB concentrations. In all experiments, the concentration of NaClO₄ was 0.02 mol L⁻¹. The results showed that the extraction was quantitative when the CTAB concentration was higher than 3.0 × 10⁻⁴ mol L⁻¹. At lower concentrations, the extraction recovery was low probably because of the inadequacy of the sorbent to adsorb the hydrophobic complex quantitatively. Therefore, a concentration of 1.0 × 10⁻³ mol L⁻¹ CTAB was chosen for the subsequent experiments.

3.5 The effect of NaClO₄ concentration

The effect of NaClO₄ as an ion-pairing agent was investigated within the range of 1.0 × 10⁻³-5.0 × 10⁻² mol L⁻¹ in the presence of 1.0 × 10⁻³ mol L⁻¹ of CTAB. The results indicated that the extraction recovery increased by increasing the NaClO₄ concentration up to 1.5 × 10⁻² mol L⁻¹ and then remained constant. These results could be explained by increasing the amount of NaClO₄, which led to the formation of more solid sorbent and, therefore, an increase in extraction recovery. So, 2.0 × 10⁻² mol L⁻¹ of NaClO₄ was chosen for the subsequent experiments.

3.6 The effect of ionic strength

For investigating the influence of ionic strength on the performance of ISSFME, various experiments were performed by adding different amounts of NaNO₃ (0.0–2.0 mol L⁻¹). Other experimental conditions were kept constant. The results confirmed that the salt addition had no significant effect on the extraction process and the recovery of cobalt (II) was quantitative. The negligible influence of the salt can be ascribed to the rapidity of the cobalt (II) chelate distribution between the involved phases as the likely result of the dispersion process. Thus, no ionic strength buffer was used for subsequent experiments. These observations suggest the possibility of the application of the method for the separation of cobalt (II) from saline solutions such as seawater.

3.7 The effect of extraction time

In our study, extraction time is defined as the time interval between the injection of the ion-pairing agent (NaClO₄) and the start of centrifugation. Similar results were observed with extraction times between 1 and 30 min. No significant effect was observed on the extraction recovery when the extraction time was increased. This may be due to the fact that the large contact surface area between the fine particles of the sorbent and the aqueous phase results in a very rapid adsorption of the analyte on the surface of the dispersed solid sorbent. Thus, 1 min (a short extraction time) was selected in subsequent experiments.

3.8 The effect of coexisting ions

The effects of common coexisting ions in natural water samples on the recovery of cobalt (II) were also studied. In these experiments, 50 mL of solutions containing 50.0 μg L⁻¹ of cobalt (II) and various amounts of interfering ions were treated according to the recommended procedure. Given spices were considered to interfere if it resulted in a ±5% variation of the AAS signal. The results showed that, in excess of 100,000-fold of Li⁺, K⁺, Na⁺, F⁻, Cl⁻, Br⁻, I⁻, SO₄²⁻, 10,000-fold of Ca²⁺, Mg²⁺, Ba²⁺, Sr²⁺, PO₄^3-, 1000-fold of Mn²⁺, Zn²⁺, Cd²⁺, 500-fold of Cr³⁺, Ag⁺, Pb²⁺, Al³⁺, 100-fold of Ni²⁺, Cu²⁺ and 50-fold of Fe²⁺, Fe³⁺ ions had no significant interferences in the extraction and determination of cobalt (II). According to the results, the major ions in the real samples have no obvious influence on cobalt (II) ISSFME under the selected conditions.

3.9 Analytical figures of merit

Under the optimum conditions, the ISSFME procedure was applied for the extraction and preconcentration of various standard solutions of cobalt (II) and the calibration graph was linear within the range of 2–100 μg L⁻¹. The LOD, defined as 3S_b/m (where S_b and m are the standard deviation of the blank and the slope of the calibration graph, respectively), was found to be 0.8 μg L⁻¹. The preconcentration factor that was calculated by dividing the aqueous phase volume to the final volume of the diluted phase was 50. Repeatability was carried out by spiking blank samples at the concentration of 50 μg L⁻¹, and the relative standard deviation (RSD) for ten replicate experiments was 2.3%.

3.10 Analysis of real samples

Food samples (spinach and black tea) and real water samples (tap water, mineral water, river water, and sea water) were subjected to the proposed method to evaluate the concentration of cobalt (II). The obtained results are given in Tables 1 and 2. Recovery studies were also carried out after it was spiked to samples of known concentrations of cobalt (II) at levels of 20 and 50 μg L⁻¹. The recovery values calculated for the added standards ranged from 97% to 102%, thus confirming the accuracy of the procedure and its independence from the matrix effects. These results confirm the validity of the proposed preconcentration method.

Furthermore, the accuracy of the developed method was evaluated by applying the recovery experiments for the determination of cobalt (II) in water SRMs. The following standard reference materials were used to verify the accuracy of the proposed method: NIST SRM 1643e (National Institute of Standard and Technology, Trace elements in water) and SRM 1640a (Trace elements in natural water). The results listed in Table 3 reveal that there is a good agreement between the obtained results and certified values and indicate that the proposed procedure was helpful for the determination of cobalt (II) in real samples with complicated matrices.

3.11 Comparing ISSFME with other methods

A comparison of the presented method with other reported preconcentration methods for cobalt (II) determination in water samples is given in Table 4. The presented method has low LOD, high-enrichment factor, good RSD and these characteristics are comparable or even better than most of the other methods in Table 4. All these results indicate that ISSFME is a reproducible, simple, and low cost technique that can be used for the preconcentration of metal ions like cobalt from water samples.