Vortex-assisted low density solvent and surfactant based dispersive liquid-liquid microextraction for sensitive spectrophotometric determination of cobalt

Abstract

This study describes the development of vortex-assisted low density solvent and surfactant based dispersive liquid-liquid microextraction (VALS-DLLME) for Co(ii) prior to its spectrophotometric detection. The method consisted of the complexation of Co(ii) with pyrocatechol violet (PV) followed by the preconcentration of the Co(II)-PV complex using VALS-DLLME and then an absorption measurement at 600 nm. The optimum conditions for complex formation were a 1 : 3 mole ratio of Co(ii) and PV at pH 7.5, while the conditions for VALS-DLLME were 300 μL 1-dodecanol as extraction solvent, and 300 μL acetonitrile as dispersive solvent under a vortex for 20 s with the addition of cationic surfactant (0.02 mmol L^-1 CTAB). Under the optimum conditions, good linearity was in the range of 0.1-10 mg L^-1, the enrichment factor (EF) was 13.5 and the low limit of detection (LOD) was 0.04 mg L^-1. The method was applied to the analysis of Co(ii) in water, green leaf vegetable and vitamin B₁₂ samples. The proposed method provided good recoveries in the range of 86-104%, which were comparable to those obtained from flame atomic absorption spectrophotometry.

Fig. 1. (a) Absorption spectra of Co(ii)–PV complex at different concentrations of Co(ii) obtained from direct analysis (left) and with VALS-DLLME (right). Conditions of complexation: molar ratio (Co(ii) : PV) of 1 : 3, phosphate buffer pH 7.5. Condition of VALS-DLLME: 300 μL 1-dodecanol, 0.02 mmol L⁻¹ CTAB, 300 μL acetonitrile, 0.2 g Na₂SO₄, vortex at 3200 rpm for 40 s. (b) Effect of the PV concentration. Conditions: 1.00 mg L⁻¹ Co(ii) and phosphate buffer pH 7.5. (c) Effect of pH. Conditions: 1.00 mg L⁻¹ Co(ii) and 0.06 mmol L⁻¹ PV in phosphate buffer.

Fig. 2. (a) Effect of surfactant. Conditions for VALS-DLLME: 300 μL 1-dodecanol, 0.02 mmol L⁻¹ surfactant, vortex at 3200 rpm for 40 s. (b) Effect of CTAB concentration. Conditions: as described in (a) except CTAB concentrations were varied.

Fig. 3. (a) Effect of extraction solvent. Conditions for VALS-DLLME: 300 μL 1-dodecanol, 0.02 mmol L⁻¹ CTAB, vortex at 3200 rpm for 40 s. (b) Effect of extraction solvent volume. Conditions: as described in (a) except volumes of 1-dodecanol were varied.

Fig. 4. (a) Effect of dispersive solvent. Conditions: 300 μL 1-dodecanol as extraction solvent, 300 μL dispersive solvent, 0.02 mmol L⁻¹ CTAB, vortex time at 3200 rpm for 40 s. (b) Effect of dispersive solvent volume. Conditions: as described in (a) except volumes of acetonitrile were varied.

Fig. 5. (a) Effect of salt addition. Conditions: 300 μL 1-dodecanol as extraction solvent, 300 μL acetonitrile as dispersive solvent, 0.02 mmol L⁻¹ CTAB, 0.1 g (1.0% w/v) salt, vortex at 3200 rpm for 40 s. (b) Effect of amount of salt. Conditions: as described in (a) except amounts of Na₂SO₄ were varied.