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.