Capillary electrophoresis with dual C4D/UV detection for simultaneously determining major metal cations and whey proteins in milk

Abstract

A reliable and simple CE method with dual C⁴D and UV detection modes for simultaneous determination of major metal cations and whey proteins in milk samples was developed. Sample pretreatment comprised dilution, acidification to pH 4.55 with 10 mM AcOH and centrifugation. The complete separation of metal cations K⁺, Ca²⁺, Na⁺, and Mg²⁺ and whey proteins α-Lac, and β-Lg could be achieved respectively within 10 min and 20 min in a simple BGE composed of 1.0 M AcOH, 12 mM L-His and 2 mM 18-crown-6 with pH 2.74 at a voltage/current of +15 kV/12.5 μA. The samples were injected hydrodynamically by a pressure of 50 mbar for 5 s, the excitation voltage and excitation frequency of the C⁴D detector were 80 V and 1000 kHz, respectively and the detection wavelength of UV detection was set at 200 nm. In cation analysis, the range of the detection limit was 0.05–0.10 mg L⁻¹ for C⁴D detection and 0.10–0.50 mg L⁻¹ for UV detection, respectively, and the relative standard deviations (RSD%, n = 5) of intraday and interday analysis were 0.37–0.55% and 0.46–0.79% for the relative migration time, and 2.51–4.12% and 3.65–4.91% for the peak area, respectively. In whey protein analysis, the detection limits of β-Lg and α-Lac analysis were 5 mg L⁻¹ and 3 mg L⁻¹, respectively and the relative standard deviations (RSD%, n = 5) of intraday and interday analysis were 0.29–0.31% and 0.43–0.48% for the migration time and 2.89–3.25% and 3.29–4.18% for the peak area, respectively. The content of four major metal cations and two whey proteins in various types of milk samples was obtained. The results indicated that the content of metal cations varied little in milk samples of different brands and prices, while the content of whey proteins, as thermosensitive active proteins, varied greatly among different heat-treated milk samples.