Metal-Phase Microextraction (MPME) as a Novel Solvent-Free and Green Sample Preparation Technique: Determination of Cadmium in Infant Formula and Real Water Samples

In this paper, metal-phase microextraction (MPME) was developed as a new solvent-free microextraction method based on iron. This technique is much safer in comparison with the liquid-phase and solid-phase microextraction procedures due to the elimination of organic solvents. In this method, NaBH₄ is added to the sample solution containing Fe(II) ions. Therefore, metal ion analyte and Fe(II) ions are converted to zero-valent metals. During the aggregation of the zero-valent iron atoms, the analyte as the zero-valent form is trapped into the iron phase. Stirring during the procedure accelerates the aggregation of iron particles, leading to the formation of large particles, which can be easily separated using an applied external magnetic field. The extraction phase containing the analyte was dissolved in 200 µL of hydrochloric acid 6 N. In order to evaluate the performance of MPME, cadmium ion was selected as a test analyte and determined in real water samples and infant formula using a flame atomic absorption spectrometer. All affecting parameters, including pH, potassium hydrogen phthalate (KHP) concentration, Fe(II) and NaBH₄ concentration, reaction time, and temperature, were investigated. Under the optimum conditions (pH 4, KHP 1 g L⁻¹, NaBH₄ 0.6 g L⁻¹, Fe²⁺ 12.5 mg L⁻¹, reaction time 4 min, and temperature 55 °C), the limit of detection (LOD) and the relative standard deviation (RSD) were 0.10 ng mL⁻¹ and 2.3%, respectively. The interfering effect of HCO₃⁻ and NO₃⁻ was eliminated by adding larger amounts of KHP and NaBH₄, respectively. Relative recoveries of cadmium from the mineral, tap, and well water were 108.1, 102.2, and 115.3%. The infant food analysis according to ISO17025:2005 and MPME showed a cadmium concentration of 36 ± 6 ng g⁻¹ and 40 ± 7 ng g⁻¹, respectively, indicating that the two methods are not significantly different at the significance level of 0.05.