Study on simultaneous determination of zinc and gluconate in nutritional supplements by CE-C4D

Nutritional supplements providing bioavailable essential minerals, such as zinc (Zn) and magnesium (Mg), in forms of absorbable salts (e.g., gluconate, stearate, lactate, etc.) have been widely used. However, it should be noted that alongside these supplements, some excipients, including inorganic salts like zinc sulfate and magnesium sulfate, are frequently employed in pharmaceutical and nutraceutical products. Conventional analytical methods for total metal contents could not distinguish actual concentrations of bioavailable forms, suggesting the need for simultaneous analysis of both these essential minerals and their corresponding salts. In this study, we applied the capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C4D) method to simultaneously measure zinc and gluconate in nutritional supplements. The optimal analytical parameters comprise: (1) 10 mM Tris/Ace electrolyte solution at pH 5.0; (2) silica capillary with an internal diameter of 50 μm and an effective length of 30 cm; (3) separation voltage of +15 kV; (4) hydrodynamic injection mode with siphoning technique at 10 cm height and injection time of 20 s. With these conditions, limit of detections of 1.0 mg/L for zinc and 1.5 mg/L for gluconate were achieved. The method has good repeatability (RSD < 3%) and good recovery (89-103% for zinc and 88-107% for gluconate). The validated method was applied to determine zinc and gluconate concentrations in three commercially available nutritional supplement samples. To ascertain the validity of the results, cross-validation was performed using the high-performance liquid chromatography with photodiode array detection (HPLCPDA) for gluconate and the inductively coupled plasma optical emission spectrometry (ICPOES) for zinc, thereby reinforcing the reliability of the CE-C4D method.