Selenomethionine as a catalyst in Ni(2+) reduction: applications to selenomethionine determination in pharmaceuticals

Selenomethionine (Se-Met) is a selenium amino acid which is widely used as a food supplement for humans and livestock. Se-Met itself is not electrochemically active at mercury electrodes. However, we demonstrate here that it can be amenable to electrochemical detection by letting it to act as a catalyst in Ni(2+) reduction reaction in a slightly alkaline solution (0.01 M borax, pH 9). Under these conditions, the catalytic reduction of Ni(2+) at a HMDE occurs according to the general mechanism [1] and results in a DPV peak at -0.75 V vs.Ag|AgCl, KCl(3.4 M).The peak current is proportional to Se-Met concentration within the concentration range from 0.1 to 10 micromol/L, which is suitable for Se-Met determination in pharmaceutical products. This reaction is free of interference from ions like nitrate, chloride, sulfate, and selenite. The last one may also be a component of food supplement and the reaction here described enables one to discriminate between the organic Se form (Se-Met) and the inorganic one (selenite) because the last one is not a catalyst in Ni(2+) reduction. The catalytic reaction involves (Ni2+) coordination by Se-Met. That is why the interference of transition metal ions and complexing compounds was also investigated, with an emphasis on food supplement components. These components may lower the peak current to some extent, but the linear current-concentration relation is still valid under such circumstances and the standard addition method can be employed for Se-Met determination. The interference of surface active compounds (SAC) is more severe and represents a critical feature because Se-Met tablets contain such compounds. In this case, CSDPV is the method of choice because Se-Met co-adsorbs with the SAC and, in this state, induces a catalytic (Ni2+) reduction peak at -0.55 V. Standard addition method can be employed for Se-Met determination in an aqueous extract by this method (5 min deposition at -0.4 V, 0.01 M borax). The results were in a fair agreement with atomic absorption spectrometry data.