In this work, a method employing coprecipitation procedure with magnesium hydroxide as a carrier for separation/preconcentration of arsenic(V), cadmium(II), chromium(III), cobalt(II), copper(II), manganese(VII), mercury(II), nickel(II), and vanadium(V) was developed. The method was applied in fish and shellfish samples, and the elements were determined by microwave-induced plasma optical emission spectrometry (MIP OES). Complete two-level factorial design for four variables and a mixture design were employed to evaluate and optimize the factors involved in this system. Under optimized conditions, the accuracy of the procedure was assessed using certified reference material, obtaining recoveries of 92 to 105% for the elements studied. The proposed method presented the detection limits in the range of 0.01–0.33 μg g⁻¹, and enrichment factors of 5; 17; 6; 25; 24; 25; 25; 25; and 13 for the analytes As⁵⁺, Cd²⁺, Cr³⁺, Co²⁺, Cu²⁺, Mn⁷⁺, Hg²⁺, Ni²⁺, and V⁵⁺, respectively. The applied chemometric tools were effective to optimize the important variables involved in the developed coprecipitation route. The proposed method presented simplicity, low cost, high sensitivity, allowing the determination of the analytes by MIP OES.