The extraction properties and the selectivity of silica modified with zirconium phosphate (SiZrP) and propylimidazol (SiIm) for cadmium, lead and mercury were evaluated. The optimization of these parameters was carried out by a complete 2³ factorial design (consisting of two levels and three factors), using the intensity of energy dispersive X-ray fluorescence (EDXRF) as response factor. SiZrP adsorbed the three metallic species, while SiIm adsorbed mercury selectively. This difference was assigned to the particular adsorption process involved in each instance. The interaction factor between pH and silica type was significant, suggesting that an unidimensional study could not have optimized the system. Using small amounts of the adsorbent matrices (0.2 g) resulted in better sensitivity, as well as an important saving of chemicals.