Main existing interferences of inorganic species found in raw propolis, such as Zn(II) and Cu(II) were investigated to test the selectivity of BiFE. The cations to be tested were introduced into a solution of 30 μg L−1 Pb(II). Zn(II) yielded a peak at about −1.18 V as shown in Fig. 4. Zn(II) had no effect up to about 300 μg L−1. Then, the signal of Pb(II) decreases smoothly. At a 900 μg L−1 level, a diminution on Pb(II) peak height of about 5% was found (see Fig. 4).