It is known that the lines of exciton (X) and exciton trapped by an ionized donor (D+, X) are often very close which makes very difficult their experimental identification. In order to facilitate their distinction in spherical quantum dots, we investigate the effect of an applied magnetic field studying the binding energy of the complex (D+, X) as function of dot size and the ionized donor position. Our calculation is using a variational approach taking into account the interactions between all charge carriers. Our results show that the complex is more sensitive to the magnetic field than the exciton and that the energy of the exciton is not sufficiently affected when the ionized donor is placed at the edge of the quantum dot. Finally our results are in a fairly good agreement with experimental data and can predict the behaviour law of the diamagnetic effect relating to (D+, X).