In this work was studied the effect of the spectral interference produced by modifier NH4H2PO4 on Tl, Pb and Cd when it is used in the presence of a magnetic field. It was found that this modifier produces a spectral interference on the Tl to 276,8 nm, due to the splitting of the PO molecule that produces the magnetic field applied by the background correction system, which adversely affects its mass characteristic and limit of detection. Mg(NO3)2 increases this interference with the Tl, but it improves its limit of detection. The use of EC-THGA atomizer reduces the interference of phosphate on the three analytes.