Isotope effect under the influence of a radial electric field is examined in a helical magnetic field configuration. We perform global gyrokinetic simulations with additional poloidal rotations to estimate quasi-linear heat flux due to ion temperature gradient mode under the mixing length model. In single-ion-species plasmas, the mass number dependency of heat flux agree with gyro-Bohm scaling in the absence of a radial electric field. Favorable mass number dependencies violating gyro-Bohm scaling are observed in the presence of a radial electric field or a heavy hydrogen component in multi-ion-species plasmas. The radial electric field and the heavy hydrogen component similarly affect the heat flux through mode stabilization and an increase of wavelength. Poloidal Mach number characterizes the transition from unfavorable to favorable mass number dependency under radial electric fields. For a given Mach number relevant to the neoclassical transport, the quasi-linear heat flux is observed to be independent of mass number. The present results are potentially relevant to the violation of gyro-Bohm scaling observed in the recent deuterium experiments in the Large Helical Device.