  In the context of quantum field theory (QFT), we compute the amplitudes of
weak interaction processes such as $ W^{+} \rightarrow e^{+} + \nu_{e}$ and $
W^{+} \rightarrow e^{+} + \nu_{\mu}$ by using different representations of
flavor states for mixed neutrinos. Analyzing the short time limit of the above
amplitudes, we find that the neutrino states defined in QFT as eigenstates of
the flavor charges lead to results consistent with lepton charge conservation.
On the contrary, the Pontecorvo flavor states produce a violation of lepton
charge in the vertex, which is in contrast with what expected at tree level in
the Standard Model.
