Moments of lepton spectrum in B decays and the $m_b - m_c$ quark mass difference

It is argued that the quark mass difference $m_b-m_c$ can be extracted with a high accuracy from experimental data on ratia of moments of lepton energy spectrum in semileptonic decays of $B$ mesons. Theoretical expressions for the moments are presented, which include perturbative as well as non-perturbative corrections.