  The spontaneous emission spectrum of a multi-level atom or molecule with
nonreciprocal transition is investigated. It is shown that the nonreciprocal
transition can lead to the elimination of a spectral line in the spontaneous
emission spectrum. As an application, we show that nonreciprocal transition
arises from the phase-related driving fields in chiral molecules with cyclic
three-level transitions, and the elimination of a spectral line induced by
nonreciprocal transition provides us a method to determine the enantiomeric
excess for the chiral molecules without requiring the enantio-pure samples.
