A method for mass spectrometry measurement of molecular species in a complex mixture, the method comprising the following steps: a. ionising an analyte in complex mixture to form a precursor ion (A, D) of an ionised species of interest and at least one interfering ion coming from the complex mixture, the precursor ion (A, D) of said ionised species of interest and said at least one interfering ion having a same mass-to-charge ratio mlz, where m represents the mass and z the electric charge number of the precursor ion (A, D) of said ionised species of interest; b. activating the precursor ion (A, D) of said ionised species of interest and of the at least one interfering ion of same mass-to-charge ratio, by interaction with a beam of neutral species, ions, electrons or photons, having a predetermined energy as a function of the energy of ionisation of the precursor ion (A, D) of said ionised species of interest or of the energy required for the detachment of electron from the precursor ion (A, D) of said ionised species of interest, said activation being adapted to produce a product-ion (B, C, E, F), by charge transition with no fragmentation of the precursor ion (A, D) of said ionised species of interest, the product-ion (B, C, E, F) having the same mass m as the precursor ion (A, D) of said ionised species of interest and an electric charge z' such that z' is a non-zero integer number different from z, z' being preferably chosen among z + 1, z+2, z+3, z-1, z-2, z-3, so that the product-ion (B, C, E, F) of said ionised species of interest has a mass-to-charge ratio mlz' different from the mass-to-charge ratio of the product-ion(s) stemming from the activation of the at least one interfering ion; c. separating the product-ion (B, C, E, F) of the ionised species of interest having said predetermined mass-to-charge ratio m/z' specific to the ionised species of interest; d. selectively detecting the product-ion (B, C, F, E) of the ionised species of interest having said predetermined mass-to-charge ratio mlz' specific to the ionised species of interest. a. ionising an analyte in complex mixture to form a precursor ion (A, D) of an ionised species of interest and at least one interfering ion coming from the complex mixture, the precursor ion (A, D) of said ionised species of interest and said at least one interfering ion having a same mass-to-charge ratio mlz, where m represents the mass and z the electric charge number of the precursor ion (A, D) of said ionised species of interest; b. activating the precursor ion (A, D) of said ionised species of interest and of the at least one interfering ion of same mass-to-charge ratio, by interaction with a beam of neutral species, ions, electrons or photons, having a predetermined energy as a function of the energy of ionisation of the precursor ion (A, D) of said ionised species of interest or of the energy required for the detachment of electron from the precursor ion (A, D) of said ionised species of interest, said activation being adapted to produce a product-ion (B, C, E, F), by charge transition with no fragmentation of the precursor ion (A, D) of said ionised species of interest, the product-ion (B, C, E, F) having the same mass m as the precursor ion (A, D) of said ionised species of interest and an electric charge z' such that z' is a non-zero integer number different from z, z' being preferably chosen among z + 1, z+2, z+3, z-1, z-2, z-3, so that the product-ion (B, C, E, F) of said ionised species of interest has a mass-to-charge ratio mlz' different from the mass-to-charge ratio of the product-ion(s) stemming from the activation of the at least one interfering ion; c. separating the product-ion (B, C, E, F) of the ionised species of interest having said predetermined mass-to-charge ratio m/z' specific to the ionised species of interest; d. selectively detecting the product-ion (B, C, F, E) of the ionised species of interest having said predetermined mass-to-charge ratio mlz' specific to the ionised species of interest.