Electrospray ionization techniques are used to generate reagents that ionize analytes for mass spectrometric analysis by charge transfer. Such techniques may be performed under ambient conditions. Suitable precursors for such reagents include ionizable nonpolar solvents, such as toluene or xylenes, polar solvents, such as water or alchohols, inert gases, such as helium or nitrogen, or combinations thereof. Environmental conditions in the ionization chamber of the mass spectrograph can be manipulated to generate a selected ion of an analyte in preference to other ions.