The invention generally relates to systems and methods for on-line reaction monitoring. In certain embodiments, the invention provides systems that include a reaction vessel having an outlet, a quantitation unit coupled to the outlet and configured to introduce internal standard and solvent into reaction solution flowed from the reaction vessel, one or more ion generating devices that receive flow from the quantitation unit, and a mass spectrometer. In certain embodiments, the invention provides systems for multiple reaction monitoring that include a plurality of reaction vessels, a plurality of ion generating devices, a plurality of channels, each channel coupling a reaction vessel to an ion generating device, an actuator coupled to the plurality of ion generating devices to thereby allow movement of the plurality of ion generating devices, and a mass spectrometer.