A mass spectroscopic reaction-monitoring method including: forcing charge-laden liquid drops to move along a traveling path; exposing to a laser beam a region to be formed of a liquid sample surface, the laser beam having an irradiation energy sufficient to cause analytes present behind the liquid sample surface to be desorbed to fly along a flying path; introducing to the region at successive points of time a liquid sample containing one reactant that undergoes an ongoing chemical reaction as a first analyte to form one product as a second analyte; and positioning the liquid sample surface relative to the laser beam at each point of time such that the flying path intersects the traveling path for enabling occlusion of at least one of the first and second analytes in at least one charge-laden liquid drop to thereby form at least a corresponding one of first and second ionized analytes.