text: The stopped-flow method depends on the existence of spectroscopic properties that can be used for following the reaction. When that is not the case quenched flow provides an alternative that uses conventional chemical methods for analysis. Instead of a mechanical stopping system the reaction is stopped by quenching, the products being delivered to a recipient that stops the reaction immediately, either by instantaneous freezing or by denaturing the enzyme with a chemical denaturant or exposing the sample to a denaturing light source. As in the continuous-flow method, the time between mixing and quenching can be varied by varying the length of the tube.
  The pulsed quenched flow method introduced by Alan Fersht and Ross Jakes overcomes the need for a long tube. The reaction is initiated exactly as in a stopped-flow experiment, but there is a third syringe that brings about quenching a definite and preset time after the initiation.
  Quenched flow has both advantages and disadvantages with respect to stopped flow. On the one hand, chemical analysis makes it clear what process is being measured, whereas it may not always be obvious what process a spectroscopic signal represents. On the other hand, quenched flow is much more laborious, as each point along the time course must be determined separately. The image at left for catalysis  by nitrogenase from Klebsiella pneumoniae illustrates both of these points: the agreement in half times indicates that the absorbance at 420 nm measured the release of P, but the quenched-flow experiment required 11 data points.