Stopped-flow spectrometry allows chemical kinetics of fast reactions (with half times of the order of milliseconds) to be studied in solution. It was first used primarily to study  enzyme-catalyzed reactions. Then the stopped-flow rapidly found its place in almost all biochemistry, biophysics, and chemistry laboratories with a need to follow chemical reactions in the millisecond time scale. 
In its simplest form, a stopped-flow mixes two solutions. Small volumes of solutions are rapidly and continuously driven into a high-efficiency mixer. This mixing process then initiates an extremely fast reaction. The newly mixed solution travels to the observation cell and pushes out the contents of the cell (the solution remaining from the previous experiment or from necessary washing steps). The time required for this solution to pass from the mixing point to the observation point is known as dead time. The minimum injection volume will depend on the volume of the mixing cell. Once enough solution has been injected to completely remove the previous solution, the instrument reaches a stationary state and the flow can be stopped. Depending on the syringe drive technology, the flow stop is achieved by using a stop valve called the hard-stop or by using a stop syringe.  The stopped-flow also sends a ‘start signal’ to the detector called the trigger so the reaction can be observed. The timing of the trigger is usually software controlled so the user can trigger at the same time the flow stops or a few milliseconds before the stop to check the stationary state has been reached.