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A stopped-flow instrument is a rapid mixing device used to study the chemical kinetics of fast reactions in solution. In stopped-flow technique, the solutions are forced from syringes into a mixing chamber. After a very short period of flow, perhaps a few ms, the flow is stopped suddenly when the observation cell is filled by an opposing piston that is linked to a sensing switch that triggers the measuring device. Small volumes of solutions are used, and the kinetic equations for modeling the reactions are equivalent to those used in conventional methods in which concentration and time are measured. Stopped-flow is useful for studying fast reactions that have half-lives as short as a few milliseconds.
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After two or more solutions containing the reagents are mixed, they are studied by whatever experimental methods are deemed suitable. Different forms of spectroscopy and scattering of radiation are common methods used. The dead time is the time between the end of mixing the two solutions and the beginning of observation of the kinetics of the reaction. The usual dead time of a stopped flow apparatus is 1-2 milliseconds but some new devices have been developed that have dead times of 0.3-0.6 ms.
A stopped-flow instrument coupled to either a circular dichroism spectrometer or a fluorescence spectrometer is often used in the field of protein folding, to observe rapid unfolding and/or refolding of proteins.
List of stopped-flow techniques
Quenched-flow
In a quenched-flow instrument, the reaction is stopped after a certain amount of time has passed after mixing. The stopping of the reaction is called quenching and it can be achieved by various means, for example by mixing with another solution, which stops the reaction (chemical quenching), quickly lowering the temperature (freeze quenching) or even by exposing the sample to light of a certain wavelength (optical quenching).