Stopped flow

A stopped flow instrument is a rapid mixing device used to study the chemical kinetics of a reaction in solution. 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 it takes to mix the two solutions and is thus the shortest time point, where it is possible to measure kinetics using this method. The usual dead time of a stopped flow apparatus is a half of a millisecond but some new devices are being developed that have dead times of 40 microseconds.

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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 refolding of proteins.

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).