StoppedflowTechnik

StoppedflowTechnik, or the stopped-flow technique, is a rapid-mixing method used to study fast chemical and biochemical reactions. In this approach, reactants are quickly combined and the flow is stopped within a dedicated observation chamber, allowing time-resolved measurements of the reaction progress.

In a typical setup, solutions are loaded into fast-acting syringes or pumps and injected into a mixing zone with very small dead volume. The resulting mixture enters an observation cell where the flow is halted. The reaction is then monitored spectroscopically—commonly by UV-visible absorbance or fluorescence, but other detectors such as Raman or light scattering may be used—while time is recorded from milliseconds to seconds, depending on hardware.

Applications include pre-steady-state kinetics of enzymes, protein folding, ligand binding, electron transfer, and fast chemical reactions. It provides access to early reaction steps that are not detectable by conventional steady-state methods. Detection relies on optically active changes, requiring chromophores or labels in the reacting species.

Advantages include high time resolution, efficient use of reagents due to small volumes, and direct observation of early kinetic events. Limitations include a finite mixing dead time setting the lower limit of measurable times, potential issues with complete mixing in very fast reactions, and the need for suitable optical signals or labels. The equipment can be complex and costly, and analysis often requires kinetic modeling.

History and variants: The technique emerged in the 1960s–1970s as a tool for physical chemistry and biophysics to study rapid reactions. Variants and related approaches, such as quench-flow or rapid-mixing stopped-flow, extend capabilities to different time scales or detection schemes.