Stopped-flow method time range

The measurement of fast interfacial reactions is very difficult, since the reaction has started just after contact of the two phases. The HSS method could measure a reaction several seconds after the contact, and the two-phase stopped-flow method could measure the reaction in the range from a few tenths of a millisecond to several hundred milliseconds. The micro two-phase sheath flow method can measure reactions as fast as < 1 ms [10]. A schematic drawing of the laser-induced fluorescence measurement in the sheath flow system is shown in Fig. 3. An inner organic phase and an outer aqueous phase flow with the same line velocity. The fluorescence at the interface is observed as a function of the distance from the end of the fused-silica capillary. The distance is converted into time. 

There is a range of ways by which one can incorporate ESI in TRMS measurements (Table 4.2). For instance, one can conduct off-line analysis of discrete samples collected from a reaction chamber by ESI-MS. Samples obtained from a reactor can be purified, diluted, or separated. ESI also provides a convenient way to directly transfer liquid-phase dynamic samples into the gas phase while enabling ionization of the target analyte species (e.g., reactants). Some reaction-ESI-MS interfaces took advantage of stopped-flow incubation [78-80]. Here, the reaction is initiated by mixing two solutions (e.g., substrate and catalyst), and it is transferred to a reaction vessel. Subsequently, the resulting reaction mixture is infused to the ESI interface [80]. The time window covered by the early implementations of this approach extended from a few to a few tens of seconds [79]. Therefore, this method may be suitable for studying liquid-phase phenomena at moderate rates. (For an overview of the stopped-flow methods, see also [1].)... 

For decades the electrochemical techniques, i.e., potential, current, or charge step methods such as chronoamperometry, -r chronocoulometry, chrono-potentiometry, coulostatic techniques were considered as fast techniques, and only with other pulse techniques such as temperature jump (T-jump) introduced by Eigen [i] or flash-photolysis method invented by Norrish and Porter [ii], much shorter time ranges became accessible. (For these achievements Eigen, Norrish, and Porter shared the 1964 Nobel Prize.) The advanced versions of flash-photolysis allow to study fast homogeneous reactions, even in the picosecond and femtosecond ranges [hi] (Zewail, A.H., Nobel Prize in Chemistry, 1999). Several other techniques have been elaborated for the study of rapid reactions, e.g., flow techniques (stopped-flow method), ultrasorhc methods, pressure jump, pH-jump, NMR methods. 

In the stopped-flow method, two fluids are forced into a mixing chamber as in the continuous-flow method. After a steady state is attained the flow of solutions into the chamber is suddenly stopped and the concentration of a product or reactant is determined spectrophotometrically as a function of time as the system approaches equilibrium. The mixing chamber is used as a spectrophotometer cell. Figure 11.7 schematically shows a stopped-flow apparatus. Flow systems have been designed that can mix two liquids in a tenth of a millisecond, so that reactions with half-lives ranging from 1 millisecond to 1 second can be studied by either of the two flow methods. Spectrophotometers can be built that record concentrations very quickly, so the response of the spectrophotometer does not limit this method. 

In order to use the stopped-flow technique, the reaction under study must have a convenient absorbance or fluorescence that can be measured spectrophotometri-cally. Another method, called rapid quench or quench-flow, operates for enzymatic systems having no component (reactant or product) that can be spectrally monitored in real time. The quench-flow is a very finely tuned, computer-controlled machine that is designed to mix enzyme and reactants very rapidly to start the enzymatic reaction, and then quench it after a defined time. The time course of the reaction can then be analyzed by electrophoretic methods. The reaction time currently ranges from about 5 ms to several seconds. 

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