Stopped flow method principle

From the beginning of the 1980s, some effective experimental approaches based on new principles have been invented for the study of interfacial reactions in solvent extraction chemistry. Recently, some methods were developed from our laboratory, the highspeed stirring (HSS) method [4,5], the two-phase stopped flow method [6], the capillary plate method [7], the reflection spectrometry [8], and the centrifugal liquid membrane (CLM) method [9]. 

The stopped-flow method was introduced by Roughton in 19344 and greatly improved by B. Chance some six years later.5 The principle is illustrated in Figure 4.3.6 In contrast to the setup in the continuous-flow system, the two driving syringes are compressed to express about 50 to 200 pL from each, and then they are mechanically stopped. Suppose that there is an observation point 1 cm from... 

Flow injection analysis (F1A). In this technique, introduced by Ruzicka and Hansen, a small amount of sample is injected into a liquid flow (see Fig. 5.16), which apart from being automated is normally continuous, but can include the use of stopped-flow, merging zones extraction techniques in addition to FIA scanning and methods based on intermittent pumping89. The principles of FIA and the versions just mentioned will now be briefly discussed on the basis of the excellent review of Ruzicka and Hansen89 in order to understand the appli-cational possibilities of electrochemical detection in this technique. 

This analyser is a computer-controlled automated batch analyser, using a stop-flow principle to analyse for pH, conductivity, turbidity and colour. TTie principle of analysis for each module is based on the recommended methods as detailed in the Examination of Waters and Associated Materials issued by the Standing Committee of Analysts of the Department of the Environment. The temperature of the sample hquid flow is measured in order that temperature-compensated results of pH and conductivity can be quoted. 

There exist in principle two general methods for carrying out HPLC-NMR on-flow and stopped-flow experiments. 

A stopped-flow experiment is quite simple in principle. The apparatus allows the rapid mixing of two or more solutions, which then flow into an observation cell while the previous contents are flushed and replaced with freshly mixed reactants (Fig. 1). A stop syringe is used to limit the volume of solution expended with each measurement and also serves to abruptly stop the flow and to trigger simultaneously a computer to start data collection. Thus, if one were to watch from the point of view of the photodetector, one would first see the solution flow into the observation cell and then abruptly stop. The reaction is followed as the solution ages after the flow stops. The time resolution of the method... 

In this chapter we mainly limit ourselves to the time region from several femtoseconds to milliseconds and to processes that are induced by illumination with light. One could roughly divide this time region into two parts, from femtoseconds to nanoseconds and from nanoseconds to milliseconds. The latter time scale can be investigated with electronic detection, and the described methods can also be used for processes that are initiated in other ways, for example, in stopped-flow experiments where reactants are rapidly mixed or pulse-radiolysis experiments where a short electron pulse induces a chemical reaction. After an introduction to the basic principles of transient absorption, we first treat slow transient absorption measurements (nanoseconds-milliseconds). 

So far, examples to illustrate experimental methods for following the time course of the approach to steady states and of their kinetic interpretation have been restricted to enzymes which do not have a natural chromophore attached to the protein although reference has been made to the classic studies of Chance with peroxidase (see p. 142). Qearly the application of these techniques to the study of enzymes with built in chromophores, such as the prosthetic groups riboflavine, pyridoxal phosphate or haem, contributed considerably to the elucidation of reaction mechanisms. However, the progress in the identification of the number and character of intermediates depended more on the improvements of spectral resolution of stopped-flow equipment than on any kinetic principles additional to those enunciated above. This is illustrated, for instance, by the progress made between the first transient kinetic study of the flavoprotein xanthine oxidase by Gutfreund Sturtevant (1959) and the much more detailed spectral analysis of intermediates by Olson et al. (1974) and Porras, Olson Palmer (1981). 

In some apparatuses, very similar to stopped-flow the flow of reactants is continuous (continuous flow methods). The observation of the reaction is integrated. An apparatus based on this principle has been described that allows the study of systems with half-time of reaction as short as 5 )xs. ... 

Most modern osmometers do not require a static equilibrium to be reached and they use the principle of stopping the flow of solvent through the membrane by applying an external pressure to the polymer solution. This method has the advantage of much greater speed and there are several commercial instruments now available that use this dynamic equilibrium principle. ... 

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