Stopped flow technique

Stopped-Flow Technique

Perhaps the most frequently used rapid kinetics techniques is stopped-flow. Small volumes of

solutions are rapidly driven from syringes into a high efficiency mixer to initiate a fast

reaction. The resultant reaction volume then displaces the contents of an observation cell thus

filling it with freshly mixed reagents. The volume injected is limited by the stop syringe

which provides the “stopped

-

flow”. Just prior to stopping, a steady state flow is achieved.

The solution entering the flow cell is only milliseconds old. The age of this reaction volume

is also known as the dead time of the stopped-flow system. As the solution fills the stopping

syringe, the plunger hits a block, causing the flow to be stopped instantaneously. Using

appropriate techniques, the kinetics of the reaction can be measured in the cell.

Traditionally stopped-flow has lent itself to UV/Vis detection methods

–

both by absorption

and fluorescence spectroscopy and these are still the most common methods employed. TgK

Scientific offer a comprehensive range of stopped-flow equipment from simple stopped-flow

accessories that can be used with existing absorbance and fluorescence spectrometers to

complete, self-contained computer controlled stopped-flow systems with dedicated

acquisition and analysis software.

Mixing options for stopped- flow include multi-mixing (also known as double mixing or

sequential mixing) where more than two solutions are mixed together, variable ratio mixing

using different size syringes and microvolume mixing for applications where only very small

quantities of reactants are available.

The environment of the reagents is also extremely important and so equipment is available

for anaerobic applications, sub-zero temperature and high pressure stopped-flow applications.

An extensive range of software packages is also available for the interpretation and analysis

of the kinetic data. In addition to equipment for absorbance and fluorescence measurements,

Hi-Tech offers dedicated stopped-flow systems for circular dichroism, conductivity and FT-

IR measurement.

Stopped- Flow - A technique for studying rapid chemical kinetics

Stopped-flow is one of a number of techniques used to study the kinetics of reactions in

solution. In the simplest form of the technique, two reactant solutions are rapidly mixed by

being forced into a mixing chamber, and then through an observation cell. At some point in

time, the flow is suddenly stopped, and the reaction monitored using a suitable spectroscopic

probe, such as absorbance, fluorescence or circular dichroism. The change in the

spectroscopic signal as a function of time is recorded.

Figure 1 shows the schematic of a typical stopped-flow apparatus. The stopping mechanism

in this example is a stop-syringe. The flow fills the stop-syringe, until the plunger hits the

trigger- switch. This simultaneously stops the flow and starts the data acquisition.

The rate constants which define the reaction kinetics, can be measured by fitting the data

using a suitable model.

The performance of a stopped-flow instrument is determined to a large extent by the dead-

time. This is defined as the minimum time after the reactant have mixed that the observation

starts. The dead-time is essentially the age of the reaction as it enters the observation cell. The

limiting factor in the dead-time of a particular stopped-flow apparatus is determined by the

distance between the mixer and the cell, and the final velocity of the flow at the instant the

flow is stopped. Another factor which can affect the dead-time is the efficiency of the mixer.

Typically, dedicated stopped-flow instruments can achieve dead-times in the region of a

millisecond. Using ultra-small observation cells, dead times of less than 0.5ms can be

achieved. Typically however, the true experimental limitation is the rate of mixing of

solutions of the two reactants.