Pulse accelerators

Proton inventory technique. 21.9-220 Pseudo-first-order kinetics, 16 Pulse-accelerated-flow method. 255 Pulse radiolysis, 266-268 Pump-probe technique. 266... 

A significant technical development is the pulsed-accelerated-flow (PAF) method, which is similar to the stopped-flow method but allows much more rapid reactions to be observed (1). Margerum s group has been the principal exponent of the method, and they have recently refined the technique to enable temperature-dependent studies. They have reported on the use of the method to obtain activation parameters for the outer-sphere electron transfer reaction between [Ti Clf ] and [W(CN)8]4. This reaction has a rate constant of 1x108M 1s 1 at 25°C, which is too fast for conventional stopped-flow methods. Since the reaction has a large driving force it is also unsuitable for observation by rapid relaxation methods. 

Fig. 3.2 The operation of flow methods. The distance x and the combined flow rate govern the time that elapses between mixing and when the combined solutions reach the observation, or quenching, point. In the stopped flow method, observation is made as near to the mixer as is feasible, and monitoring occurs after the solutions are stopped. In the pulsed accelerated flow method, observation is within the mixer.

Pulsed continuous flow is a method in which continuous flow is established for a short time. This method can reduce reagent consumption to 5 ml, and fast jet mixers have lowered the accessible reaction half-time to the 10 ps range. Pulsed accelerated flow may be viewed as an adaptation of pulsed continuous flow in which the flow rate through the mixer and observation chamber is varied during the course of one kinetic run. This method can be used for reactions with half-times down to 10 ps. This method is limited to first-order reaction conditions. 

Feist H, Koep M, Reich H. (1971) A current transformer and gated integrator for measurement of week currents from pulsed accelerators. Nud Instr and Meth 97 319-321. 

Figure 2. Top and end views of a radial mixer/observation cell used in pulse-accelerated-flow spectrometer. W, windows, A and B, reactants. Reproduced with permission from S. A. Jacobs, M. T. Nemeth, G.W. Kramer, T. Y. Ridley, and D. W. Margerum, Anal. Chem. 1984, 56, 1058. Copyright 1984 American Chemical Society.

The pulsed-flow method evolved further into the pulsed-accelerated-flow method [7], Here, the solutions have a range of flow velocities owing to the constant flow... 

The limit for the measured rate constants is determined by the mixing rate and the instrument s dead time, defined as the time required for the solution to travel from the mixing chamber to the observation point. Nowadays, half-times in the millisecond range can be measured routinely. An extension of accessible rates up to 2000 s through algebraic corrections for mixing effects was discussed [11]. Under the assumption that the behavior of the solution at short times after mixing in the stopped-flow is described by the same equations that were found applicable for pulsed-accelerated flow, the precise rate constant can be obtained from a set of experiments carried out under pseudo-first-order conditions by use of Eq. 10. 

The development of high fluxes of low-energy neutrons from pulsed accelerator sources, such as IPNS at Argonne National Laboratory (USA), KEK (Japan) and, especially, ISIS at Rutherford Appleton Laboratory (UK) provides a most suitable probe for studies of the vibrational dynamics of hydrogenous materials. The advantages of neutron scattering for the study of molecular dynamics stem from their remarkable properties. 

The current measurements revealed a mean cyclonic circulation following the bathymetric contours. The average duration of a complete cycle was estimated to be about 60 days along the 220 m isobath. The circulation lasted longer than 1 year. There is observational evidence that each new inflow pulse accelerated the ongoing deep circulation (Hagen and Feistel, 2001). 

Early studies (1936-1950) of neutron scattering used radium-beryllium neutron sources but their low neutron flux prevented exploitation of neutron scattering as a spectroscopic technique [4]. Today neutrons are either extracted from a nuclear reactor or generated at a pulsed, accelerator-based spallation source. The exploitation of neutrons from nuclear reactors in structural studies and spectroscopy dates from the 1950s and from pulsed sources from the 1970s. A useful summary of the development of neutron sources is given in [5]. 

High-intensity electric field pulses accelerated osmotic dehydration of carrot [18], A Fickian diffusion coefficient for water and solute increased exponentially with electric field strength. This effect was attributed to increased cell wall permeability, which was also manifested by the softening of product. 

Figure 2. A comparison of steady-state (reactor) and pulsed (accelerator) techniques for neutron studies of liquids and amorphous solids. L7J...

Pulsed acceleration of ions perpendicular to their direction of travel into a time-of-flight mass spectrometer. Ions may be extracted from a directional ion source, drift tube or miz separation stage. 

An interesting approach to MS-MS is the combination of an ion-trap storage device and a reflectron time-of-flight instrument. While initially the ion trap was used only to store ions from the continuous ion source before the pulsed acceleration to the TOF analyser, it has recently been demonstrated by the group of Lubman that MS-MS in the ion trap before TOF mass analysis results in additional possibilities. 

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