Fast Kinetic Measurements Research Articles

Measurement of fast desorption kinetics of D 2, from tungsten by laser induced thermal desorption

A laser heating technique for studying fast surface processes has been applied in an initial study to the thermal desorption of D 2 from a polycrystalline tungsten sample. This technique is a means for measuring surface reactions at rates, concentrations, and temperatures that approach conditions of technical interest, but with the high degree of definition and control made possible with an ultrahigh vacuum apparatus. The method is analogous to the fast temperature jump method used for studying reactions in condensed phases, and can sort out elementary processes that have differing activation energies. The variation of total flux desorbed with maximum surface temperature reached and initial surface coverage serves, with the aid of a model kinetic rate expression, to determine the desorption rate parameters. It is shown that the desorption of D 2 from W at rates of 5 × 10 7 monolayers/sec is governed by the same kinetics as obtained by extrapolating previous measurements made at a rate about 10 5 times slower. The surface is subjected to a sufficiently fast and large temperature rise to desorb surface atoms or molecules in a time short compared to the range of flight times to a mass spectrometer detector. In this way the velocity distribution of the desorbing species may be determined. This along with the surface temperature history gives additional information on the reaction rate model and also whether the species are emerging in translational thermal equilibrium with the surface. In the present experiments a significant number of desorbatedesorbate collisions occur. Corrections are made for the collision effects in the interpretation of the data. It is shown how modifications of the technique can be made to substantially eliminate these effects. The present conditions were laser pulse width of 3 × 10 −8 sec and surface temperature rise of 300 to 3000 K.

Kinetic evidence for intermediate states in the unfolding of chymotrypsinogen A

Fast kinetic measurements show that the reversible unfolding of chymotrypsinogen A at low pH is more complex than a two-state reaction. The kinetics of exposure of buried tryptophan groups at 293 nm show a compound fast phase (in milliseconds) in addition to the known slow unfolding reaction (in seconds). The fast reactions apparently are part of the unfolding process, since they are negligible in temperature jumps that end below or begin above the transition zone. The data are similar in many respects to those reported recently for ribonuclease A. Taken together, the two sets of results suggest that: (a) there may be a common pattern of kinetic behavior in the thermal unfolding of several small proteins (cf. Pohl 1968 a, b; 1969); (b) fast kinetic methods are especially sensitive for detecting intermediates in unfolding; and (c) other protein unfolding reactions that have been described as two-state reactions may also show observable intermediates in the millisecond time range.

Standardisation of dilute aqueous bromine solutions for reaction kinetics by incremental potentiometric titration with thiosulphate

Aqueous bromine in the concentration range 10 -1–10 -6 M oxidizes thiosulphate quantitatively to sulphate. This reaction can be used to standardise very dilute bromine solutions by addition of a measured but insufficient quantity of relatively concentrated standard thiosulphate solution. The fall in bromine-bromide redox potential at an inert indicator electrode caused by the addition allows the final bromine concentration to be found. The method can be applied in the measurement of fast bromination kinetics.