ABSTRACT
A pulsed-accelerated-flow spectrophotometer with UV-visible capability is described that permits measurement of pseudo-first-order rate constants as large as 500 000 s(-)(1) (t(1/2) = 1.4 µs). Chemical rate processes are resolved from physical mixing rate processes by variation of flow velocities under conditions of turbulent flow. Two mixing processes are found in the mixing/observation tube. One mixing rate constant, valid for the full length of the tube, is directly proportional to the flow velocity. The other mixing behavior, proportional to the square of the flow velocity, is found only in the immediate vicinity of the 10 inlet reactant streams that collide with one another in the middle of the observation tube. Contributions from the latter mixing become more important for very fast reactions that take place close to the inlet jets. These mixing models and improved signal/noise permit the measurement of rate constants for very fast reactions. Applications of the PAF method to electron-transfer, proton-transfer, hydrolysis, and non-metal redox reactions are reported for pseudo-first-order and second-order reactions.
ABSTRACT
The coupling of a caesium ion source to a triple quadrupole mass spectrometer is described. The potential of this combination for examining thermally labile, non-volatile molecules, such as thiamine hydrochloride, is examined. Emphasis is placed on the advantages the various scanning modes of tandem mass spectrometry provide in ion structure elucidation and the investigation of desorption ionization mechanisms. Use of the caesium ion source for desorption of neutral molecules which are chemically ionized by an auxiliary gas is demonstrated. This procedure may be especially useful for examining non-volatile, non-ionic samples.
