Time-zero kinetics

Time-zero Kinetics. At any given particle size and feed concentration, it was found that the dependence of butene conversion on space time can be captured reasonably well by a first order kinetic model. The concentration of isobutane is assumed to be... 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

A successful modification to the technique involves delayed pulsed-field extraction which allows discrimination between zero and near-zero kinetic energy electrons. About 1 ps after the laser pulse has produced photoelectrons, a small voltage pulse is applied. This has the effect of amplifying the differences in fhe velocities of fhe phofoelecfrons and allows easy discrimination befween fhem as a resulf of fhe differenf times of arrival af fhe defector. In fhis way only fhe elections which originally had zero kinetic energy following ionization can be counted to give fhe ZEKE-PE specfmm. 

FIGURE 9.6 Apparatus for time resolution kinetic in FT-measurements. Quick ampoule sampling, internal references, accurate control of flows and pressure, precise zero time, accurate mixing of flows, and catalyst powder on inert particles. 

Figure 5.11 A constant velocity Mossbauer experiment reveals the kinetics of the denitridation of an iron nitride in different gases at 525 K. The negative part of the time scale gives the transmission of the most intense peak of the nitride at time zero the gas atmosphere is changed to the desired gas. Denitridation occurs relatively fast in H2, but is retarded by CO, whereas the nitride is stable in an inert gas such as helium (from Hummel etal. [33]).

Upon the administration of 1 gram of an aminoglycoside every 12 hours, the Cmj v, was found to be 8 mcg/mL. The plasma concentration at time zero was 63 mcg/mL and elimination rate constant was 0.14 hour 1. If it is desired to increase the Cmin ss to 10 mcg/mL, what should be the dose of the drug, and the new Cmax ss Assume that the drug follows linear kinetics. 

Figure 6.14. Kinetics of 02 secretion by activated neutrophils. Neutrophil suspensions (5 x 10s/ml) were suspended in RPMI 1640 medium containing 75 /JM cytochrome c. In (b) and (d), suspensions contained 100 nM staurosporine. At time zero, suspensions in (a) and (b) were stimulated by the addition of 1 /iM fMet-Leu-Phe, whilst suspensions in (c) and (d) were stimulated by the addition of 0.1 fi g/ml PM A. Reference cuvettes were identical to the sample cuvettes, but additionally contained 30 jUg/ml SOD. The bar marker represents a A4 of 0.03 in (a) and (b), or 0.08 in (c) and (d). Similar results were obtained using the more specific protein kinase C inhibitor, bisindolylmaleimide.

The aqueous phase is usually contained in a metal capillary. The capillary is pushed into the lower part of the cell, containing the organic phase, at time zero of the extraction. The method examines the reaction during very short time intervals (from 0.05-1 s) and is relatively simple, as the two phases are not stirred. The consumption of the reactants is very small (about 20 mm per experiment) and permits one to make many determinations in a relatively short time (about 10 kinetic curves per hour). Unfortunately, the method proves inapplicable for concentrated aqueous solutions and when many species are involved in the extraction reaction. 

In this section, a brief description of the necessary experiments to identify the kinetic parameters of a seeded naphthalene-toluene batch crystallization system is presented. Details about the experimental apparatus and procedure are given by Witkowski (12). Operating conditions are selected so that the supersaturation level is kept within the metastable region to prevent homogeneous nucleation. To enhance the probability of secondary nucleation, sieved naphthalene seed particles are introduced into the system at time zero. 

A very interesting and complex protonation mechanism has been snggested for the hydride cluster [W3S4H3(dmpe)3]PF6 in CH2CI2 solutions. In the presence of an excess of HCl, a careful kinetic study of the process in eq. (10.4) by the stopped-flow technique [9] has revealed three kinetically distinguishable steps very fast, fast, and slow, with rate constants A 1, ki, and k3. The kinetic order in the initial hydride cluster in the slow step has been measured as 1. At the same time, rate constants k and A 2 have corresponded to a second-order dependence on acid concentration, while the third step has shown a zero kinetic order on HCl. The rate constants have been determined as A i =2.41 x 10 M-2/s, k2 = 1.03 X 10 M /s, A 3 = 4 X 10 s . Note that the protonation process becomes simple at lower concentrations of HCl. Under these conditions it shows a single step with a first kinetic order on the acid. 

The ratio of the enantiomeric benzyl amide products was determined by analyzing a diluted aliquot of the quenched reaction mixture by HPLC using a chiral stationary phase column (Chiralcel OD, Daicel Chemical Co.). Since racemization is a pseudo-first-order kinetic process, these data (along with the time zero value) are sufficient for determination of the intrinsic rate of racemization kR. The half-life for racemization lRU2 can be directly calculated from the l/d ratio (or % enantiomeric excess, %ee) where t was the time of benzylamine addition (the delay time) ... 

It expresses the velocity (v) of a single-substrate reaction (Equation C1. 1.1) in terms of substrate concentration at time zero ([S]) and the kinetic constants KM and V. is defined as the limiting maximal velocity for the reaction, which is observed when all of the enzyme is present as ES. KM, known as the Michaelis constant, is a pseudoequilibrium constant, which equals the concentration of substrate at which the reaction velocity equals one-half Vtrax (Figure Cl. 1.1). 

In a typical run, the nitrile 1 (30 mmol) and phthalic acid 2 (36 mmol) were introduced into the reactor, and heated under stirring. In the kinetic studies, time zero is taken at complete dissolution of the phthalic acid. At the desired reaction time, the reactor was rapidly cooled in a water-ice mixture and then chloroform (30 mL) was added. The mixture was stirred for 5 min and then the solid was filtered off. The chloroform solution contains the unchanged nitrile 1, the amide and the carboxylic acid 3. The residual solid contains unchanged phthalic acid 2, phthalimide 4, and as the major component, phthalic anhydride 5. The volume of the chloroform solution was adjusted to 50 mL and naphthalene was added as an internal standard. The resulting solution was analyzed by GLC. 

Figure 4. Electrolyte (150 mequiv CaCl.) injected under DPL-DPPA mixed films. Kinetic curves of AV of films containing 10 wt % (upper panel) and 50 wt % (lower panel) DPPA at three different values of x (2,10, and 20 dyn/cm). Aqueous hypophase, pH 5.6, 25°C. The mixed-lipia film at the indicated pressure was spread first on distilled H 0 the electrolyte then was injected beneath at time zero. Error as in Figure 2.

The kinetic rate constants are kg and kc. We can analytically solve these differential equations, assuming that we start at time zero with only reactant A (CA0) ... 

The integration of these mass balances, often with the help of the computer because the nonlinearity of the equation does not allow for an analytical solution, allows for a calculation of the changes in the concentrations of the molecules over time, given all concentration at time zero, values of all the kinetic parameters, and a description of the interaction of the network (the system ) with its environment. 

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