Selectivity stopped flow

Figure 31. Selected stopped-flow data for the disappearance of (HPX)Fe acyl peroxide (X, ) 416 nm and the concomitant appearance of (HP X)Fe =0 (Xmax) 678 nm. Global analysis of the full spectral window (400-700 nm) for the disappearance and appearance traces using a first-order kinetic model gives kohs = (1-9 0.1) x 10 s for 0—0 bond heterolysis.

Inherently, the FI A stopped-flow procedure should be an ideal vehicle to determine reaction rates and rate laws, provided that an experimental approach could be designed that allows resolving the individual contributions of physical dispersion and chemical kinetics. A comprehensive treatment of this problem was recently described by Hungerford et al. [838], who pointed out that although the single-line stopped-flow system (Fig. 4.15a cf. Fig. 4.11) allows optimization of solution conditions for measurement during a selected stopped-flow time interval (fs) by choosing... 

Espenson has shown that the reaction of c/j-Co(en)2(N3)2 with takes place by an inner-sphere mechanism. This Co(III) complex was selected for investigation because it is particularly reactive towards and also the dissociation of monoazido vanadium(lll) is relatively slow. At low concentrations (2-20 X 10 M) the second-order rate coefficient is 32.9 l.mole . sec at 25 °C, [H ] = 0.10 M and [i = 1.0 M. At higher concentrations ( 0.1 M), using a stopped-flow apparatus, the kinetics are apparently first order at 520 mfi, a wavelength where shows negligible absorbance. The rate coefficient under... 

Stopped-flow experiments, illustrated by pathway A of Figure 7.8, are better suited for sample components present at low concentrations since a separated analyte can be selected and held in the NMR probe allowing extensive signal averaging. Execution of stopped-flow acquisition requires calibration of the exact length of time it takes... 

Both the capillary LC and NMR are controlled by the interface software, which enables the operator to use the UV-detector output for peak selection. Only peaks of interest can be subjected to NMR analysis, while minor or unimportant compounds can be directed to waste. NMR acquisition can take place in either on-flow or stop-flow mode. The combination of capillary LC and NMR is suitable for sample-limited applications (e.g., proteomics) and allows for low nanograms detection. 

There are four general modes of operation for LC-NMR on-flow, direct stop-flow, time-sliced and loop collection/transfer. The mode selected will depend on the level and complexity of the analyte and also on the NMR information required. All modes of LC-NMR can be run under full automation for LC peak-picking, LC peak transfer to storage loops or NMR flow cell, and NMR detection [46],... 

Since asterosaponin mixtures consist of a variety of compounds with different combinations of reoccurring structural features (oligosaccharide or steroidal side-chains), this approach has already yielded valuable information for identif-ing known constituents and for selecting compounds of interest for further investigations, such as 2D stop-flow NMR spectra (see Section 5.1.2.5 below). 

Figure 6.5 Stop-flow NMR spectra of selected peaks in fraction A. Reprinted with permission from Preiss, A., Sanger, U., Karfich, N., Levsen, K. and Miigge, C., Anal. Chem., 72, 992-998 (2000). Copyright (2000) American Chemical Society...

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