Stop flow procedure

An example of the LC-NMR stop-flow procedure is provided in the analysis of polyphe-nolics from the Chilean plant Gentiana ottonis (Gentianaceae). Preliminary LC-UV analysis... 

In this radiometric detection approach, the detection limit could be lowered by using a stopped-flow procedure that captured 89% of the eluted "Tc within a 2.5-mL flow-through counting cell. Continuous-flow and stopped-flow detector traces are... 

Figure 2.2 Effect of the stop-flow procedure on a chromatographic separation (a) chromatogram with 14 stops for NMR measurements of 0.5-1 h each, with a total run time of 9.5 h (a) uninterrupted chromatogram...

Figure 7. Focusing FFF fractogram of 9.87- and 40.1-iim PS latex microspheres in diluted Percoll density gradient generated by the electric potential of 32 mV and using the stop-flow procedure for relaxation. (Reproduced with permission from reference 13. Copyright 1993 Springer-Verlag.)...

The frit-inlet injection device was originally apphed to the conventional symmetrical channel in order to bypass the stop-flow procedure [6]. However, the lowest axial flow rate that can be manipulated in a frit-inlet symmetrical system is hmited, because the total axial flow rate becomes the sum of the injection flow rate and frit flow rate, and the incoming cross-flow penetrates through the bottom wall at the same rate. The relatively high axial flow rate in a symmetrical system needs a very high cross-flow rate in order to separate relatively low-retaining materials, such as proteins or low-molecular-weight components. 

The injection volume of the conventional stop-flow sample injection technique is limited typically to 2-20 /zL. This procedure is carried out using a sample loop and injecting the sample plug just onto the channel carried with the channel flow before stopping that flow and letting the sample attain its equilibrium distribution relative to the accumulation wall (relaxation). Sedimentation FFF (SdFFF) is the only technique where the field is not applied during the complete stop-flow procedure, but the centrifuge is started as soon as the sample is injected and the channel flow switched away for practical reasons. 

Regarding the influence of the Schlieren effect on analytical procedures, the effect of differences between the temperature of the ambient environment and the sample on the absorbance was originally reported in relation to stopped-flow procedures [84]. The systematic deviation in absorbance was proportional to the temperature difference and was also dependent on the optical properties of the instrument. The effect also manifested itself when identical solutions at different temperatures were mixed [85] a linear dependence of the measurement on the temperature... 

Using a computer-controlled FIA system, certain technical arrangements can be implemented in order to raise the sensitivity of the assays. Stopped flow procedures where the sample is kept in contact with the immobilized enzyme preparation for a longer time than the passage takes is one such arrangement being used [37]. Reversal of flow rate in order to improve mixing in the enzyme... 

The generation of O2 from potassium superoxide was also applied to stop-flow procedures. In this method O2 was dissolved in dimethyl sulfoxide and stabilized in 18-crown-6-polyether. This method is useful for mechanistic studies indeed, McClune and Fee (1976) were able to obtain catalytic rate constants for bovine copper/zinc superoxide dismutase as a function of pH in various buffers. More recently the mechanism of catalysis and of anion inhibition of iron superoxide dismutase from E. coli have been examined by this method using a specially constructed stop-flow spectrophotometer (Bull and Fee, 1985). A limitation of the method is that the pre-equilibrium state cannot be properly investigated because of the time resolution of the stop-flow equipment (== 5 msec). 

Another interesting example of the FIA stopped-flow procedure is the. determination of sulfur dioxide in wine [71], based on the well-known West-Gaeke [4.2] method, in which a purple compound formed by the... 

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... 

LC-NMR One information-rich spectral technique that is more suited to the liquid mobile phase of HPLC than to the vapor phase of GC is NMR. LC-NMR has been implemented, but it has significant limitations. To obtain interpretable spectra of unknowns, concentrations in the measurement cell must be higher than with other detectors. The cell must be smaller than the usual NMR tube, so for any but the very highest concentrations of analytes, FT-NMR acquisition is preferred, with each eluted peak being retained in the measurement cell by a stopped-flow procedure similar to that employed to increase sensitivity in GC-IR (Section 12.8.2). Expensive deuterated mobile phase solvents are required for proton NMR, which mandates the use of low mobile phase volume flow columns narrow bore or even capillary HPLC. LC-NMR is expensive to implement and not readily available from commercial vendors at this time. 

Micro-TFFF was also applied to separate colloidal particles.The experimental conditions were optimized with respect to the relaxation phenomena and related stop-flow procedure, and to the accumulation wall tem-perature. An original method was proposed for the determination of the sign of the Soret coefficient, based on the comparison of the retentions of the studied species in horizontally and vertically positioned micro-TFFF channels. The change of the retention in these two positions of the channel is due to the appearance of a convective flow, superposed to the forced flow of the carrier liquid, in the vertical channel. 

The main important details of these procedures for AA determination are contained in Table 18.8. As can be seen in Table 18.8, among these flow analysis techniques, stopped-flow procedures are the most applied to AA determination, being mostly kinetic enzymatic determinations. BI methodologies for AA determination use spectrophotometric detection and a commercial flow cell, which is filled with appropriate solid beads, works as a flow-through chemical sensor integrating online reaction, retention, and detection on the solid-phase disposable beads. 

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