Gradient techniques stop-flow

S. Olsen, J. Ruzicka, E.H. Hansen, Gradient techniques in flow injection analysis. Stopped-flow measurement of the activity of lactate dehydrogenase with electronic dilution, Anal. Chim. Acta 136 (1982) 101. 

However, some techniques of FI have incorporated a gradient chamber into the flow line, particularly where stopped-flow methods of FI are involved... 

The direct coupling of HPLC with NMR spectroscopy has waited for a number of technological developments to make it a feasible routine technique. Since then, major advances have made the routine use of on-line NMR detection of HPLC fractions a useful adjunct to the armoury of analytical methods. " Experiments can be carried out in one of three modes, direct on-line NMR detection of the HPLC eluent (on-flow), a stopped-flow approach or finally the eluted fractions can be stored in capillary tubes for later recall for detailed NMR spectroscopic studies. No compromise needs to be made in the chromatographic conditions, and programmed gradient elution profiles can be accommodated. The extension of the method to nanolitre scale separations has also been discussed. ... 

The flexibility of this methodology allows for a variety of applications from determinations with no additional chemical reaction to sequential reactions involving five or six reactants from gradient techniques to stopped-flow or kinetic determinations from incorporation of liquid-iiquid extraction units to Insertion of gas samples. 

Except for FIA titrations, the stopped-flow method [20, 264] is, the most frequently used gradient technique. The most practical approach to reaction rate measurement is to halt the reaction mixture within the observation volume of the flowthrough detector by stopping the flow and to measure the change of the signal with time. This is readily accomplished by the FIA stopped-flow technique (Chapter 4.3), which relies on a combination of a suitable dispersion of the sample material within the reagent stream and on the subsequent reaction rate measurement during the stopped-flow period. 

Being suitable for conventional FIA procedures, as well as for gradient techniques and stopped-flow measurements, the hydrodynamic injection variations presented above are also ideally suited for microminiaturization. 

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