Chromatograms rate theory

Analytical information taken from a chromatogram has almost exclusively involved either retention data (retention times, capacity factors, etc.) for peak identification or peak heights and peak areas for quantitative assessment. The width of the peak has been rarely used for analytical purposes, except occasionally to obtain approximate values for peak areas. Nevertheless, as seen from the Rate Theory, the peak width is inversely proportional to the solute diffusivity which, in turn, is a function of the solute molecular weight. It follows that for high molecular weight materials, particularly those that cannot be volatalized in the ionization source of a mass spectrometer, peak width measurement offers an approximate source of molecular weight data for very intractable solutes. 

X 0.75 cm) Ve i = 28 ml = 50 ml eluent 0.05 M NaCI flow rate 0.80 ml/min detection Optilab 903 interferometric differential refractometer applied sample mass/volume 200 /tl of 2-mg/ml aqueous solutions sum of individual chromatograms (theory —) and (theory/experimental) ratio (—) plotted for quantification of deviations in separation performance between narrow distributed samples and broad distributed samples. 

The final information needed is a search pattern and a definition of the independent variable to be searched. Usual variables are flow rate, %B, and %C %A is assumed to be a dependent variable (100%—the sum of the other solvent percentages). One commercial search pattern starts with all the variables at zero, then systematically changes one variable by a preset percentage and walks incrementally through all possible values, then repeats for the next variable. Once all injections and chromatograms have been run, each run is inspected and the best value is selected. I call this the infinite monkey theory of methods development you will find it uses a lot of time, reagents, and paper. 

The experimental optimization procedures outlined above can be replaced with others based on computer simulations [64,65], which make use of the chromatographic theory and of one or two prior experiments intended to define critical parameters such as the sample, mobile phase, column, temperature, flow-rate and pressure. Simulated chromatograms are obtained for different experimental conditions (column dimensions, particle size, mobile phase composition, flow-rate, temperature, etc.) until the required resolution is achieved. In essence, the procedure is similar to experimental optimization, although the chromatograph functioning is replaced with programming. The information obtained can be checked experimentally or be used for designing new approached to experimental optimization. 

Theories of paper chromatography are controversial (173,261-272) and, like that of its adsorption counterpart, they are incomplete in that no theory of this intricate process takes into account all known factors. Martin and Synge (174) drew a parallelism with fractional distillation and introduced the theoretical plate concept to chromatography. The rate of movement of a substance on a paper chromatogram may be defined by its R/ value (71), which is the ratio of the distance moved by the substance to that moved by the solvent front measured from the point of application of the substance. This value remains fairly constant for any one compound under given conditions of temperature, solvent composition, solute concentraticm, pH, and paper, but it cannot be relied upon for purposes of identification (24,70,71, 159). 

Once the competitive isotherms for solutes are known, the theories of chromatography now allow the rapid optimisation of production rate and, through computer simulations, also allow the rapid prediction of preparative chromatograms under any desired conditions. These facilitate the full optimisation of a production scale method. 

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