Diffusion chromatographic measurement

In the kinetic studies of the adsorption process, the mass transport of the analyte to the binding sites is an important parameter to account for. Several theoretical descriptions of the chromatographic process are proposed to overcome this difficulty. Many complementary experiments are now needed to ascertain the kinetic measurements. Similar problems are found in the applications of the surface plasmon resonance technology (SPR) for association rate constant measurements. In both techniques the adsorption studies are carried out in a flow system, on surfaces with immobilized ligands. The role of the external diffusion limitations in the analysis of SPR assays has often been mentioned, and the technique is yet considered as giving an estimate of the adsorption rate constant. It is thus important to correlate the SPR data with results obtained from independent experiments, such as those from chromatographic measurements. 

Table 12. Diffusion coefficients from gas chromatographic measurements on low density polyethylene (705)...

E. Grushka, E.J. Kikta, Extension of the chromatographic broadening method of measuring diffusion coefficients to liquid systems. I. Diffusion coefficients of some alkylbenzenes in chloroform, J. Phys. Chem. 78 (1974) 2297. 

The retention volumes on poly(vinyl chloride) powders were correlated with their capacity for incorporating the plasticizer [153]. The diffusion of plasticizer in the polymer powder being controlled by the external smface, by the plasticizer diffusion coefficient and by a shape factor, some connection with gas chromatographic measurements is to be expected. The plasticizer adsorption was found to take place only at a temperature slightly above T,. 

In a two-bulb capillary diffusion apparatus like that in Pig. 3.1-2, one bulb contains 75% H2 and 25% CgHs, and the other contains 65% H2, 34.9% CeHe, and 0.1% radioactively tagged CgHs. The system is at 0°C. We can measure diffusion in one of two ways. First, we can measure the concentration change of all the benzene using a gas chromatograph. Second, we can measure the concentration difference of the radioactive isotopes. How different are these results To answer this problem, let 1 be... 

The solvent used was 5 %v/v ethyl acetate in n-hexane at a flow rate of 0.5 ml/min. Each solute was dissolved in the mobile phase at a concentration appropriate to its extinction coefficient. Each determination was carried out in triplicate and, if any individual measurement differed by more than 3% from either or both replicates, then further replicate samples were injected. All peaks were symmetrical (i.e., the asymmetry ratio was less than 1.1). The efficiency of each solute peak was taken as four times the square of the ratio of the retention time in seconds to the peak width in seconds measured at 0.6065 of the peak height. The diffusivities obtained for 69 different solutes are included with other physical and chromatographic properties in table 1. The diffusivity values are included here as they can be useful in many theoretical studies and there is a dearth of such data available in the literature (particularly for the type of solutes and solvents commonly used in LC separations). 

To day peak widths are rarely used in chromatographic analysis except for the purpose of calculating peak areas. Peak widths, however, can provide a means of measuring the diffusivity of a solute which is a function of the molecular weight. Consequently, if a reliable relationship between diffusivity and molecular weight can be identified, then the molecular weight of the solute can be assessed. Peak widths of solutes eluted from an open tube can give very precise values of diffusivity. There are a number of equations that purport to relate diffusivity to... 

In a typical pulse experiment, a pulse of known size, shape and composition is introduced to a reactor, preferably one with a simple flow pattern, either plug flow or well mixed. The response to the perturbation is then measured behind the reactor. A thermal conductivity detector can be used to compare the shape of the peaks before and after the reactor. This is usually done in the case of non-reacting systems, and moment analysis of the response curve can give information on diffusivities, mass transfer coefficients and adsorption constants. The typical pulse experiment in a reacting system traditionally uses GC analysis by leading the effluent from the reactor directly into a gas chromatographic column. This method yields conversions and selectivities for the total pulse, the time coordinate is lost. 

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