RADIAL MEASUREMENT OF HYDRODYNAMICALLY GENERATED CONCENTRATION PROFILES FOR MOLECULAR-WEIGHT DETERMINATION

Hydrodynamically generated concentration gradients, obtained by injecting a plug of sample in a straight cylindrical tube, are manipulated by changing flow rate and tube length. The ratio of axial velocity to radial velocity experienced by the analyte due to the combined effect of diffusion and convection in the flow through tube dictates the shape of the concentration profile. The radial concentration gradient is sensitively measured by a refractive index gradient (RIG) detector. The data are converted into an asymmetry ratio which is correlated to the diffusion coefficient of the analyte and hence to the molecular weight. The effect of flow rate and tube length were extensively studied, ultimately, to determine the molecular weight of poly(ethylene glycols) (PEGs) ranging in molecular weight from 200 to 8000 g mol-1. The range of molecular weight that could be determined with a given set of experimental conditions was determined with a relative uncertainty In molecular weight of 4% for one trial. The sensitivity of the RIG signal depended upon the relative position of the narrow, collimated probe beam with respect to the center of a flow cell. Hence, the dynamic range of molecular weight sensing of PEGs can be selected by changing the position of the probe beam, flow rate, or tube length. Measurement of the axial concentration profile as a function of radial position was demonstrated using the RIG detector.