MOLECULAR-WEIGHT SENSING OF POLYETHYLENE GLYCOLS BY FLOW-INJECTION ANALYSIS AND REFRACTIVE-INDEX GRADIENT DETECTION

Fundamental aspects of a novel molecular weight sensor (MWS) will be described with application to polyethylene glycol (PEG) analysis in the range of 1470 g mole-1 to 12 600 mole-1. The MWS incorporates controlled dispersion due to convection and diffusion contributions resulting from flow of a sample plug through a narrow diameter tube and flow cell. The controlled dispersion results in a concentration profile that is a strong function of the analyte diffusion coefficient and is readily correlated to molecular weight. The concentration profile is sensitively measured by a refractive index gradient (RIG) detector. This detector provides excellent sensitivity without requiring analyte absorbance. The data is readily converted to an asymmetry ratio (AR), a quality parameter that is independent of analyte concentration between the range of 100 ppm to 800 ppm injected PEG. The asymmetry ratio is the ratio of the relative maximum to the relative minimum on the RIG detected signal. The concentration gradient at the maximum and minimum were found to be dominated by different dependencies on the analyte diffusion coefficient. When the asymmetry ratio is calculated, the diffusion effects constructively add to the sensitivity of the molecular weight prediction. The precision of the molecular weight determination for 1470 g mole-1 PEG was 117 g mole-1 (n = 5 trials).