Size-Exclusion Chromatography in the Measurement of Concentration and Molecular Weight of Some EOR Polymers

Abstract

Summary Procedures that involve the use of size exclusion chromatography (SEC) for the measurement of concentration and weight-averaged molecular weight, M¯w, of some EOR polymers were developed and found to give improved detectability, accuracy, and/or efficiency. The separation of polymer from low-molecular-weight impurities by size allows unambiguous detection of polymer properties such as concentration and M¯w. A combination of an SEC column of a pore size small enough to exclude the polymer totally and a mobile phase of ionic strength of 1.5 was found suitable for the separation of Polyacrylamide, partially hydrolyzed Polyacrylamide, cationic Polyacrylamide derivative, and xanthan polysaccharide from impurities. Concentration detection of the separated polymer sample with a variable-wavelength ultraviolet (UV) detector was found to give superior detectability over detection by refractive index difference. A wavelength of 214 nm [2,140 Å] was used for the detection of these polymers on the basis of the spectra of samples purified by dialysis. With the active polymer assay determined by reprecipitation into a nonsolvent, the detection limit by UV was determined to be <0.1 μg/cm3 for Polyacrylamide and a cationic Polyacrylamide derivative, <0.2 μg/cm3 for partially hydrolyzed Polyacrylamide, and <0.7 μg/cm3 for a xanthan polysaccharide. The linear calibration range was up to 500 μg/cm3. The precision of the concentration measurement was better than 4% for Polyacrylamide and its derivative and 5% for polysaccharide at a 95% confidence level. On-line measurement of M¯w of four different types of EOR polymers was made by connecting a low-angle laser-light-scattering (LALLS) detector to the SEC system and interfacing the detectors to a computer. The polymer sample was dispersed in the SEC column to give a concentration profile while being separated from the impurities. Continuous measurement of concentration and Rayleigh ratio of the dispersed slug by UV and LALLS detectors allows the calculation of the M¯w for the whole sample. The precision of this measurement is expected to be better than ±5% at a 95% confidence level. The M¯w determined for a Polyacrylamide molecular-weight standard was in good agreement with the reported value (0.98 million compared to 1 million). Such a procedure substantially minimizes the interference by dust and requires only a single injection, thereby improving both the accuracy and efficiency of the M¯w measurement.