Determination of degree of substitution of chemically modified xanthan by Raman and FT-IR spectroscopic methods

Abstract

Many chemical modifications such as carboxymethylation and sulfation contribute to the improvement in the functional properties of polysaccharides, and enhance their applications in various industries. Degrees of carboxymethylation and sulfation are measured traditionally by wet chemistry methods, colorimetry, and turbidimetry, which are time-consuming, susceptible to interference from residual impurities, and destructive. Our objective was to develop novel analytical techniques based on Raman and Fourier-transform infrared spectroscopies to measure the extent of modification in chemically modified non-starch polysaccharides. Xanthan was either carboxymethylated or sulfated to various degrees by treatment with sodium monochloroacetate and sulfur trioxide-DMF complex respectively. Raman spectra were recorded by a renishaw raman imaging microscope equipped with a 514nm Argon ion laser. The laser was focused on the solid polysaccharide samples without any prior sample preparation. Infrared spectra were obtained for samples prepared in a form of potassium bromide discs using a BIO-RAD FTS 165 spectrophotometer. Our results showed that the Raman and IR spectra of different modified xanthan samples showed characteristic marker bands. For carboxymethylation, both the 1610cm^-1Raman band and 1607cm^-1IR band attributed to the COO^- carbonyl stretch can be selected. Similarly, sulfation gave rise to the new 986cm^-1Raman marker band and an 1138cm^-1 IR band, representing the symmetric SO₂and C-O-S stretches respectively. The calibration curves were constructed using the ratio of the intensity of these selected bands to that of an internal standard band (in 800cm^-1 - 900cm^-1region C-C stretch) versus modification levels determined by wet chemistry methods. Linear correlation coefficients (R²) equal to or higher than 0.96 were obtained. The data suggest that simple and efficient analytical methods based on vibrational spectroscopies can be developed to monitor the degree of chemical modification in food polysaccharides.