Simultaneous quantification of residual lignin and hexenuronic acid from chemical pulps with UV resonance Raman spectroscopy and multivariate calibration

Ultraviolet resonance Raman (UVRR) spectroscopy offers a highly sensitive and fluorescence-free method for the determination of lignin and hexenuronic acid (HexA)directly from unbleached and fully bleached chemical pulps. The UVRR data were calibrated using the partial least squares regression method to obtain good predictive ability for the concentrations of lignin and HexA in pulps simultaneously. The concentration of lignin was predicted fairly accurately (standard error of prediction, SEP 0.6%) from unbleached and fully bleached chemical softwood pulps in the range 0.4-5.6%. The concentration of HexA was more difficult to predict and the SEP was 7.0 mmol/kg. When only oxygen and peroxide bleached pulps were studied, the SEP was 4.4 mmol/kg for the calibration of HexA (range was 0-41 mmol/kg). For unknown samples, a wide-range calibration is first recommended, after which a narrow-range calibration can be used to obtain a more accurate estimation of analyte concentrations.