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Summary A direct and rapid method for determining the average molecular weight (MW) and content of lignin and hemicelluloses in spent cooking liquor by high performance size-exclusion chromatography (HPSEC) using a cartridge glass column and 2.5 mM NaOH as an eluent was developed. A Uv290 detector in line with a RI detector was applied to characterise lignin and hemicelluloses, respectively, from liquor during the kraft cooking of grey and black alder wood. During the main delignification stage, the concentration of high-MW wood components in liquor changed through the maximum. The MW values of hemicelluloses in the liquor decreased to DPn = 30 ± 5 and DPw = 55 ± 5. The M̄ n and M̄ w of lignin in spent kraft cooking liquor during the main and final delignification stages did not exceed 3000 and 5000, respectively.

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