Determination of carboxyl groups in wood fibers by headspace gas chromatography

The phase reaction conversion (PRC) headspace gas chromatographic (HSGC) technique was employed to develop a method for the determination of the content of carboxyl groups in wood fibers. Acid treatment of the wood fibers using hydrochloric was applied to convert carboxyl groups to carboxyl acids. Bicarbonate solution is then used to react with carboxyl acids on the treated fibers in the headspace of a testing vial to form carbon dioxide that is analyzed by a thermal conductivity detector using gas chromatography. The conversion reaction was conducted at 60C for about 10 minutes to achieve near complete conversion. The contribution to GC detector signal from carbon dioxide formed by residual hydrochloric acid on wood fibers can be accounted for from the known experimental parameters. The effect of carbon dioxide in the headspace of the testing vial air was calibrated or can be eliminated by purging the testing vial using nitrogen before experiment. The measured contents of carboxyl groups in 8 wood fiber samples were in good agreement with those measured by a titration method. The present method is accurate, rapid, and automated. INTRODUCTION Carboxylic acid groups, COOH, represent the ionexchange capacity of cellulose materials, i.e., the ability to absorb metallic cations during processing and are responsible for wood fiber swelling, conformability, and thus contribute to the bounding of fibers. They also have the ability to improve the adsorption of retention aids in papermaking. The stability and electric properties of paper depend on the amount of metal ions bound by the carboxyl groups on wood fibers. On the other hand, the absorbed cations by the carboxylic acid groups contribute to a discoloration mechanism for fiber and paper during paper drying. These acid groups also play an important role for wood fiber modification since they are fairly reactive and strategic sites for addition and substitution reactions. Finally, they can increase viscosity and decrease fiber solubility of specialty grade dissolving pulps. Therefore, the quantification of carboxyl groups on wood fibers is of paramount importance for both fundamental and applied studies. The traditional methods for quantifying the total carboxylic acid group concentration in wood fibers or polymers are mainly based on either acid-based titration [1-4], or complex titration using EDTA [5]. A detailed comparison of carboxylic acid group content in wood fibers measured by these methods was reviewed by Wilson [6]. It was found that all these methods are not only complicated and time-consuming, but also demonstrated large variance among themselves even when they were conducted within the same laboratory. Headspace gas chromatography (HSGC) has been widely used for analysis of volatile species in complex matrix samples. Many applications based on HSGC have been published in the textbooks [7-9] and review articles [10-12]. HSGC can also be applied to analyze some nonvolatile species that could be converted to volatile specie through chemical reactions. In a previous study [13], we have reported a phase reaction conversion (PRC) HSGC method for the determination of carbonate, a nonvolatile species, in wood pulping spent liquors, based on converting the carbonate in the sample liquor into carbon dioxide through acidification and measured by gas chromatography equipped with a thermal conductivity detector (TCD). METHDOLOGY The phase reaction headspace GC method along with other technical issues have been described and discussed in detail in our previous study [13]. The key is to convert the species of interest in the condensed phase into gas phase that can be analyzed by GC in a headspace. The content of the species of interest in a condensed phase can be determined through mass balance in a one step reaction (1) using eq. (2). rR + bB(condensed) ↔ pP + qQ(gas) (1)