Fast chromatographic methods for determining aliphatic carboxylic acids in black liquors

Abstract

Two fast quantitative analysis methods and one qualitative analysis method were developed for the analysis of the low-molecular-mass (LMM) aliphatic carboxylic acids present in alkaline spent liquors from wood and non-wood delignification. The methods were applied to monitor the formation of the acid constituents during laboratory-made kraft cooks. In the first part of the research, a high-performance liquid chromatographic (HPLC) separation method with atmospheric pressure chemical ionization mass spectrometry (APCI-MS) was developed and validated for the analysis of the main LMM aliphatic carboxylic acids in aqueous media. In the second part, the applicability of the method developed was tested for the determination of the most abundant LMM aliphatic mono- and dicarboxylic acids present in softwood (pine kraft and spruce kraft), hardwood (birch kraft and aspen kraft), and non-wood (bamboo soda-AQ (anthraquinone), bamboo kraft, wheat straw soda-AQ, and wheat straw kraft) black liquors. Even more robust and simpler methods than HPLC-APCI-MS might be useful for industrial applications. Ion chromatography (IC) with conductivity detection (CD) offers a versatile, simple, and rapid method for determining a multitude of their key anions. Therefore, an ion chromatographic method with suppressed conductivity detection was developed and validated for the quantitative determination of several LMM aliphatic mono- and dicarboxylic acids as their carboxylate anions together with some inorganic anions (chloride, sulfate, and thiosulfate). To confirm the identification of some carboxylate anions that lack commercial model substances, a complementary qualitative IC method with suppressed electrospray ionization mass spectrometry (ESI-MS) was also developed. The method validation process for IC-CD was carried out with a mixture of carboxylic acids and inorganic anions in aqueous media and in seven different types of wood and non-wood black liquor samples. The validation results clearly indicated that, with respect to selectivity, linearity, limits of detection and quantification, precision, and accuracy, the IC-CD method showed good applicability in the determination of the LMM aliphatic carboxylic acids in black liquors. The developed HPLC-APCI-MS and IC-CD methods are potential tools for the fast determination of the acids for many practical purposes. It has been noticed that there is a clear relationship between the acid formation and the progress of delignification. Therefore, the formation of these acid by-products, which are the main alkaline degradation products of carbohydrates, was investigated. It was observed that a correlation exists between the formation of certain acids and the H-factor. Instead of using absolute amounts, practical information was obtained by plotting the concentration ratios of the selected acid products against the parameters characterizing delignification. This procedure eliminates the difficulty of measuring the absolute concentration (i.e., the use of an internal standard), and means that chromatographic peak areas, being proportional to the concentrations, could be used directly. Acid ratios were measured with HPLC-APCI-MS and ICCD and no significant difference was observed between the results obtained with these two methods. The correlation between the acid ratio and the total yield and kappa number of the pulp was also examined. When the same acid ratios as in the H-factor case were used, the linear correlations were obtained.