Determination of low-molecular-mass aliphatic carboxylic acids and inorganic anions from kraft black liquors by ion chromatography

Abstract

An ion chromatographic (IC) method with suppressed conductivity detection (CD) was developed and validated for the quantitative determination of several low-molecular-mass aliphatic mono- and dicarboxylic acids as their carboxylate anions together with some inorganic anions (chloride, sulfate, and thiosulfate) from kraft black liquors. To confirm the identification of some carboxylate anions which lack commercial model substances, a qualitative IC method with suppressed electrospray ionization mass spectrometry (ESI-MS) was also developed. The separations were performed on an IonPac AS 11-HC anion-exchange column operated at 25 °C within 25 min by a gradient elution with aqueous potassium hydroxide (suppressed CD in the AutoRegen mode) or sodium hydroxide (suppressed ESI-MS in the pressurized bottle mode). In the validation process a mixture of carboxylic acids and inorganic anions in aqueous media and in seven different types of wood and non-wood black liquor samples were quantitatively analyzed by IC-CD. As a result, calibration lines with correlation coefficients of 1.00 for all analytes were achieved at a concentration range from 0.05 to 105 mg L −1. In black liquor samples intra-day ( n = 6) precision values ranged from 0.9 to 5%. Day-to-day ( n 1 = 3) and intermediate precision values were less than 5% for all other compounds except sulfate and thiosulfate. The variability in the thiosulfate and sulfate results is due in large part to the oxidation of sulfide and thiosulfate, respectively. Recoveries were close to 100% with standard deviations less than 8%. Depending of the analyte, the limits of detection and quantification were, respectively, between 1 and 8 μg L −1 and between 3 and 27 μg L −1 for standard compounds in aqueous media and between 6 and 106 μg L −1 and between 14 and 148 μg L −1 for black liquor samples. These 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 determinations described above.