Ultralow Sample Volume Cupric Sulfate Oxidation Method for the Analysis of Dissolved Lignin

A novel cupric sulfate (CuSO₄) oxidation method was developed to enable the analysis of dissolved lignin phenols in small volumes of open ocean seawater (<200 mL for deep ocean) or river water (<17 mL) samples. Dissolved lignin phenols were isolated by automated reversed-phase (octadecyl) extraction from seawater before oxidation, whereas for freshwater samples, oxidation was performed without prior extraction. Optimized reaction conditions using alkaline CuSO₄ at 150 °C effectively limited losses of lignin phenols and suppressed side reactions at 5-100 μg of sample organic carbon. The method yielded up to ∼33% higher lignin phenol concentrations and 24-36% lower acid/aldehyde ratios of lignin phenols than existing CuO oxidation methods. The microscale design (200 μL reaction volume) resulted in extremely low blanks allowing accurate and precise quantification of lignin phenols. A comparison of silica-based octadecyl-bonded sorbents (C18) with copolymer-type sorbents (PPL) indicated that sorbent type affected concentrations and diagnostic ratios of dissolved lignin phenols. For robust intercomparability of measured lignin phenol concentrations and in consideration of method detection limits for quantification, a constant sample organic carbon content of ∼30 μg C and the addition of 150 μg ascorbic acid-C are recommended. Small sample volumes avoid time-consuming extraction steps in the field, and the simplified oxidation and sample processing procedures make the new method ideal for high-throughput analysis of dissolved lignin phenols in marine and freshwater environments.