The molecular level determination of black carbon in marine dissolved organic matter

Abstract

Products from incomplete combustion of organic carbon (black carbon) are ubiquitous in the environment. However, the available information on black carbon in marine dissolved organic matter (DOM), one of the largest organic carbon pools, is extremely scarce. The objective of this study was to introduce an analytical technique for the routine determination of black carbon in marine DOM. Dissolved black carbon was isolated from seawater via solid phase extraction. The salt free extracts were oxidized with nitric acid in a microwave digestion system. Oxidation temperature and duration were optimized to yield maximum amounts of benzenepolycarboxylic acids (BPCAs) from condensed aromatic structures. BPCAs were separated and quantified with high-performance liquid chromatography and diode-array detection (HPLC–DAD). Minimal sample processing and no derivatization are required which makes the method practical for routine analysis. The technique was tested on a suite of reference materials and applied to coastal and open ocean DOM. One half liter of seawater from the open Gulf of Mexico was sufficient for reliable analysis. Black carbon free DOM from algal cultures and organic tissues did not yield detectable amounts of BPCA (<0.02% of total dissolved organic carbon, DOC), whereas all seawater samples yielded significant amounts of BPCA. An algorithm was developed for the transformation of BPCA into black carbon concentrations in the ocean. Black carbon concentrations decreased from near to offshore from 3.7 to 0.8 mM C (2.6–0.9% of DOC). The proportion of the individual BPCAs provided basic structural information on dissolved black carbon. Low proportions of hexacarboxylic acids (B6CA) in marine DOM is consistent with the relatively small size (seven condensed rings) deduced from mass spectrometry data.