An improved thermal oxidation method for the quantification of soot/graphitic black carbon in sediments and soils

Recent findings have Confirmed the importance of black carbon (BC) in the g lobal biogeochemical cycles of carbon and oxygen through its important cont ribution to the slowly cycling organic carbon (OC) pool. Yet, most BC deter mination methods published to date measure operationally defined BC fractio ns, oftentimes with a high potential for artifacts and a lack of specificit y for one of the two major forms of the BC continuum, soot/graphitic BC (GB C) and char/charcoal BG (CBC). This paper describes a method that reduces t he potential for artifacts to accurately and selectively measure the concen tration of GBC in complex mineral and organic matrixes. Marine and lacustri ne sediments, river sediments, suspended particles, and a marine plankton s ample were first demineralized with a mixture of hydrochloric (HCL) and hyd rofluoric (HF) acids to expose any biochemical entrapped in a mineral matri x. The hydrolyzable organic matter fraction (mostly proteins and carbohydra tes) was then removed with O-2-free trifluoroacetic acid and HCl, after whi ch the non-GBC, non-hydrolyzable OC fraction was finally removed by thermal oxidation at 375 degreesC for 24 h. The specificity of the method for GBC was assessed with pure CBC and GBC samples. Detection limit and GBC recover y,in spiked samples were 10 mg kg(-1) and similar to 85%, respectively. Typ ical GBC concentrations measured in a series of natural samples ranged from < 10 mg kg(-l) in marine plankton to 0.19% in a riverine sample, These con centrations were lower by as much as 3 orders of magnitude than those obtai ned by thermal oxidation without demineralization and removal of hydrolyzab le organic matter. The improvements presented in this work allow for the ac curate and precise measurement of GBC in complex organic and mineral matrix es by eliminating the interference caused by the presence of CBC, residual non-BC OC and minerals, or by the formation of condensation products that c ould account for as much as 4-6% of total OC, Combined to stable and radioi sotope analysis, this improved method should permit quantitative assessment s of the role and dynamics of GBC in the global geochemical cycles of car b on and oxygen.