A standard protocol for NaBH4 reduction of CDOM and HS. (31st March 2016)

• Record Type: Journal Article
• Authors: Schendorf, Tara Marie; Del Vecchio, Rossana; Koech, Kevin; Blough, Neil V.
• Is Part Of: Limnology and oceanography, methods. Volume 14:Number 6(2016:Jun.)
• Page Start: 414
• Page End: 423
• Publication Date: 2016-03-31
• Subjects: Limnology -- Methodology -- Periodicals; Oceanography -- Methodology -- Periodicals; 551.48
• DOI: 10.1002/lom3.10100
• Languages: English
• ISSNs: 1541-5856

Abstract

Abstract: Sodium borohydride (NaBH4 ) selectively reduces ketones/aldehydes to alcohols and quinones to hydroquinones and has been used to reduce these carbonyl moieties within humic substances (HS) and chromophoric dissolved organic matter (CDOM) to gain insight into their contributions to the optical/photochemical properties of these materials. To further investigate the effects of borohydride reduction on the absorption and emission properties of HS and to develop a standard protocol for this reduction, the effects of increasing borohydride concentrations were examined over the course of 96 h. Time courses for the reduction revealed the presence of two kinetically distinguishable pools of carbonyls that were irreversibly reduced. A third pool appears to be reversibly reduced at a low mass excess of NaBH4, but contributes much less to HS absorption. Reduced samples can be adjusted to pH 7 and passed through a size exclusion column (Sephadex G‐10) to remove borate salts, thus producing salt‐free samples that can be used for further analyses. A 25‐fold mass excess of NaBH4 relative to the HS mass produced the largest changes in optical properties without generating excessive amounts of reaction products. Although smaller additional changes are observed at higher mass excesses, the removal of excess borate becomes substantially more difficult and time consuming. While most changes are complete by 48 h, our results suggest that the reaction should be allowed to proceed for 96 h in the dark at room temperature to ensure that additional changes in the optical signals are not observed.