Dissolved Organic Carbon in Oligotrophic Waters: Experiments on Sample Preservation, Storage and Analysis

L. M. Tupas, B. N. Popp and D. M. Karl

Abstract

The determination of the abundance of dissolved organic carbon (DOC) in seawater is dependent on discrete water samples collected from predetermined depths in the water column. We evaluated different methods for the preservation and storage of seawater samples for analysis of its DOC content and recommend a method for determing the running system blank of the high temperature catalytic oxidation (HTCO) instrument.

DOC in oligotrophic ocean water was adequately preserved in polypropylene and polyethylene plastic containers, and glass ampoules under frozen conditions. This method is relatively straightforward, non-contaminating and allows for analysis of chemical constituents other than DOC on the same sample. DOC measurements from frozen were the same as fresh samples analyzed on ship. Filtration of oligotrophic water samples is not necessary before storage.

For the Ionics 555 TOC analyzer, we determined a system running blank of 20 µM C for a 100 µl injection of seawater. This seawater blank must be subtracted from the DOC content of unknown samples. The blank from seawater is not due to sodium, chloride or sulfate ions. Using this method for DOC analysis, we have found surface waters at Station ALOHA to contain about 90-115 µm C. DOC concentrations decreased with depth to about 50 µm C at 500 m and remained relatively constant at greater depths.