Dissolved organic carbon methods: a critical review

Abstract

According to the most recent high-temperature catalytic oxidation (HTCO) analyses, the dissolved organic carbon content of seawater can be divided into material oxidizable, and hence measurable, by the standard wet chemical and photochemical methods, and material which is not oxidizable by either chemical or photochemical methods, but can be measured by HTCO methods. Both of these categories contain materials which are biologically extremely labile, disappearing within hours or days in unpreserved samples. A major problem in all methods is preservation of this labile material. As all of the methods are routinely performed, the amount of the labile material included in the measurement is unknown, and may vary between total inclusion, for samples analyzed as soon as they are drawn, to total exclusion, for samples stored at room temperature and analyzed some weeks later. This fraction may make up a third to a half of the total dissolved organic carbon (DOC). A less labile, chemically resistant fraction can be measured as the difference between HTCO and wet oxidation results for seawater below the oxygen minimum layer. This material must be available to some extent to biological, but not to chemical degradation. This requirement sets limits on the possible composition of the material. The most pressing needs for future work in this field are reliable preservation methods, which will allow measurement of the labile materials, suitable primary standards, to permit calibration of analytical systems, and water sufficiently free of organic carbon to be usable in determining instrumental blanks.