Water Analysis: Organic Carbon Determinations

Abstract

Abstract The determination of total organic carbon and dissolved organic carbon (TOC and DOC) is one of the most important parameters in water quality and environmental analysis today. TOC is composed of particulate organic carbon, DOC and volatile organic carbon, (POC, DOC and VOC). In most waters, DOC is the dominant component of TOC. POC consists of living and nonliving organic particles and can occasionally become quantitatively important. POC, DOC and VOC are operationally defined by the methods used to separate them. POC and DOC are usually separated from each other by filtration, which can introduce errors due to contamination and filter clogging. Other major errors in TOC and DOC analyses are related to contamination and losses during sampling, sample storage and sample manipulation. In particular, removal of inorganic carbon by either acidification with sparging or by acidification with evaporation to dryness results in variable loss of VOC. Commonly used TOC and DOC methods fall into two approaches: wet oxidation and high‐temperature combustion (HTC). In both approaches, organic matter is oxidized to CO 2 , which is then usually determined by nondispersive infrared (NDIR) absorbance. Two wet oxidation methods are in common use: wet chemical oxidation (WCO) and ultraviolet oxidation (UVO). In WCO, a strong chemical oxidant, usually persulfate, is added to the aqueous sample and the digestion is usually carried out batchwise in a reactor at an elevated temperature. In contrast, UVO digestion is commonly carried out in a quartz coil surrounding a mercury vapor lamp. Thus, UVO methods can be readily automated using flow‐injection systems. Two types of HTC methods are currently in use: dry combustion and direct aqueous injection. In dry combustion methods, the sample is acidified, dried and the residue is combusted at high temperature in a sealed tube, usually in the presence of a catalyst. This method is advantageous for the analysis of large samples. In direct aqueous injection HTC methods, samples are injected into a high‐temperature column (600–900°C), which usually contains a catalyst. In recent years, the latter method has become widely used for the analysis of seawater and other waters because it appears to have a greater oxidation efficiency and precision than most of the other methods, and it can be readily automated. However, automated HTC instruments have problems in the analysis of saline samples, in particular, salt deposits in the sample injection system, memory (or carry‐over) effects, and system blank evaluation. Recently, improvements in the HTC injection system and column design have addressed these problems.