Moisture Management
Control moisture in Total Organic Carbon Analyzers
Total organic carbon (TOC) is a valuable parameter since it is present in every sample that is analyzed by laboratories in the analytical, environmental and pharmaceutical industries. TOC is part of what is referred to as total carbon (TC). TC consists of TOC and inorganic carbon (IC). TOC can be further subdivided into purgeable organic carbon (POC) and non-purgeable organic carbon (NPOC), depending on the type of carbon and solubility in water.
TOC is mainly measured by using either catalytic combustion oxidation or wet chemical persulfate oxidation. Catalytic combustion oxidation involves combusting the sample at a high temperature in the presence of a catalyst, while wet chemical persulfate oxidation involves mixing the sample with a persulfate oxidizer, heated at 80oC, and UV irradiated (1,2). During either process, moisture might be generated from the oxidation process. The oxidation process converts the carbon within the sample to carbon dioxide, which is later detected by either conductivity or non-dispersive infrared (NDIR) detectors.
NDIR detectors are considered more stable detectors than conductivity detectors and, as a result, are more popular. NDIR detectors are less susceptible to the interferences affecting sensitivity and results that afflict conductivity detectors. Interferences that affect the conductivity detectors include pH, temperature, and ions in solution that lead to high recoveries. Also, conductivity detectors are affected by interferences from gases such as chlorine dioxide, sulfur dioxide and other noxious gases.
Fig. 1: Peltier Cooler Schematic
Whether measuring TC, IC, TOC, POC, or NPOC, using an NDIR ensures good reproducibility and accuracy of the data. During the oxidation process, moisture might be generated as mentioned earlier. The generated moisture can contaminate the detector and increase the required maintenance. Moisture can be removed using different dehumidifiers, such as the Peltier cooler and Permeation dryer.
Fig. 2: A Schematic Diagram of the Concept of Permeation Selectivity (magnified)
The Peltier cooler (Figure 1) is sometimes referred to as either the thermoelectric dryer or the cold trap (3). The carrier gas and the sample are passed through a tube that is cooled by the peltier cooling system. The peltier cooler keeps the tube at a temperature of about 1oC. The temperature is controlled automatically by the instrument and is maintained at 1oC to prevent any freezing. As a result, as the carrier gas and sample moves through the tube, water vapor condenses and is removed from the system by dropping into the drain vessel (Figure 1). As no filters or membranes are involved, no clogging occurs, and back flow of the generated moisture can not take place. Consequently, the peltier cooler requires zero maintenance. Such type of dehumidifier is economical in terms of electrical usage, as no temperature adjustment is required. In addition, the peltier cooler encompasses no moving parts. As a result, the dehumidifier generally lasts the lifetime of the analyzer. With the temperature maintained at 1oC, the Peltier cooler is a better moisture removal dehumidifier than, for example, the permeation dryer, which operates at room temperature.
The permeation dryer (Figure 2) uses membranes for moisture removal based on the concept of selective permeation. The gas sample is passed through a tube membrane that is selectively permeable. As a result, the moisture selectively permeates through the membrane while the gas sample passes through to be detected. Such membranes require filtration of the gas sample beforehand to prevent any effects caused by contaminants and moisture. Accordingly, the filters used must be replaced regularly. In addition, clogging of the membrane may occur if the filters do not work properly, which may result in increased maintenance. Replacement of the membrane is difficult to predict, and the end of its life time is usually indicated by poor results.
TOC analyzers consist mainly of three main systems that perform the sampling, oxidation, and detection. In order for such systems to work properly, manufacturers of TOC analyzers must utilize not only the best systems on the market, but also must include in the analyzers the best equipment on the market to ensure the systems operate to their fullest with the least amount of maintenance. With these considerations, the TOC analyzer user will be confident that the obtained TOC value is a true value.�
REFERENCES
TOC-V CPH/CPN and TOC-Control V Software User Manual. Shimadzu Corporation: Process and Environmental Instrumentation Division. Japan, Kyoto, 2001.
TOC-V WP and TOC-Control V Software User Manual. Shimadzu Corporation: Process and Environmental Instrumentation Division. Japan, Kyoto, 2001.
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