TOC Analyzer Membrane Conductivity Detection

A TOC analyzer is capable of measuring the carbon dioxide formed by the oxidation of organic carbon and acidification of inorganic carbon. The carbon dioxide produced can be measured by two different methods: conductivity and non-infrared dispersion.

Direct Conductivity

A TOC analyzer that makes use of a direct conductivity detector offers the most convenient and compact design available. However, it loses its effectiveness in conductivity measurements of over 50 ppm/C TOC as these are not uniformly proportional to the TOC of the sample and show a lot of variation with the specific species that contain the carbon. The conductivity compensation errors relating to temperature and TOC concentration are not well-known but are very important. As in most cases it is not practical for a TOC analyzer to ascertain all the chemicals in the sample that needs to be analyzed and understand their effects, it is equally unlikely to compensate for errors due to TOC concentration conductivity and temperature.

Membrane Conductivity Detection

Membrane conductivity detection is a variation used for improving the accuracy of analysis in a TOC analyzer. In this method hydrophobic gas permeation membranes are used within the TOC analyzer so that the dissolved carbon dioxide gas passes to the “zero” water in a more ‘selective’ manner for subsequent conductivity measurement. While this technique has been helpful in some ways, membranes are not without their own particular limitations. For example, they provide a location for secondary chemical reactions, which have a tendency by a poorly designed TOC analyzer to display ‘false negatives’, which prove to be far worse than “false positives” in critical applications. Some other problems that are related to membranes include micro leaks, clogging, flow problems, true selectivity and microbial growth. Certain amines are allowed to pass; this increases the conductivity of the water loops.

Most frustrating problem with the use membrane methods is the inability of these to get back to operational performance after an overload occurs that over-ranges the TOC analyzer. Most of the times, it would be hours before a TOC analyzer with membranes returns to reliable service and recalibration. Small variations in pH value are also common contributors to inaccuracy on the data displayed by the TOC analyzer. The interference of the organic material that is not fully oxidized would remain, presenting the error of carbon dioxide detection interference on the part of the TOC analyzer.