TOC Analyzer Techniques

One of the techniques that a modern TOC analyzer uses to measure total organic carbon is the conductivity method. There are two techniques used to measure conductivity, direct and membrane.

Direct conductivity is a cheap and simple means of analyzing carbon dioxide. TOC analyzers that use this method usually have fast analysis time. Membrane conductivity is a more robust measurement technique than direct conductivity, but the time taken for analyzing the data makes it a slow one.  In both methods, the sample is analysed before and after the oxidizing process; the TOC analyzer relates the difference in both measurements to the TOC of the sample. The oxidization of the sample generates carbon dioxide and other gases. The dissolved carbon dioxide forms a weak acid; this acid formation creates a change in the conductivity of the initial sample, which is proportional to the TOC in the sample.

TOC analyzers that use the conductivity analysis method supposed that nothing except carbon dioxide is contained within the solution. In situations where this supposition is true, the calculation of TOC made by this differential measurement is valid. However, depending on the chemicals contained in the sample and their individual oxidation products, a positive or a negative interference to the actual TOC value may be present; this interference induces an analytical error in some TOC analyzers. Some species that cause interference are Cl-, HCO3-, SO32- and H+. Some TOC analyzers manufacturers have managed to solve this problem through the use of their proprietary methods.

The membrane conductivity TOC analyzer is an attempt to improve the direct conductivity method. In this approach, hydrophobic gas permeation membranes are used in an effort to make carbon dioxide gas passage more ‘selective’. This advancement in conductivity technology has helped some TOC analyzer manufacturers in resolving some issues; however, membranes have limitations as well. The use of membranes by some TOC analyzers creates problems, such as true selectivity, clogging, and they also accommodate chemical reactions, which have a tendency of displaying ‘false negatives,’ something that is significantly worse than “false positives” in critical applications. Some TOC analyzers pose problems like blockage, micro leaks and dead spots can also occur due to the use of membranes. The most disconcerting aspect of the use of membrane conductivity TOC analyzer is the recovery time needed after the instrument gets over-ranged due to overloading or “spilling”.