A TOC analyzer can be used for two purposes: oxidation of organic carbon in water to carbon dioxide and measurement of the carbon dioxide produced. The difference between different TOC analyzers lies in the methods used for oxidation and detection. There are three common methods used by TOC analyzers for carbon dioxide detection, two are conductometric methods: membrane conductometric and direct conductometric, and the third is non-dispersive infrared (NDIR).
TOC analyzer that uses a conductometric method can be used for the measurement of carbon dioxide in the liquid phase. The calibration of conductometric detectors is stable and their sensitivity is high. The main difference between membrane and direct detectors is that the latter are susceptible to interference from ionic contamination, halogenated organic material, acids and bases.
In membrane conductivity, the membrane forms a protective barrier to interfering ions, enabling the exclusive analysis of carbon dioxide. Unlike a TOC analyzer that uses NDIR detection, the conductivity detection method displays an extremely stable calibration and is not susceptible to considerable drift over time. This means that the TOC analyzer can be calibrated less frequently without compromising on analytical performance.
In order to make conductometric measurements more stable and reliable, a membrane that passes only the carbon dioxide produced by the oxidation of organics can be used. The prevention of compounds, such as acids and bases from interfering with the carbon dioxide makes membrane conductivity a useful method; however, there are some disadvantages of using membranes as well. For instance, ‘false negatives’ can appear because membranes present secondary sites for other chemical reactions. This can be a far bigger problem than ‘false positives’ in critical applications. Growth of micro organisms is also a potential problem. The worst problem that the user of membrane conductivity TOC analyzer faces is the inability of membrane methods to become operational in the event of an overload or ‘spill’ that over ranges the TOC analyzer. The recovery time is often in hours.
A TOC analyzer with NDIR detector can be used to measure carbon dioxide in the gas phase. The best quality of NDIR is that it is a direct method that specifically measures carbon dioxide. Static Pressurized Concentration (SPC) is a new NDIR technology. In this method, the detector is pressurized, and once the gases contained in the detector reach equilibrium, carbon dioxide concentration is analyzed. A TOC analyzer that utilizes this process measures all of the oxidation products contained in the sample in a single reading.