Total Organic Carbon Analyzers (TOC)
Generally, all total organic carbon (TOC) analyzers employ the same basic technique. A liquid sample is initially introduced to an inorganic carbon (IC) removal stage, where acid is added to the sample. At this point the IC is converted into carbon dioxide gas that is stripped out of the liquid by a sparge carrier gas. The remaining inorganic carbon-free sample is then oxidized and the carbon dioxide generated from the oxidation process is directly related to the TOC in the sample.
The analysis methods total organic carbon analyzers use to oxidize and detect the organic carbon may be combustion, UV persulfate oxidation, ozone promoted, or UV fluorescence. With the combustion method, analysis is determined when carbon compounds are combusted in an oxygen-rich environment, resulting in the complete conversion of carbon-to-carbon dioxide. In UV persulfate oxidation, the carbon dioxide is purged from the sample and then detected by a detector calibrated to directly display the mass of carbon dioxide measured. This mass is proportional to the mass of analyte in the sample. Persulfate reacts with organic carbon in the sample at 100 degrees Celsius to form carbon dioxide that is purged from the sample and detected. The ozone promoted method oxidizes the carbon by exposing it to ozone. UV fluorescence is a direct measurement of aromatic hydrocarbons in water. Fluorescence occurs when a molecule absorbs an "excitation" energy of one wavelength to be measured as concentration of the hydrocarbon. This may also be referred to as spectrophotometry or colorimetry.
One method for introducing the sample to the analyzer is via syringe. Another is loop sampling, in which the sample loop introduction system allows repeatable analysis over a wide range of concentrations while avoiding the inherent dead volumes of syringe-based systems. On-line total organic carbon analyzer systems have an analyzer that is mounted in a process line and the sample is introduced via a connection to the process. Vial auto samplers are another way to introduce the sample. The liquid-sample transfer auto-sampler removes specific sample volumes from a standard vial and transfers the sample to the common analysis vessel. A sample carousel is loaded with up to fifty vials and placed in the auto-sampler for unattended analysis. In addition to measuring total organic carbon, total organic carbon analyzers may sometimes be used to detect total carbon, total inorganic carbon, and purgeable and nonpurgeable organic carbon.
One of the most important specifications for total organic carbon analyzers is the measuring range. Carrier gas flow rate, average analysis time and process temperature are important as well. Accuracy and resolution are important to consider also.
User interfaces for total organic carbon analyzers can be analog, with dials, switches and potentiometers; digital, with menus, keypads and buttons; or computer interfaces connected to the analyzers by a serial or parallel or other connection. Displays can be analog meters, numeric or alphanumeric digital displays or video terminal. Outputs from TOC analyzers, aside from computer-readable data, can be analog voltage, current or frequency. Another option is to have the output be an alarm or change in state of switches.
Typical features that may be available for some models of TOC analyzers are measurement of solid samples and display of temperature and resistivity. Other handy features are programmability, self-calibration, data storage options, self-test diagnostics and extreme environment ratings.
