TOC Analyzer Measurement Method
Regardless of the method used for total organic carbon (TOC) analysis by various types of TOC analyzers, most common methods of TOC analysis can be divided into three stages: acidification, oxidation, and detection and quantification.
Acidification
Acidification of a sample is done so that the inorganic carbon (IC) and purgeable organic carbon (POC) can be removed from it. In TC-IC (total carbon-inorganic carbon) analysis the gases are released to the detector for measurement, and in non-purgeable organic carbon TOC (NPOC) analysis the gases are released to air.
Oxidation
Oxidization is the next stage in the process and involves the release of carbon within the TOC analyzer’s chamber cell in the remaining sample in the form of carbon dioxide (CO2) and other gases. Oxidation can be performed with the help of many processes including: High Temperature Combustion and High Temperature Catalytic Oxidation (HTCO).
High Temperature Combustion
The process involves the combustion of samples at high temperature of 1,350o C in an oxygen-rich atmosphere. The total carbon present in the sample is converted to carbon dioxide; it then moves through scrubber tubes to eliminate interferences such as chlorine gas, and water vapor to give a better reading by the TOC analyzer.
High Temperature Catalytic Oxidation
In HTCO, the sample is injected onto a platinum catalyst within the TOC analyzer with the help of a manual or automatic process. The temperature kept for the process is 680o C and the atmosphere is oxygen-rich. Carbon dioxide produced by the process is measured by using a non-dispersive infrared (NDIR) detector by the TOC analyzer.
Detection and Quantification
Detection and quantification is the final and most vital stage of the TOC analyzer’s analysis of total organic carbon. Detection in modern TOC analyzers is commonly performed either by conductivity or non-dispersive infrared (NDIR) method.
Conductivity
Most common TOC analyzers uses either one of two types of conductivity detectors: direct and membrane. Using a direct conductor is a simple and cheap method of measuring CO2. The advantages of this method are that good oxidation of organics takes place, carrier gas is not required, and the parts per billion (ppb) ranges are good. However, the analytical range that you get with this detector is very limited.
Although, the TOC analyzer technology used for membrane conductivity is the same as direct conductivity, it is more robust and analyses slowly. In both TOC analyzers’ analysis methods, conductivity of the sample is analyzed twice i.e. before and after oxidization; the difference between the two measurements is related to the total organic carbon of the sample.