Basic operating principles of the TOC Analyser

The TOC analyser is used for the measurement organic carbon pollution in water samples from municipal and industrial sources. TOC is a popular analytical technique that forms the basis of many industry standard analytical methods. Carbon can be present in water samples from organic and inorganic sources. Total Organic Carbon (TOC) consists of living organisms, decaying matter and synthetic/organic products such as petrol, coal, plastics, etc. However water can also contain carbon from inorganic sources. Inorganic Carbon (IC) is comprised of carbonates and bicarbonates, dissolved CO₂ elemental carbon, cyanides etc. Carbon is analysed in the TOC analyser by first oxidising it to form carbon dioxide, which is then detected using a non-dispersive infrared (NDIR) detector. There are two types of TOC analyser which use either high temperature catalytic combustion oxidation or UV-persulphate oxidation methods.

High temperature catalytic combustion oxidation TOC analyser

The sample is injected into a furnace operating at 680°C -1000°C. The combustion tube contains a bed of a proprietary platinum are often used to promote better oxidation. This is the preferred method for the analysis of waste water, drinking and surface water, ground water, sea water, and other hard to oxidize matrices. It offers excellent oxidation of all organics, the ability to analyse salt samples and works well with particulate samples. The method does tend to have relatively high background levels making it less effective at low parts per billion (ppb) levels.

UV-Persulfate oxidation TOC analyser

In UV/Persulphate methods, UV light is the main oxidiser but the oxidation power of the reaction is enhanced by the addition of a persulphate compound. The UV/ Persulphate oxidation method is generally best suited for the pharmaceutical and biotechnological industries. This is primarily due to low instrument background and the large sample volumes that may be tested for TOC content. This method offers excellent precision at low ppb levels.

Measurement options

The TOC analyser is a versatile instrument that offers a number of measurement options.

• Total carbon can be measured by placing the sample directly into the analyser without pre-treatment.
• TOC can be measured directly by first removing inorganic carbon by acidification and sparging. The remaining carbon is then measured as TOC. This method, however, also removes purgeable organic carbon (volatile and semi volatile organic materials), but since this generally represents 1% or less of total carbon in a sample, it is considered negligible.
• IC can be analysed in liquid samples by acidifying with an inorganic acid to pH 3 or lower, and then sparging with a stream of inert gas. The acidification converts carbonates and bicarbonates to carbon dioxide, which is then removed along with dissolved CO₂ by the gas stream, and measured to provide an IC value.

TOC can also be measured by difference or by sum. TOC measurement by Difference measures TC and IC individually and the difference between these two measurements is rigorously TOC. TOC measurement by Sum measures nonpurgeable organic carbon and purgeable organic carbon independently. The sum of these measurements is rigorously TOC.

If you would like to learn more, contact a member of the Lambda team today.