Common Methods for Total Organic Carbon (TOC) Analysis in Water

Total Organic Carbon (TOC) is a crucial parameter for assessing water quality, indicating the amount of organic matter present. Accurate TOC measurement is vital in environmental monitoring, drinking water safety, wastewater treatment, and industrial process control. Several well-established methods are commonly employed:

1. High-Temperature Combustion (HTC) followed by NDIR Detection:

• Principle: A measured water sample is injected into a high-temperature combustion chamber (typically 680°C to 1200°C) in an oxygen-rich atmosphere. All carbon compounds (organic and inorganic) are oxidized to carbon dioxide (CO₂).

• Detection: The resulting CO₂ gas is carried by a carrier gas stream to a Non-Dispersive Infrared (NDIR) detector. The NDIR sensor measures the concentration of CO₂ by detecting its specific infrared absorption, which is directly proportional to the Total Carbon (TC) content.

• TOC Determination: To specifically measure TOC, the sample must first be acidified (usually with phosphoric or nitric acid) and sparged (purged with inert gas) before injection. This step removes Inorganic Carbon (IC = carbonates, bicarbonates, CO₂) as CO₂ gas. The remaining carbon measured after acidification/sparging is the Non-Purgeable Organic Carbon (NPOC), which is considered equivalent to TOC for most natural and treated waters. TOC = TC - IC.

2. UV-Persulfate Oxidation followed by NDIR Detection:

• Principle: A measured sample is acidified and sparged to remove IC. Then, a strong oxidizing agent (usually persulfate, S₂O₈²⁻) is added. The mixture is exposed to Ultraviolet (UV) light. The UV radiation generates highly reactive radicals from the persulfate, which rapidly oxidize the organic carbon compounds to CO₂.

• Detection: The generated CO₂ is either diffused across a membrane or carried by a gas stream to an NDIR detector for quantification.

• Characteristics: This method generally operates at lower temperatures than HTC. It's highly effective for dissolved organic carbon (DOC) and is widely used in online/instrumental analyzers. It also requires the IC removal step for TOC measurement.

Key Considerations for Both Methods:

• IC Removal: Accurate TOC measurement always requires the removal or separate quantification of inorganic carbon (IC) when it's present, which is achieved by acidification and sparging in the NPOC approach.

• Sample Preparation: Samples with particulates often require filtration (typically 0.45 µm) to measure Dissolved Organic Carbon (DOC). Homogenization may be needed for samples with significant suspended solids if Total Organic Carbon (including particulates) is desired.

• Detection: NDIR detection is the standard and highly reliable method for quantifying the CO₂ produced in both oxidation techniques due to its sensitivity and specificity.

• Applications: These methods are the backbone of laboratory TOC analyzers and many online/process monitoring instruments used globally by regulatory bodies, utilities, and industries.

In summary, High-Temperature Combustion and UV-Persulfate Oxidation, both coupled with acidification/sparging for IC removal and NDIR detection for CO₂ quantification, represent the dominant and most reliable techniques for measuring Total Organic Carbon in diverse water matrices.