High-temperature combustion (HTC) is a total organic carbon (TOC) analysis method in which the water sample is injected into a catalytic combustion tube maintained at 680 to 1,200 °C. At these temperatures, all organic and inorganic carbon compounds are oxidised to carbon dioxide (CO₂) and water in the presence of high-purity oxygen or synthetic air as the carrier gas. The CO₂ produced is measured by non-dispersive infrared (NDIR) detection. HTC is defined by Standard Methods 5310B and is standardised in ASTM D7573.
The combustion furnace in an HTC instrument contains a catalyst — typically platinum (Pt) supported on quartz wool, aluminium oxide, or a similar substrate, or copper oxide (CuO) — that promotes complete oxidation at the operating temperature. The carrier gas is oxygen or high-purity synthetic air, which serves as both the oxidant and the sweep gas that carries CO₂ from the furnace to the NDIR detector. Sample volumes are small (typically 20 µL to several hundred µL, commonly 50–200 µL depending on the instrument) and are injected by syringe or autosampler directly into the furnace inlet.
Because HTC operates at such high temperatures with an oxidising atmosphere, it achieves more complete decomposition of refractory organic compounds than wet chemical methods. Highly condensed aromatic structures (such as humic acids in complex matrices), char particles, microplastics, and certain industrial chemicals that resist persulfate or UV-persulfate oxidation are fully combusted at 680 to 1,200 °C. This makes HTC the method of choice for wastewater, industrial effluent, environmental water with high or variable organic loads, and any sample matrix where completeness of oxidation is in doubt.
Total Carbon (TC) — not just organic carbon — is measured in the combustion step, because inorganic carbonate and bicarbonate are also oxidised at furnace temperatures. To determine TOC, the sample is either acidified and sparged to remove inorganic carbon before injection (giving a result equivalent to NPOC), or TC is measured on the whole sample and Total Inorganic Carbon (TIC) is measured separately by acidification and sparging without combustion, with TOC calculated by difference (TOC = TC − TIC).
The main practical trade-offs of HTC relative to wet chemical methods are higher capital cost, higher operating cost (oxygen gas supply and catalyst replacement), and greater instrument maintenance complexity. For Australian laboratories working primarily with drinking water, pharmaceutical water, or routine environmental compliance samples — where organic loads are moderate and matrices are relatively clean — heated persulfate instruments often provide comparable accuracy at lower cost and complexity. HTC is most justified when sample matrices are complex, refractory organics are expected, or when the highest possible analytical confidence in completeness of oxidation is required.
Key Points
- Combustion furnace at 680–1,200 °C with Pt or CuO catalyst; O₂ or synthetic air as carrier gas
- Most complete oxidation of any TOC method — including refractory organics and complex matrices
- TC is measured; TOC obtained by acidification/sparging (NPOC approach) or TC minus TIC difference
- Higher capital and operating cost than wet chemical methods; requires gas supply
- Method of choice for wastewater, industrial effluent, and samples with unknown or refractory organics
Relevant Standards
- Standard Methods 5310B (high-temperature combustion method for TOC)
- ASTM D7573 (standard test method for total carbon and organic carbon in water by high-temperature catalytic combustion and infrared detection)
- ISO 8245 (water quality — guidelines for TOC and DOC determination, covers all oxidation methods)
- USEPA 415.3 (permits combustion or persulfate-UV oxidation with NDIR or conductimetric detection)
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Frequently Asked Questions
Why does high-temperature combustion give more complete TOC results than persulfate methods?
At 680–1,200 °C in an oxidising atmosphere, all carbon-containing compounds — including refractory organics such as humic acids, char particles, and condensed aromatic structures — are thermally decomposed and oxidised to CO₂. Wet chemical methods (heated persulfate and UV-persulfate) rely on radical chemistry at much lower temperatures, which may not fully break down very stable or high-molecular-weight organic structures. If your sample contains unknown or complex organics, HTC provides the highest confidence in complete oxidation.
Does high-temperature combustion require oxygen gas?
Most HTC TOC instruments require either high-purity compressed oxygen or high-purity synthetic air as both the oxidant and the carrier gas that sweeps CO₂ to the detector. This represents an ongoing operating cost and logistical consideration that wet chemical TOC methods (heated persulfate, UV-persulfate) avoid, since those methods use inert nitrogen or air for sparging only and do not consume oxidant gas during the measurement.
When should I choose high-temperature combustion over heated persulfate for TOC?
Choose HTC when your samples contain refractory organics (complex wastewater, highly coloured environmental water, industrial process streams), when you need to certify completeness of oxidation, or when the organic matrix is unknown or highly variable. For drinking water, pharmaceutical water, and most environmental compliance work with clean matrices, heated persulfate typically provides equivalent results at lower cost and complexity.
What catalysts are used in high-temperature combustion TOC furnaces?
The most common catalysts are platinum (Pt) supported on quartz wool or aluminium oxide, and copper oxide (CuO). Platinum catalysts are highly active but can be poisoned by chloride at high concentrations, which can be an issue for saline or process water samples. Copper oxide and alternative catalyst formulations are used in instruments designed for high-chloride matrices.