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Thermo-optical analysis (TOA) is a quantitative analysis technique that can be used to determine the concentration of elemental carbon in a sample. It is a technique that is currently being considered by the European standardization body (CEN) as a reference method for determining the concentrations of elemental carbon and organic carbon in ambient air environments.
Carbon-based materials make up a significant fraction (and often dominant fraction) of particulate matter in the air and need to be accounted for. The broad composition of carbon in the atmosphere has attracted attention from climate, air pollution, and health research communities, driving the need to establish new official protocols for their analysis.
Not all of the carbon in the air falls into the elemental carbon category - some are made up of organic carbon and inorganic carbon. While many of the other carbonaceous forms are produced by photo-oxidation or polymerization of carbon in the atmosphere, elemental carbon is exclusively from a primary origin. It is often emitted due to the incomplete combustion of carbon-based fuels.
Some of the most common fuel sources include wood and fossil fuels, which are used in houses and industrial processes, and standard forms of elemental carbon include black carbon, soot, and light-absorbing carbon. However, these sub-classes can sometimes overlap, as their light-absorbing and optical properties govern their definitions. This also means that different elemental carbon terms are often used interchangeably.
Thermal, optical analyses rely on the optical behavior of carbon in a particulate sample to distinguish when the elemental carbon thermally separates from the organic carbon in the sample. This is typically carried out by carefully and continuously monitoring the optical absorbance of the sample during analysis. This controlled approach also enables the elemental carbon from oxidizing during the analysis, and it can adjust for the char that forms via a pyrolytic conversion of organics into elemental carbon during the heating process. Below, we’re going to go into more detail about the analysis process.
Performing the Analysis
The analysis is initially performed in an oxygen-free atmosphere, and helium is often the atmospheric gas of choice. The carbon sample is heated in different incremental steps to remove the organic carbon from the sample without affecting the elemental carbon. The final heating cycle sends the temperature of the carbon sample to around 700 °C, where the inorganic carbon species in the sample decompose.
In this heating phase, some of the organic carbon will be transformed into elemental carbon (up to 30%) by pyrolytic conversion, but this can be accounted for by continuously monitoring the sample with a laser (and subsequently, the laser transmission during the conversion process).
The organic compounds in the sample will become vaporized at these temperatures and become oxidized into carbon dioxide. The carbon dioxide is then mixed with the helium gas flow and transported to a methanator oven, where it is reduced into methane and identified using a detector (commonly a flame ionization detector).
The sample is then cooled down (to around 525 °C), and the gaseous environment is changed from a pure helium atmosphere to a 2% oxygen/ 98% helium atmosphere. The temperature is increased again, but this time to 850 °C, and at this point, the elemental carbon (and any pyrolyzed organic carbon) become oxidized into carbon dioxide due to the now oxygenated atmosphere. This carbon dioxide is also converted into methane and detected.
Once all of the carbon in the sample has been oxidized, a known volume and concentration of methane is introduced into the sample oven, and this provides calibration to measure the sample against. Using the laser transmission and detector data, the relative concentrations of both elemental carbon and organic carbon in the sample can be calculated.
Like any technique, there are some limits to the analysis, and there is always an element of error in the calculation. Thermal-optical analyses of elemental carbon typically have a standard deviation error of 4-6%, which equates to between 1 and 15 µg cm-2. To ensure that the accuracy doesn’t fall outside of the ideal values, reference tests can be performed using known concentrations of carbon-based materials (such as sucrose). The lower detection limits of a thermal-optical method are around 0.2 µgcm-2, so if lower detection limits are needed for a specific sample, then trace element analysis methods are required.