22. Melting Point Indicator for Calibrating Thermal/Optical Carbon Analyzers

The thermal/optical method has been widely used for analyzing carbon content in ambient and source particulate samples. Particles deposited onto filter substrates (e.g., quartz, glass, or aluminum) are heated to designated temperatures in either inert or oxidizing environment, and carbon evolved is converted to or and quantified. The more volatile carbon fraction that evolves at a lower temperature is referred to as organic carbon (OC), in contrast to element carbon (EC) that is oxidized only at a higher temperature. The choice of combustion temperatures is important to the thermal/optical analysis since 1) the partition of OC and EC depends on the temperature protocols and 2) carbonaceous material from various sources evolves or charres at different temperatures due to different molecular structures. The appropriate selection of temperature maximizes information regarding source profiles. In most cases the temperature is determined by a thermocouple placed near the sample. However, due to the inhomogeneity of temperature in the furnace, different thermal properties of the sample and the sensor, and delay response time of the thermocouple, the thermocouple reading may not represent the actual sample temperature nor the ramping rate. Sample temperatures are related to thermocouple temperature using a melting point standard, the quick-drying non-flammable liquid Tempilaq. A Tempilaq coating is spread on the filter, and its change in appereance on melting is detected by monitoring filter reflectance and transmittance. The thermocouple readings are simultaneously registered, under both heating and cooling, and compared with the known Tempilaq melting point temperature. Typical temperature uncertainties, both systematic and statistical, are estimated. Different versions of the thermal/optical carbon analyzer are calibrated.