Aerosols are solid particles or liquid droplets in a gas, mostly air. The particles can be classified according to their size, whereby particles with an aerodynamic diameter of less than 2.5 µm are called fine particulate matter or PM2.5. The atmospheric PM fine may stem from different natural sources, such as sea salt, volcanic activity, spores or pollen, but also from various anthropogenic emission sources, most likely traffic, industrial incineration plants or residential wood combustion (etc.) and can also be formed by secondary reactions in the atmosphere. Due to their small aerodynamic diameter PM2.5 particles may penetrate deeply in to the lung thus health effects may occur. Furthermore, particles may influence the earth’s radiation balance and so affect the climate. That’s why a comprehensive chemical characterization of particulate matter is of growing interest [1].

Modified thermal-optical carbon analysis

Thermal-optical carbon analysis (TOCA) is one standard method for the characterization of PM fine. Collected on quartz fibre filters, PM is subjected to a specific temperature program and organic matter is desorbed and pyrolyzed in four thermal fractions in an inert helium atmosphere. The evolved compounds are oxidized to CO2 and water subsequently by a manganese dioxide catalyst. The formed carbon dioxide is reduced catalytically by a nickel catalyst and molecular hydrogen to methane which is quantified by a flame ionization detector (FID) afterwards. Carbon that can be detected in the first four fractions is referred as organic carbon (OC), where the carbon that can only be detected in a He/O2 atmosphere is referred as elemental carbon (EC) [2].

By a modification of the TOCA instrument a minor part of the not yet oxidized sample gas flow is transferred to a photoionization time-of-flight mass spectrometer [3]. Evolved compounds are ionized softly and can be detected almost fragment free, which provides an insight into chemistry behind OC and results in a fingerprint for each fraction. A deeper description of this analytical method and different ionization mechanisms can be found in the article On-line combustion analysis.