The selection and application of analytical methods for the measurement of trace amounts of dicarboxylic acids in the air
Van Heerden, Anke
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Carbonaceous aerosol components which consist of organic compounds (OC) and black carbon (BC) account for a large fraction of atmospheric particulate matter. Most information available on the abundance, properties, and effects of these components so far is based on measurement data of total carbon (TC = OC + BC). This data is increasingly complemented by measurements of water soluble organic carbon (WSOC), its macromolecular fraction (MWSOC), and individual organic compounds due to its environmental significance. WSOC are usually highly polar, oxygenated compounds containing two or more COOH, C=O and/or OH functional groups such as hydroxyamines, amino acids, polyalcohols, sugars, dicarboxylic acids, ketocarboxylic acids and dicarbonyls. These compounds contribute to the ability of particles to act as cloud condensation nuclei (CCN) and dicarboxylic acids especially can potentially affect the global climate by scattering incoming solar radiation, which counteracts the global warming caused by the increase of greenhouse gases. According to literature the burning of cellulose (biomass burning) generates smoke particles that were nearly 100% watersoluble. The Vaal Triangle was recently declared as the first priority area in South Africa by the Minister of Environmental Affairs and Tourism on the 21 st of April 2006. The area comprises of heavy industrial activities, one power station, several commercial operations, motor vehicles as well as many households utilizing coal as an energy source. Ambient aerosol sampling for this study was done at 3 sites in the Vaal Triangle (Vereeniging, Vanderbijlpark and Sasolburg) during the winter of 2006 and summer of 2007 with Mini-volume portable air samplers. Aerosol samples were collected on pre-fired quartz filters. Gas and Ion chromatography were applied in analyzing the aerosol filters for specific dicarboxylic acids in the WSOC fraction. However, the GC-MS method required the water extracted samples to be derivatized before injection. This multiple synthesis pathway proved difficult and errors prone with potential dicarboxylic acid loss since the dicarboxylic acids are present in ng/m3 . This meant the GC-MS was only used as a quantitative technique. An alternative ion chromatographic method of analyzing dicarboxylic acids was developed. A new Dionex ICS-3000 RFIC instrument along with its special licensed software (Virtual Column) was utilized. The Virtual Column software makes it possible to simulate possible separations of predetermined individual compounds within the WSOC fraction. The influence and impact of various parameters can be checked without wasting valuable sample. After a method was developed, it was tested practically by analyzing standard solutions. The optimized method was then used to analyze the field samples collected at the different sites. The ICS-3000 RFIC with Virtual Column proved to be a convenient and appropriate technique. It showed that the dicarboxylic acid species oxalic, malonic, succinic, glutaric and phthalic as well as inorganic ions fluoride, chloride, nitrate and sulphate were present in the air of all the sites. The chromatographic profile of all the sites also closely resembled each other, be they residential, industrial or petrochemical. However, the methodology was only developed for qualitative analysis and further studies should develop the method further to include quantitative analysis as well.