Evaporative light scattering: A novel detection method for the quantitative analysis of humic-like substances in aerosols

The chemical composition of organic atmospheric aerosols is only poorly understood. Although a significant fraction of organic aerosols consists of humic-like substances (HULIS), only little is known about this class of compound, and accurate quantification remains difficult, partly due to the lack of appropriate standards. Here, evaporative light-scattering detection (ELSD) was applied for the first time to quantify water-soluble HULIS in aerosol particles smaller than 1 mu m. This detection method was shown to be suitable for the quantification of compounds with unknown structures and lacking appropriate quantification standards. As compared to organic carbon determination of isolated HULIS, no organic carbon/organic mass (OC/OM) conversion factor needs to be applied with ELSD and therefore eliminates this significant uncertainty factor of the OC/OM method, which is frequently used to quantify HULIS. Solid-phase extraction and size-exclusion chromatography were applied to separate inorganic ions and low molecular weight compounds from HULIS before ELSD quantification. The ELSD itself provides an additional separation step where low volatility HULIS are separated from high volatility, small compounds. Electrospray ionization mass spectrometry was used to identify the molecular weight range of the compounds quantified with ELSD. The most intensive peaks were in the range of m/z 200-500, with some masses up to m/z 800. We showed that UV detection using fulvic acid as surrogate quantification standard underestimates the HULIS concentration by a factor of 1.1 to 2.5, which is in agreement with earlier studies. During a 6 week winter 2005-2006 campaign at a suburban site near Zurich, Switzerland, an average of 1.1 mu g/m(3) HULIS was found, which is about 4-6% of the total particle mass smaller than 1 mu m (PM1) and 10-35% of the organic matter in PM1.