A method for liquid spectrophotometric measurement of total and water-soluble iron and copper in ambient aerosols

Determination of transition metals in ambient aerosols is important due to their toxicity to human health. However, the traditional measurement techniques for metal analysis are often costly and require sophisticated instruments. In this study, we developed and verified relatively low-cost liquid spectrophotometric methods for the measurements of iron (Fe) and copper (Cu), the two most abundant transition metals in ambient fine particulate matter (PM2.5). For Fe analysis, we utilized a ferrozine-based colorimetric method, which has been frequently used for water-soluble (WS) Fe determination, and further extended this approach for the measurement of total Fe (water-soluble + water-insoluble). In this method, Fe is quantified through the formation of a light-absorbing ferrozine–Fe(II) complex (absorbance at 562 nm). A similar colorimetric method, which forms a bathocuproine–Cu(I) complex absorbing light at 484 nm, was developed and examined for measurement of WS and total Cu. These methods were applied to 24 h integrated filter samples collected in urban Atlanta. Based on PM2.5 ambient aerosols, total and water-soluble Fe and Cu concentrations were in good agreement with inductively coupled plasma mass spectrometry (ICP-MS) measurements (slopes 1.0±0.1, r2>0.89). The water-soluble components, operationally defined as those species in the aqueous filter extract that pass through a 0.45 µm filter, were further characterized by ultrafiltration, which showed that roughly 85 % of both the Fe and Cu in the water-soluble fraction was composed of species smaller than nominally 4 nm.