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

Abstract

Abstract. 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.

Measurement report: Abundance and fractional solubilities of aerosol metals in urban Hong Kong – insights into factors that control aerosol metal dissolution in an urban site in South China

“…The remaining 16 % of Fe and Cu colloidal particles had nominal diameters between 4 nm and 0.45 µm and may be in water-insoluble forms (e.g., Fe and Cu oxides), so they may not be "true" water-soluble species. Hence, analogous to observations made by Yang et al (2021), it is possible that the filterable metal fractions in water extracts in this study contain some metals in waterinsoluble forms with diameters smaller than 0.22 µm that penetrated through the 0.22 µm pore size filters. The last filter piece was extracted via acid digestion for chemical analysis of total metals.…”

“…The exact sizes and distribution of metal colloidal particles in the filterable metal fraction in water extracts in this study are not known. However, Yang et al (2021) recently reported that around 84 % of Fe and Cu colloidal particles that penetrated through 0.45 µm syringe filters had nominal diameters smaller than 4 nm. The remaining 16 % of Fe and Cu colloidal particles had nominal diameters between 4 nm and 0.45 µm and may be in water-insoluble forms (e.g., Fe and Cu oxides), so they may not be "true" water-soluble species.…”

“…in individual aerosol particles. In addition, a recent study by Yang et al (2021) reported that the filterable metal fractions in the water extracts may contain some metals in water-insoluble forms with small diameters that allowed them to pass through the pores of syringe filters. This would result in overestimated metal fractional solubilities, which could explain why some of the data points in Fig.…”

Assessing the Oxidative Potential of Outdoor PM2.5 in Wintertime Fairbanks, Alaska

“…This included total metals, such as magnesium, aluminum, potassium, manganese, iron, copper, zinc, and lead, and water-soluble metals. The latter were prepared by extracting filters in water followed by 0.45 μm pore PTFE filtration then ICP-MS analysis . Both water and methanol-soluble brown carbon (WS and MS BrC, respectively, i.e., light absorption at 365 nm wavelength) based on separate filter punches were determined with UV-Vis spectrophotometry .…”

Variable Valence State of Trace Elements Regulating Toxic Potencies of Inorganic Particulate Matter

“…The determination method is detailed in Text S3 and previous studies. 40,41 2.3. Toxicological Assessment and Data Analysis.…”