Environ. Sci. Technol. 2001, 35, 595-599
Heterogeneity of Platinum Group Metals in Airborne Particles SE ´ BASTIEN RAUCH,* MING LU, AND GREGORY M. MORRISON Water Environment Transport, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden
The emission of three of the platinum group metals (PGMs), Pd, Pt, and Rh, from automobile catalysts and into the urban environment is a potential concern for human health. The analysis of low concentrations of PGMs in air particles is an analytical challenge, and it is demonstrated that interferences in inductively coupled plasma-mass spectrometry (ICP-MS) can be corrected mathematically for Pt and Rh and, at higher concentrations, for Pd. The PM10 concentrations of PGMs in urban air collected in Go¨ teborg are 0.1-10 pg m-3 (Pd), 0.9-19 pg m-3 (Pt), and 0.3-4 pg m-3 (Rh), with higher values for greater traffic intensity. These concentrations provide a general background for the urban atmospheric environment, and the Pt:Rh ratio (4.2: 1) agrees with known automobile catalyst composition. The analysis of filters from 10 parallel samplers indicated the significant heterogeneity of PGMs in samples of urban air particles. Scanning laser ablation ICP-MS provided the µm spatial resolution required to identify individual particles. Ablation across the filter surface demonstrated the presence of relatively few particles with a high concentration of PGMs. These occasional high concentration particles, which might be overlooked in conventional air sampling with total analysis, have a high Pt:Rh ratio (12.3: 1), which can be attributed to catalyst aging during vehicle operation.
Introduction Platinum group metals (PGMs) occur naturally at very low concentrations (1), a situation which is changing with the release of PGMs from automobile catalysts (2-4). Pd, Pt, and Rh are the main active components in automobile catalysts, which are now mandatory on new cars in most developed countries and are being introduced into developing countries. PGM emissions lie in the ng km-1 range and are the result of mechanical abrasion of PGM containing washcoat particles (4). While the automobile catalyst has made an important contribution to automobile pollution control for over two decades, a potential problem appears with the release of PGMs from the automobile catalyst into the environment (2, 3). Environmental concentrations of PGMs have not been shown to directly affect ecosystems or result in direct health risks (5). However, Pt was bioaccumulated by rats exposed to a model substance which resembled Pt containing particles emitted by automobiles (6). Pt salts are among the most potent sensitizers known (7), and Pt was found to react with DNA (8). Further investigation is still needed to determine the human toxicity of Pd and Rh, as relatively high concentrations have been reported for samples of urine (9) and blood (10). * Corresponding author phone: +46 31 772 2123; fax: +46 31 772 2128; e-mail:
[email protected]. 10.1021/es000048c CCC: $20.00 Published on Web 12/23/2000
2001 American Chemical Society
Elevated PGM concentrations have been found in the urban and roadside environment (11-14), although few studies have dealt with PGMs in airborne particles (15-21) and most of these focus on Pt alone. In 1974 the Pt concentration in air was reported to be lower than 0.05 pg m-3 near a freeway in California (15). However, more recent studies in Germany have shown that the total Pt concentration in air along a highway range from 0.02 to 5.1 pg m-3 with Pt mainly present in particles ranging from 0.5 to 8 µm, larger particles having a lower Pt concentration (17). The proportion of soluble platinum in air particles varied from 30% to 43%. A mean Pt concentration of 0.5 pg m-3 has been measured inside Munich city buses and tramways during regular rides, with a clear correlation with traffic density (18). Recently, concentrations of 21.2-85.7 pg m-3 (Pd), 7.8-38.8 pg m-3 (Pt) and 2.2-5.8 pg m-3 (Rh) (21) and of 3.1-15.5 pg m-3 (Pt) and not detected-9.32 pg m-3 (Rh) (20) were reported in Rome and Madrid, respectively. There are severe analytical difficulties with the analysis of PGMs in air particles; low concentrations combined with interferences being the major difficulties. Several techniques have been applied to the determination of Pt in airborne particles including instrumental neutron activation analysis, INAA (19), cathodic stripping voltammetry, CSV (17), and inductively coupled plasma-mass spectrometry, ICP-MS (20, 21). While all three techniques are sufficiently sensitive, they all suffer from interference. INAA determination is prone to spectral interference (19). CSV has proven sensitive for both Pt and Rh with detection limits down to lower ng l-1 or even pg l-1 concentrations but has severe interferences from organic matter (22). ICP-MS provides detection limits down to lower ng l-1 concentrations and can directly analyze Pd, Pt, and Rh, although ICP-MS also suffers from both nonspectral and spectral interferences (23). Nonspectral interference can easily be controlled through the use of an appropriate internal standard, while spectral interference can be avoided by applying a sufficiently high spectral resolution (24), matrix separation (16), or mathematical correction (20, 25). In this article the heterogeneity of PGM distribution in airborne particles collected in urban areas is investigated. PGMs were determined in filters collected with an impactor and constructed samplers at sites with varying traffic intensities. Further, heterogeneity was investigated by the analysis of samples collected simultaneously as well as through the detailed spatially resolved analysis of filters by ICP-MS. Finally, the elemental ratio of Pt/Rh was used to indicate traffic as the major source of PGMs.
Experimental Section Sampling Method and Strategy. PGM concentrations in air particles were measured at six sites in Go¨teborg, Sweden between May and November 1999. Sampling was performed at four sites with heavy traffic intensity (30000-70000 vehicles per day) and one site with low traffic intensity (
