Scanning laser ablation-ICP-MS tracking of platinum group elements in urban particles

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Abstract

While it has now been demonstrated that platinum group elements (PGE) are released from automobile catalysts into the environment, less is known about the form in which they are emitted and transported. Here we show that scanning laser ablation-inductively coupled plasma-mass spectrometry (scanning laser ablation-ICP-MS) can identify and track individual particles released from automobile catalysts present in environmental particulates and sediments. Particles with high PGE concentrations were found in the exhaust of gasoline and diesel vehicles equipped with catalytic converters. The PGE-Ce association in individual particles provides a definitive fingerprinting for tracking catalyst particles in environmental compartments, while relative PGE signal intensity is an indication of the catalyst type. Scanning laser ablation-ICP-MS of road and aquatic sediments revealed a few PGE containing catalyst particles and it was possible to identify catalyst types for the origin of these particles.

Introduction

Automobile related pollution, including emissions of gaseous pollutant and particles, is an important stress to both environment and human health (Degobert, 1995). As a consequence, increasingly stringent regulations have been applied, followed by clean car technologies. Platinum based oxidation catalysts were introduced in the US in the mid-1970s to remove carbon monoxide and hydrocarbon emissions from automobile exhausts (Degobert, 1995, Palacios et al., 2000a). Palladium has also been used to complement or replace Pt in catalysts owing to similar properties and lower market price, although prices have been fluctuating (Degobert, 1995). Modern catalysts for gasoline engines also allow the removal of nitrogen oxides through reduction over Rh and consequently, use Pt and/or Pd combined with Rh, while diesel catalysts only use Pt and/or Pd (Degobert, 1995, Palacios et al., 2000a). It has now been demonstrated that these three platinum group elements (PGE) are released from automobile catalysts with emission rates in the ng km−1 range (Palacios et al., 2000a, Palacios et al., 2000b, König et al., 1992). PGE are emitted mostly as abraded washcoat particles with a low soluble fraction (Palacios et al., 2000a) and size ranging from sub-micrometer to several micrometers (Alt et al., 1993, Gomez et al., 2001, Zereini et al., 2001). As a result, increasing PGE concentrations have been reported in the urban environment (Helmers and Mergel, 1998, Petrucci et al., 2000, Tuit et al., 2000, Rauch et al., 2001, Jarvis et al., 2001), as well as in Greenland remote areas, such as Greenland (Barbante et al., 2001). It is therefore relevant to study the environmental behaviour of these new contaminants in order to assess their potential ecological and human health risk (Rosner and Merget, 2000).

The determination of PGE in environmental samples is fraught with difficulties owing to low concentrations and analytical interferences. Inductively coupled plasma-mass spectrometry (ICP-MS) has become the method of choice for PGE determination in environmental samples with detection limits in the lower ng l−1 range and the possibility to measure simultaneously Pd, Pt and Rh (Barefoot, 1997, Balcerzak, 1997). However, interference, especially from the formation of oxide species, is a severe difficulty when trace concentrations have to be determined (Rauch et al., 2000a, Rauch et al., 2001, Gomez et al., 2000, Köllensperger et al., 2000). Laser ablation (LA) sampling has been developed for the direct analysis of solids by ICP-MS (Russo et al., 1998, Durrant, 1999, Günther et al., 2000) and has recently been used for the quantification of PGE in road sediments, with detection limits in the ng g−1 range and lower oxide interference resulting from the absence of oxygen containing solvent (Motelica-Heino et al., 2001). In addition to quantification, LA-ICP-MS allows micrometer-resolved sampling (Garbe-Schönberg et al., 1997, Watmough et al., 1998, Lochner et al., 1999) and can be used for the multi-elemental analysis of individual sediment grains (Rauch et al., 2000b). The possibility to obtain elemental fingerprints from LA-ICP-MS (Watling et al., 1997, Watling, 1998) has been demonstrated and opens up the possibility for fingerprinting individual particles.

In the present study a tracking procedure for automobile catalyst particles based on the association of PGE with Ce, a major catalyst component (Degobert, 1995, Palacios et al., 2000a), in individual particles in automobile exhaust, road sediments and river sediments is developed and assessed.

Section snippets

Sample collection and preparation

Automobile exhaust particles, road sediments (sometimes termed road dust) and river sediments were analysed in the present investigation. Exhaust particles were collected from a gasoline engine (1.3-l Ford Fiesta Van) and a diesel engine (Seat Ibiza 1.9-l TDI) equipped with exhaust catalysts. The exhaust fumes were administered through two consecutive absorbent solutions (0.2 mol l−1 HNO3), which were filtered through a cellulose ester filter after sampling (Moldovan et al., 1999, Palacios et

Influence of laser ablation and data acquisition parameters

Compared to single point shots, which would require the selection of particles beforehand, scanning laser ablation provides an easier sampling procedure in which particles are selected randomly along the scan line with important parameters being scan speed, spot size and energy. It has already been demonstrated that elemental associations are preferably determined with a small spot size, a low energy and a slow scanning speed (Rauch et al., 2000b). However, analysis of urban sediments shows

Conclusion

PGE were found to occur as washcoat particles in the roadside and aquatic environment. These particles are emitted from automobile catalysts, deposit onto the road surface and are transported to aquatic systems through stormwater. The present study indicates that PGE containing particles remain relatively unchanged through the environment, while only a small fraction of the PGE is transformed into bioavailable species.

High resolution scanning LA-ICP-MS provides a clear association of PGE with

Acknowledgements

The authors would like to acknowledge funding from the European Community under the Environment and Climate Program (contract ENV4-CT97-0518) and from the Swedish Environmental Protection Agency.

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