Speciation of palladium in plants: Method development for investigating metabolic changes

Abstract

Since the introduction of noble metal based car catalysts in Europe, the three elements platinum, palladium, and rhodium have been a matter of concern in scientific and public discussions. Details about their respective emissions and fate in the environment are discussed in a monography (Zereini and Alt 2000). While in the beginning these discussions were focussing mainly on platinum, this changed in the 1990s due to the replacement of platinum-based car catalysts by palladium-based catalysts. In the meantime, the preferred active material for car catalysts (Pt or Pd) has changed to some extent from year to year, which is obviously due to the actual prices of the different noble metals on the world market. Therefore it is difficult to extrapolate a trend of catalyst based palladium emissions for the future, but it is already clear that the palladium concentration in the environment has increased considerably during the last decade (Schfer et al. 1999, Palacios et al. 2000, Barbante et al. 2001). Moreover, it has been shown by several authors that the bioavailability of palladium to biota, i.e. plants (Schafer et al. 1998) and animals (Artelt et al. 1999, Sures et al. 2001, Sures et al. 2002), is higher compared to that of platinum. In order to understand the relatively high uptake rates and bioavailability of palladium, speciation methods are needed, which allow the separation and identification of the respective species in environmental samples and biomatrices. Unfortunately, up to now only very few publications on palladium species are available. For example, the complexation equilibria of model palladium(II) complexes with biologically important ligands like cysteine and glutathion (Bugarcic et al. 2004) or guanine and guanosine (Zhu et al. 2004) have been measured, the influence of oxalate and acetate on environmental palladium species has been calculated (Wood and van Middlesworth 2004), and for the chemotherapeutic lipoic-acid palladium complex (Antonawich et al. 2004) even bio-medical effects have been investigated in gerbils. But in the latter case no palladium species were directly analysed. The first results on the speciation of palladium in plant material have been published in the last three years (Alt et al. 2002, Weber et al. 2004, Lesniewska et al. 2004). One reason for the lack of speciation methods for palladium lies in the fact that chemical analysis of palladium is even more difficult than that of platinum. Namely the use of the very sensitive ICP-MS is complicated by spectral interferences on all palladium isotopes. These interferences have to be removed by special sample preparation (Rauch et al. 2000, Hann et al. 2001, Bencs et al. 2003). Also adsorptive stripping voltammetry, which is successfully used for platinum (Hoppstock et al. 1989), is no alternative for palladium, because the detection limits are much higher. One solution to this problem was the introduction of a TXRF based method for reductive preconcentration and ultratrace determination of palladium (Messerschmidt et al. 2000), which allows a very selective determination of palladium with a detection limit in the low pg-range. This method was also the basis of the first investigations of palladium species in plants (Alt et al. 2002, Weber et al. 2004), which will be discussed in more detail in the following chapters.