Control of interferences in the determination of Pt, Pd and Rh in airborne particulate matter by inductively coupled plasma mass spectrometry Original Research Article

The development of methods for the determination of Pt, Pd and Rh (platinum group elements (PGEs)) in environmental samples has attracted considerable interest in recent years due to the spread of these elements into the environment mainly from cars fitted with exhaust catalysts. This work studies the direct determination of PGEs at ultratrace level in contaminated airborne samples by quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS) after mathematical correction of interferences. The interference contribution of HfO+ in 195Pt determination, of Pb2+, ArCu+, SrO+ and RbO+ in 103Rh determination and that of YO+, ArCu+ and ArGa+ in 105Pd determination was studied in order to find the best analyte/interferent signal ratio. The influence of instrumental parameters such as nebuliser gas flow rate and plasma power was evaluated. The results showed that argon gas flow rate cannot be used for interference alleviation and that, in general, a medium plasma power of 1350 W yields the best ratio, although Pb2+ formation is minimised at 1150 W. The detection limits obtained were 0.6, 3.3 and 0.9 ng l−1 for Pt, Pd and Rh, respectively, and the relative standard deviation (RSD) (50 ng l−1, n = 5) was <3%. Analysis of airborne samples collected beside the Madrid M-30 ring road on cellulose filters with PM 10 impactors and filter mineralisation in a microwave oven using aqua regia/HF solutions showed that the PGE content is at the pg m−3 level. Pt can be determined without major errors, the HfO+ contribution being about 10%. Pd determination is hampered by the conjugate factors of very low Pd concentration in these samples and a matrix with a high content of Cu and Y, whose interference contribution is enormous. The contribution of interferences to the Rh content in airborne particles is in the range of 50–75%, and after mathematical correction, the Rh content can be evaluated.