Direct Determination of Trace Noble Metals (Palladium, Platinum and Rhodium) in Automobile Catalysts by Glow Discharge Mass Spectrometry

Abstract

Most automobile catalysts consist of a CeO₂- and Al₂O₃-rich washcoat deposited on a porous, honeycombed cordierite support block. The active ingredients in the automobile catalyst are several noble metals (usually Pd, Rh and Pt) contained in the washcoat. Rapid, precise and accurate quantification of noble metals in automobile catalysts is desirable for noble metal tracking and accountability. Determination of Pd, Rh and Pt in the catalysts by ID–ICP-MS is possible only after extensive treatment with highly corrosive acids in sealed quartz tubes at high temperatures. By contrast, GDMS permits the direct determination of noble metals in the catalysts without prior dissolution. Although the auto catalyst powders are non-conducting, they may be combined with a metallic binder and analysed. Aluminium powder was chosen as the binder material for its superior gettering abilities, and because the Al mass spectrum in the vicinity of the Pd, Rh and Pt peaks was clear of interferences. Results from the GDMS analysis of noble metals in NIST Standard Reference Material 2557 Used Auto Catalyst (monolith) are precise and accurate to <10% RSD for Pd, Rh and Pt. The current generation of auto catalyst materials also contain significant amounts (≈0.3% by mass) of ZrO₂, which causes isobaric (ZrO)⁺ interference on m/z 106, 107, 108, 110 and 112. A simple mathematical peak-stripping routine was devised to eliminate the (ZrO)⁺ interference.