Inductively coupled plasma mass spectrometric determination of heavy metals in soil and sludge candidate reference materials

The present paper describes the inductively coupled plasma mass spectrometric determination of the total content and the aqua regia soluble fraction of Mn, Cu, Zn, Cd, Hg and Pb in 3 different soil and sludge candidate reference materials (CRM-142R, CRM-143R and CRM-145R) in the framework of a BCR certification campaign. The determination of the transition metals was hampered by a number of peculiar spectral interferences, at least partly induced by the matrix elements Si and Al. A digestion procedure based on HF addition solved this problem. A method to identify polyatom induced spectral overlap, basically consisting of the registration of signal vs. nebulizer gas flow rate plots is discussed. The accuracy of external calibration, using a well-chosen internal standard was evaluated. Generally the accordance with the standard additions method was better than 1%. However, it is shown that, despite the use of an internal standard with a mass number close to that of the analytes, the use of the more time-consuming standard additions method appeared to be indispensable for the determination of Cd and Hg. Finally, very accurate results were obtained as the mean relative difference with the finally certified values for the respective elements was: 0.6% (Mn), 1.6% (Cu), 3.0% (Zn), 2.7% (Cd), 2.7% (Hg) and 0.7% (Pb).