Comparison of grinding techniques for the determination of potentially toxic elements in botanical samples using ICP-MS

The determination of potentially toxic elements in botanical samples requires extensive sample preparation prior to instrumental analysis. Three grinding techniques were compared for lichen, moss, rhododendron, berberis and pine needles collected from Central Scotland. The samples were washed with deionised water and air-dried, before being ground either by mortar-and-pestle, use of a stainless-steel grinder or a hand crushing method. The ground samples were subjected to microwave-assisted digestion in nitric acid. The digests were analysed by ICP-MS. All three grinding methods produced similar results for As, Cd, Ni, Pb and Zn. For moss, a higher Cr concentration was found with use of the stainless-steel grinder, whilst a higher Cu concentration was produced by hand crushing. Higher Cu levels were also found applying the mortar-and-pestle method to lichen. For Mn, higher levels were found for pine needles with use of mortar-and-pestle, and for rhododendron with the stainless-steel grinder. Overall, the stainless-steel grinding procedure produced the most consistent results.