Validation and application of a new microwave-digestion/ICP-MS method for the analysis of trace metals in tree increment cores

Abstract

A method is presented for the routine analysis of eight trace metals (As, Cd, Co, Cr, Cu, Ni, Pb, V) in wood samples of 100–200 mg. The organic matrix is broken down by microwave digestion and the resulting solutions are analyzed by ICP-MS, after appropriate dilution, against an external multi-element calibration line. Detection limits are ~200 ng metal per g of dry wood for Cr and Cu, and as low as 10–60 ng g–1 for the other six metals. Frequent analysis of the Certified Reference Material NJV 94-5 (Wood Fuel) and a series of spike recovery experiments gave chemical yields ranging from 90% for Cr to 103% for V, but only 80% for As. Precision is generally 2–7%, yet better for Cd and Cu (~1%) and somewhat worse for As (~20%). This new method is suitable for the analysis of tree increment cores cut into sections representing 1–5 years of growth and can be readily modified to include additional metals. As a practical application, increment cores of eastern cottonwood (Populus deltoides) from Ohio (USA) were analyzed for the period 1970–2009. Pairwise comparisons of cores taken from the same tree, or from adjacent trees at the same location, illustrate some acknowledged limitations of their use as a temporal record of trace metal loading in soils.