Determination of trace elements in agricultural soil samples by inductively coupled plasma-mass spectrometry: Microwave acid digestion versus aqua regia extraction

Microwave acid digestion and aqua regia extraction techniques were investigated to obtain a simple, rapid and safe method for the determination of eight trace elements in soil (Cd, Co, Cr, Cu, Mn, Ni, Pb, and Zn). Soil samples were collected between 6 October and 3 November 2004, air dried, grinded, sieved (<63 μm) and subjected to digestion. The digests were subsequently analyzed for metal content by inductively coupled plasma-mass spectrometry (ICP-MS). Various combinations and volumes of hydrofluoric, nitric and hydrochloric acids were evaluated for the microwave acid digestion efficiency. Two certified reference materials (CRM 142, light sandy soil, and CRM 143, over fertilized soil) were used in the comparison of these digestion protocols and good agreement between the two procedures and the certified values was found. The aqua regia microwave acid digestion with a mixture of 2 ml of HNO3 and 6 ml of HCl produced the fastest, safest and accurate analytical results with a recovery of 91-110% and a precision better than 5% in most cases. Very low limits of detection were obtained, below 0.05 μg g−1, for all elements, except for Zn, 2.3 μg g−1. A number of agricultural soil samples irrigated by the Tinishu Akaki River (TAR) and its tributaries, Ethiopia, were analyzed.