Trace Element Analysis via Open‐Vessel or Microwave‐Assisted Digestion in Calcareous Mediterranean Soils

Abstract

Different digestion methods can be applied for analyzing trace elements in soils. Selection of the most useful and convenient method is critical for the adequate determination of their concentrations in each type of soil to appropriately detect potential environmental pollution. Open‐vessel digestion using a digester block (method 1) and microwave‐assisted digestion (method 2) were compared using a certified reference material (CRM) with similar properties and characteristics to calcareous Mediterranean soils. Both methods were contrasted in terms of accuracy by calculating the recovery of cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) in the CRM and precision of replicate analysis through the relative standard deviation (RSD). Recoveries from the digester block method were slightly higher than recoveries from microwave digestion because the use of perchloric acid (HClO₄), whereas RSD values for microwave digestion were generally lower but with higher measured precision than values obtained by open‐vessel digestion. The digestion methods were compared for analyzing trace elements in calcareous agricultural soils devoted to vegetable crops in the Alicante province (southeast Spain), as a representative area of the European Mediterranean region. Results of the paired‐sample t‐test showed significant differences between both methods for Co, Cr, Cu, and Pb in calcareous Mediterranean soils, whereas regression analysis indicated a good correlation between both methods for Cu, Ni, Pb, and Zn. Consequently, the choice of the digestion method is more relevant for minor elements such as Co and Cr. Microwave‐assisted digestion seems to be the best option to determine most of the trace elements in calcareous Mediterranean soils, because closed‐vessel digestion exhibited lower variability according to the RSD values obtained in the study area, particularly for Co and Cr. Trace element concentrations were in agreement with background levels, except for Cu and Pb in some soils, which seem to be related to anthropic activities.