Determination of inorganic pollutants in soil after volatilization using microwave-induced combustion

Abstract

Microwave-induced combustion (MIC) was applied for analyte volatilization from soil and subsequent determination of As, Cd and Pb by inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP-OES), and Hg by cold vapor generation inductively coupled plasma mass spectrometry (CVG-ICP-MS). Soil samples (up to 300mg) were mixed with microcrystalline cellulose, pressed as pellets and combusted in closed quartz vessels pressurized with 20bar O2. Analytes were volatilized from soil during combustion and quantitatively absorbed in a suitable solution: nitric acid (1, 2, 4 or 6molL−1) or a solution of nitric (2molL−1) and hydrochloric (1, 2 or 4molL−1) acids. Accuracy was evaluated using certified reference materials of soil (NIST 2709, San Joaquin Soil) and sediment (SUD-1, Sudbury sediment for trace elements). Agreement with certified values was better than 95% (t-test, 95% confidence level) for all analytes when 6mL of a solution of 2molL−1 HNO3 and 2molL−1 HCl was used with a reflux step of 5min. The limit of detection was 0.010, 0.002, 0.009 and 0.012μgg−1 for As, Cd, Hg and Pb, respectively using ICP-MS determination. A clear advantage of the proposed method over classical approaches is that only diluted solution is used. Moreover, a complete separation of the analytes from matrix is achieved minimizing potential interferences in ICP-MS or ICP-OES determination. Up to eight samples can be digested in a single run of only 25min, resulting in a solution suitable for the determination of all analytes by both techniques.