Mercury determination in soil by CVG-ICP-MS after volatilization using microwave-induced combustion

Microwave-induced combustion (MIC) was applied for mercury volatilization from soil with subsequent determination by cold vapor generation coupled with inductively coupled plasma mass spectrometry (CVG-ICP-MS). Samples of soil were mixed with microcrystalline cellulose (300 mg), pressed as pellets and combusted in closed quartz vessels pressurized with 20 bar of oxygen. Mercury was volatilized from the sample matrix during combustion and quantitatively absorbed in a suitable solution. The type and concentration of absorbing solution (diluted or concentrated nitric or hydrochloric acids, or even water) as well as the use of a reflux step after combustion were studied. Accuracy was evaluated using soil certified reference materials. Using 0.25 mol L−1 HNO3 as absorbing solution with a reflux step of 5 min the agreement with reference values was better than 95%. The limit of detection was 0.006 μg g−1 Hg (using 300 mg of sample mass) and the residual carbon content for MIC digests was always below 1%. The main advantage of the proposed method is related to the complete separation of the analyte from the sample matrix. Up to eight samples could be simultaneously combusted in only 25 min. Taking into account that only 6 ml of very diluted nitric acid solution (0.25 mol L−1) were used, the proposed MIC method coupled with CVG-ICP-MS can be considered in good agreement with green analytical chemistry recommendations.