Analysis of Heavy Metal Speciation and Organic Compounds in Soil Using Electrochemical Mass Spectrometry

It is of great significance to analyze the heavy metal speciation and identify the organic compounds in soil. By now, the most of existed methods can not be used for simultaneously analyzing metal-containing compounds and organic components, which require tedious sample preparation and offline separation processes before detection, increasing analytical errors and lengthening analysis time. Herein, based on our previous research, a novel electrochemical mass spectrometry (EC-MS) device was fabricated for online sequential qualitative detection of metal species and organics in soil without sample pretreatment. Firstly, the components were divided into four species, including water soluble, lipid soluble, insoluble and oxidable speciation, and then dissociated by extraction, reaction and electrolysis online. Ultrapure water and CH₃OH were employed as eluents to extract water-soluble and lipid soluble species, respectively. EDTA-2Na (CH₃OH:H₂O=1:1, V/V) solution was used as reactant to dissociate insoluble speciation by chelation. For oxidable speciation, the electrolysis was applied to transform the elements into ions using EDTA-2Na as an electrolyte. Then, the dissociated components were mixed with different charged reagents to form ions for MS detection. For example, the produced inorganic metal ions reacted with EDTA-2Na in situ to form metal-EDTA chelate, the polycyclic aromatic hydrocarbons would chelate with Ag⁺ to form Ag⁺-PAHs. For organic compounds, those were easily ionized, would be charged with CH₃OH directly. Finally, the charged ions and chelates would be transferred for ESI-MS detection in real time under negative and positive ion modes, respectively. The soil sample collected from farmers’ own vegetable fields in the suburb near the roadside was analyzed by EC-MS. The results showed that the soil sample contains water-soluble Mg, Cr, Ni, Zn, Co, Cu, Al, Pb, urea and various organic amines, lipid soluble carboxylic acids, insoluble Ni, Cr, Pb, and oxidable Cu, Pb. The heavy metals in the sample may come from animal and plant waste, the settlement of road dust and atmospheric suspended particles, etc. Amines may come from urine, pesticides, biological tissues, etc. And the presence of organic acids were related to organisms. It indicated the potential of EC-MS for the rapid qualitative species detection of heavy metals and organics in soil. This method can solve the problems, such as low sensitivity, long analysis time, complex pretreatment operation, large sample dosage, and complex operation process, which can provide a strong technical support for environmental assessment and environmental governance.