BACKGROUND Hexavalent chromium Cr(Ⅵ) is one of the basic monitoring indicators of soil in construction land. It is of great significance to carry out soil hexavalent chromium monitoring in the prevention and control of environmental pollution. At present, the standard method of hexavalent chromium in soil is flame atomic absorption spectrometry (FAAS). The FAAS method has a high detection limit (0.5mg/kg), and serious matrix effect, and cannot meet the analysis of Cr(Ⅵ) in low-concentration soil samples.

OBJECTIVES To establish a convenient and high sensitivity method for determination of low-concentration hexavalent chromium in soil.

METHODS An ion-exchange-inductively coupled plasma-mass spectrometry (ICP-MS) method was developed to determine the content of hexavalent chromium in soil by extracting hexavalent chromium with alkali solution. The resin content, mixing speed, extraction temperature and extraction time were studied. The measurement results were compared with the flame atomic absorption spectrophotometry (FAAS) method.

RESULTS The results showed that the total dissolved solids (TDS) mass fraction was reduced from 2.4% to 0.17% after the alkaline extraction solution was diluted 10 times and 3.5g of cation exchange resin was added, and the matrix interference was greatly reduced. At the same time, due to the dissolution of hydrogen ions in the ion exchange process, the pH reached a suitable detection range (7.5±0.5). The pretreatment conditions were optimized. When the extraction temperature was 90-95℃, the stirring speed was 300 rpm, and the heating time was 70 min, the extraction effect of Cr(Ⅵ) was the best, and the relative error was -1.7%. The relative standard deviation (RSD) was 3.1%-5.9%, and the average relative error was -3.8% to -1.1%. F test and t test were used to compare the test results of high, medium and low concentration standard substances by ICP-MS and FAAS, and there was no significant difference between the two methods. The method detection limit (MDL) was 0.061 mg/kg.

CONCLUSIONS Since this method adopts dilution, ion exchange, internal standard method, to reduce matrix interference, combined with the high sensitivity and good accuracy of ICP-MS, the method detection limit (MDL) is significantly lower than the detection limit of FAAS method (0.5mg/kg). This method can be used for the determination of low-concentration soil Cr(Ⅵ) samples.