BACKGROUNDThe sampling weight can be as low as milligram level when the contents of rare earth elements in rock and soil samples are determined by inductively coupled plasma-mass spectrometry (ICP-MS). However, the size of 200 mesh samples requires 100mg to ensure representativeness, which makes it difficult for ICP-MS to make full use of its advantages of high sensitivity and small sampling volume.

OBJECTIVESTo establish the method for the determination of rare earth elements in ultra-fine rock and soil samples by ICP-MS with microwave digestion.

METHODSA method was developed for the determination of rare earth elements in ultra-fine rock and soil samples by ICP-MS with microwave digestion. The ultra-fine rock and soil samples were prepared by the planetary superfine pulverizing machine with wet grinding. Some parameters influencing the preparation of ultra-fine rock and soil samples, including grinding method, grinding time and the proportion of grinding balls were optimized. Microwave digestion of ultrafine-grained rock and soil samples in a nitric acid-hydrofluoric acid-hydrogen peroxide solution system, the sampling amount was reduced to 5mg, the total acid amount was reduced to 0.25mL and the digestion time was reduced to 25 minutes.

RESULTSThe results indicated that the samples were best refined when the wet-milling was adopted with ethanol as a dispersant and the milling time was 10 minutes and 15 minutes for 200 mesh soil and rock samples, respectively. The material ratios were 7g materials, 500 grinding balls, 45mL dispersants for the preparation of ultra-fine soil samples, whereas 5g materials, 500 grinding balls, 45mL dispersants for the preparation of ultra-fine rock samples. The particle size distribution D₉₅ of soil standard substance GBW07404 and GBW07447 can be as low as 7.51μm and 7.05μm, and the D₉₅ of rock standard substance GBW07104 and GBW07121 can be 8.42μm and 8.30μm under the optimized conditions. Microwave digestion of ultrafine-grained rock sample GBW07104 in a nitric acid-hydrofluoric acid-hydrogen peroxide solution system, the sampling amount was reduced to 5mg, the total acid amount was reduced to 0.25mL, the digestion time was reduced to 25 minutes, and the measured value of rare earth elements was consistent with the certified value with the relative standard deviation is between 1.64% and 5.21%.

CONCLUSIONSThis method was used for the detection of rare earth elements in other ultra-fine standard materials (GBW07404, GBW07447 and GBW07121), which yielded the relative standard deviation of 0.17%-6.60%, meeting the first criterion of Geology and Minerals Laboratory Testing Quality Management Standards.