Determination of natural uranium and thorium in environmental samples by ETV-ICP-MS after matrix removal by on-line solid phase extraction

Abstract

An on-line solid phase extraction method has been developed for the determination of ²³⁸ U and ²³² Th biological certified reference material using inductively coupled plasma mass spectrometry (ICP-MS). Absolute detection limits were 2.7 pg and 3.1 pg for the determination of ²³⁸ U and ²³² Th respectively, both being blank limited. The result for the determination of ²³⁸ U in NASS-4 Open Ocean Sea Water was 2.13±0.28 ng ml ^–1 compared with a certified value of 2.68±0.12 ng ml ^–1 . The results for the determination of ²³⁸ U in SLRS-3 River Water was 0.043±0.002 ng ml ^–1 compared with an indicative value of 0.045 ng ml ^–1 . Results for the determination of ²³⁸ U and ²³² Th in NIST 1575 Pine Needles were 14.6±3.4 ng g ^–1 and 28.3±4.5 ng g ^–1 respectively compared with certified values of 20±4 ng g ^–1 and 37±3 ng g ^–1 , using a dry and wet ashing sample preparation method. Results for the determination of ²³⁸ U and ²³² Th in NIST 1566a oyster tissue were 121±21 ng g ^–1 and 29±8 ng g ^–1 for ²³⁸ U and ²³² Th compared to certified and indicative values of 132±12 ng g ^–1 and 40 ng g ^–1 , using the same method. When a lithium metaborate fusion method was used, results for ²³⁸ U and ²³² Th were 23.3±2.0 ng g ^–1 and 36.2±5.6 ng g ^–1 respectively in NIST 1575 Pine Needles. The application of electrothermal vaporisation ICP-MS (ETV-ICP-MS) to NASS-4 Open Ocean Sea Water gave 2.81±0.54 ng ml ^–1 and SLRS-3 River Water 0.045±0.004 ng ml ^–1 for ²³⁸ U. When the fused NIST 1575 samples were analysed using ETV-ICP-MS, results for ²³⁸ U and ²³² Th were 19.5±1.7 ng g ^–1 and 38.8±2.2 ng g ^–1 respectively. Absolute detection limits for ETV-ICP-MS were 30 fg and 9 fg for ²³⁸ U and ²³² Th respectively, both being blank limited.