Development of a high-sensitivity inductively coupled plasma mass spectrometer for actinide measurement in the femtogram range

Abstract

A quadrupole inductively coupled plasma mass spectrometer was modified and developed to achieve very low detection limits, down to a few femtograms in the actinide range. This type of performance was achieved through improvements in the interface pumping system of a conventional ICP-MS instrument, resulting approximately in a 3-fold gain in sensitivity, a 3-fold reduction in background noise, a 2-fold reduction in protenated uranium abundance and a 4-fold reduction in detection limits. A further gain in sensitivity and detection limits, by a factor of 7, was obtained using a high-efficiency desolvating nebulizer which further reduces protenated uranium abundance. The detection limit achieved with good reproducibility, using the high-efficiency desolvating nebulizer on standard solutions, is close to 1.2 fg. The performance of the new instrument is discussed and compared with that of radiometric techniques, with particular reference to plutonium, using standard solutions and real samples. The ‘half-life cut off’ between ICP-MS and alpha-spectrometry is close to 650 years. Intercomparison of plutonium measurements from 300 pg down to 35 fg in 16 environmental samples shows good agreement. The plutonium detection limits in real matrices are greater than those in clean samples and vary from 1.5 to 3 fg. The high-sensitivity ICP-MS system gives access to environmental contamination of other long-lived radionuclides, e.g., ⁹⁹Tc, ²³⁷Np and uranium isotopes, which are sometimes difficult to measure at very low levels using conventional techniques.