Ultratrace and isotope analysis of long-lived radionuclides by inductively coupled plasma quadrupole mass spectrometry using a direct injection high efficiency nebulizer

The direct injection high efficiency nebulizer (DIHEN) was explored for the ultrasensitive determination of long-lived radionuclides (Ra-226, Th-230, Np-237, U-238, Pu-239, and Am-241) and for precise isotope analysis by inductively coupled plasma mass spectrometry (ICPMS). The DIHEN was used at low solution uptake rates (1-100 mu L/min) without a spray chamber. Optimal sensitivity (e.g., U-238, 230 MHz/ppm; Th-230, 190 MHz/ppm; and Pu-239, 184 MHz/ppm) was achieved at low nebulizer gas now rates (0.16 L/min), high rf power (1450 W), and low solution uptake rates (100 mu L/min). The optimum parameters varied slightly for the two DIHENs tested. The detection limits of long-lived radionuclides in aqueous solutions varied from 0.012 to 0.11 ng/L, The sensitivity of the DIHEN was improved by a factor of 3 to 5 compared with that of a microconcentric nebulizer (MicroMist used with a minicyclonic spray chamber at a solution uptake rate of 85 mu L/min) and a factor of 1.5 to 4 compared with that of a conventional nebulizer (cross-now used with a Scott type spray chamber at a solution uptake rate of 1 mL/min). The precision of the DIHEN ranged from 0.5 to 1.7% RSD (N = 3) for all measurements at the 10 ng/L concentration level (similar to 3 pg sample size). The sensitivity decreased to 10 MHz/ppm at a solution uptake rate of 1 mu L/min, The precision was about 5% RSD at a sample size of 30 fg for each long-lived radionuclide by the DIHEN-ICPMS method. The oxide to atom ratios were less than 0.05 (except ThO+/Th+) and decreased under the optimum conditions in the following sequence: ThO+/Th+ > UO+/U+ > NpO+/Np+ > PuO+/Pu+ > AmO+/Am+ > RaO+/Ra+. Atomic and oxide ions were used as analyte ions for ultratrace and isotope analyses of long-lived radionuclides in environmental and radioactive waste samples. The analytical methods developed were applied to the determination of long-lived radionuclides and isotope ratio measurements in different radioactive waste and environmental samples using the DIHEN in combination with quadrupole ICPMS, For instance, the Pu-240/Pu-239 isotope ratio was measured in a radioactive waste sample at a plutonium concentration of 12 ng/L, This demonstrates a main advantage of DIHEN-ICPMS compared with alpha-spectrometry, which cannot be used to selectively determine Pu-239 and Pu-240 because of similar alpha energies (5.244 and 5.255 MeV, respectively).