Application of double-focusing sector field ICP mass spectrometry with shielded torch using different nebulizers for ultratrace and precise isotope analysis of long-lived radionuclides

Abstract

The capability of double-focusing sector field ICP-MS with a plasma-shielded torch using different nebulizers (a Meinhard nebulizer with a Scott-type spray chamber with a solution uptake rate of 1 ml min^–1; a MicroMist microconcentric nebulizer used with a minicyclonic spray chamber with a solution uptake rate of 0.085 ml min^–1; an ultrasonic nebulizer with a solution uptake rate of 2 ml min^–1; and a direct injection high-efficiency nebulizer with a solution uptake rate of 0.085 ml min^–1) for the introduction of radioactive sample solutions into the ICP was investigated. The total amount of analyte for each long-lived radionuclide (²²⁶Ra, ²³⁰Th, ²³⁷Np, ²³⁸U, ²³⁹Pu and ²⁴¹Am; concentration of each was 1 ng l^–1 in the aqueous solution) using different nebulizers was 5 pg for the Meinhard nebulizer, 0.4 pg for the MicroMist microconcentric nebulizer and 10 pg for the ultrasonic nebulizer. The application of the shielded torch yielded an increase in sensitivity for all these nebulizers of up to a factor of 5 compared with the original configuration without a shielded torch. Sensitivities of about 2000 MHz ppm^–1 were measured for the radionuclides investigated (except for ²²⁶Ra) using the MicroMist microconcentric nebulizer with a shielded torch. The detection limits were in the sub-pg l^–1 range and the precision ranged from 1 to 2% RSD (n=5) for the 1 ng l^–1 concentration level (0.4 pg sample size). A further increase in sensitivity for long-lived radionuclides of nearly one order of magnitude in comparison with the MicroMist microconcentric nebulizer was observed using ultrasonic nebulization, but the amount of analyte required was significantly higher (by a factor of 25). In contrast, the direct injection high-efficiency nebulizer (DIHEN) in double-focusing sector field ICP-MS (DF-ICP-MS) with a shielded torch resulted in a decrease in sensitivity in comparison with the unshielded torch because of a higher water load due to the direct injection of aqueous solution into the plasma. At low solution uptake rates (down to several µl min^–1), the uranium solutions were analyzed by DIHEN-ICP-MS using a double-focusing sector field instrument with higher sensitivity than quadrupole-based ICP-MS. Flow injection was used for sample introduction to measure small sample volumes of radioactive waste solutions (20 µl). The determination of ²³⁷Np at a concentration of 10 ng l^–1 by flow injection DF-ICP-MS was possible with a precision of 2.0% (RSD, n=5). In order to avoid mass spectral interferences and matrix effects long-lived radionuclides (e.g., of U, Th and ⁹⁹Tc) were separated from the radioactive waste matrix by liquid-liquid extraction or ion exchange. The methods developed for the precise determination of the concentration and isotopic ratios of long-lived radionuclides were applied to aqueous standard solutions and radioactive wastes by double-focusing sector field ICP-MS. The precision of Pu isotopic analysis by double-focusing ICP-MS with a shielded torch was 0.2, 2 and 14% for 1000, 100 and 10 pg l^–1 (amount of analyte: 500, 50 and 5 fg), respectively.

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