Determination of ultra-trace level plutonium isotopes in soil samples by triple-quadrupole inductively coupled plasma-mass spectrometry with mass-shift mode combined with UTEVA chromatographic separation

Although triple-quadrupole inductively coupled plasma-mass spectrometry (ICP-MS/MS) has become an attractive technique for the measurement of long-lived radionuclides, the abundance sensitivity, isobaric and polyatomic ions interferences seriously restrict the application. The spectral peak tailing and uranium hydrides (UH+, UH2+) from U-238 have a serious influence on the accurate measurement of Pu-239 and Pu-240, especially for the ultra-trace level plutonium isotopes in the higher uranium sample. A new method was developed using ICPMS/MS measurement in mass-shift mode with collision-reaction gas combined with a chemical separation procedure. As O-2 readily converted Pu+ ion to PuO2+, while disassociated the interfering diatomic ions of interfering elements (U, Pb, Hg, Tl, etc.), the interferences from these elements were completely eliminated if plutonium was detected as PuO2+ at the m/z more than 270. By the mass filter in MS/MS mode combined with O-2 as reaction gas the lower peak tailing of U-238(+) (<5 x 10(-12)) was significantly suppressed. By this way, the (UO2H+)-U-238/(UO2+)-U-238 atomic ratio was reduced to 4.82 x 10(9), which is significantly lower than that of other collision-reaction gas modes. Interferences from Pb, Hg and Tl polyatomic ions were also completely eliminated. Thus, accurate measurement of ultra-trace level 239Pu in high uranium sample solutions with the Pu-239/U-238 concentration ratio of 10(-10) was achieved by the mass-shift mode with 0.15 mL/min O-2/He + 12.0 mL/min He as collision-reaction gas, and high elimination efficiency of uranium interferences up to 10(14) can be obtained by combination with the chemical separation using a single UTEVA resin column. The developed method can be applied to accurately determine the fg level Pu-239 in high uranium samples, such as large-size deep seawater, deep soil and sediment, uranium debris of nuclear fuel.