Determination of ultra-low level 241Am in soil and sediment using chemical separation and triple quadrupole inductively coupled plasma mass spectrometry measurement with He-NH3 as collision-reaction gas

Inductively coupled plasma mass spectrometry (ICP-MS) has been becoming a competitive technique for the measurement of trace americium isotopes, but the isobaric and polyatomic ions interference (Pu-241, (PbCl)-Pb-206-Cl-35, (PbCl)-Pb-204-Cl-37, etc.) will deteriorate the analysis accuracy for soil and sediment samples with high concentration of interfering elements (e.g., Pb). This study developed a novel analysis method to determine Am-241 using ICP-MS with tandem quadrupoles and collision/reaction cell. The interference of isobaric and polyatomic ions was effectively removed by the mass filter of quadrupole and reactions with NH3, with the contribution efficiency of interfering elements at m/z 241 or 243 lower than 1 x 10(-8), and the measurement sensitivity in this mode relatively (1170 Mcps/(mg/L)) higher than in other modes. The detection limit of 0.091 fg/g for Am-241 was achieved, 3 times better than other types of ICP-MS (Q-ICP-MS, SF-ICP-MS, etc.). The collision focusing by He and the chemical reaction with NH3 played an important role in the improvement of Am sensitivity and elimination of polyatomic ions. This study suggested that the presence of Cl- could significantly increase the polyatomic ions interference ((PbCl)-Pb-206-Cl-35, (PbCl)-Pb-204-Cl-37, (PbCl)-Pb-208-Cl-35, (PbCl)-Pb-206-Cl-37, etc.) because of the high Pb concentration in the soil or sediment samples, and thus should be completely removed. The developed method had been validated with two certified reference materials of soil (IAEA-375 and IAEA-Soil-6) and successfully applied to measure Am-241 concentrations in seven soil samples collected in different regions of China and one sediment sample collected in Denmark.

Automated summary

This study developed a novel analysis method using ICP-MS to determine Am-241 with high sensitivity and a low detection limit, by effectively eliminating the interference of isobaric and polyatomic ions. The presence of Cl was found to significantly increase polyatomic ions interference, emphasizing the need for complete removal.