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

Highlights: • ICP-MS/MS using He-NH₃ as collision-reaction gas was first utilized to measure the trace-level Am in environmental samples. • The detection limit of ²⁴¹Am (0.091 fg/g) using ICP-MS/MS with He-NH₃ is better than conventional ICP-MS. • Polyatomic ions from the elements (Pb, etc.) is effectively removed by collision focusing and chemical reaction. 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 (²⁴¹Pu, ²⁰⁶Pb³⁵Cl, ²⁰⁴Pb³⁷Cl, 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 ²⁴¹Am 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 × 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 ²⁴¹Am 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 (²⁰⁶Pb³⁵Cl, ²⁰⁴Pb³⁷Cl, ²⁰⁸Pb³⁵Cl, ²⁰⁶Pb³⁷Cl, 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 ²⁴¹Am concentrations in seven soil samples collected in different regions of China and one sediment sample collected in Denmark.