Triple-Quadrupole Inductively Coupled Plasma-Mass Spectrometry with a High-Efficiency Sample Introduction System for Ultratrace Determination of ¹³⁵Cs and ¹³⁷Cs in Environmental Samples at Femtogram Levels

High yield fission products, ¹³⁵Cs and ¹³⁷Cs, have entered the environment as a result of anthropogenic nuclear activities. Analytical methods for ultratrace measurement of ¹³⁵Cs and ¹³⁷Cs are required for environmental geochemical and nuclear forensics studies. Here we report a highly sensitive method combining a desolvation sample introduction system (APEX-Q) with triple-quadrupole inductively coupled plasma mass spectrometry (AEPX-ICPMS/MS) for the determination of ¹³⁵Cs and ¹³⁵Cs/¹³⁷Cs isotope ratio at femtogram levels. Using this system, we introduced only selected ions into the collision/reaction cell to react with N₂O, significantly reducing the isobaric interferences (¹³⁵Ba⁺ and ¹³⁷Ba⁺) and polyatomic interferences (^95,97Mo⁴⁰Ar⁺, ¹¹⁹Sn¹⁶O⁺, and ¹²¹Sb¹⁶O⁺). Compared to the instrument setup of ICPMS/MS, the APEX-ICPMS/MS enables a 10-fold sensitivity increase. In addition, an effective chemical separation scheme consisting of ammonium molybdophosphate (AMP) Cs-selective adsorption and two-stage ion-exchange chromatographic separation was developed to remove major matrix and interfering elements from environmental samples (10–40 g). This separation method showed high decontamination factors (10⁴–10⁷) for major matrix elements (Al, Ca, K, Mg, Na, and Si) and interfering elements (Ba, Mo, Sb, and Sn). The high sensitivity of APEX-ICPMS/MS and the effective removal sample matrix allowed reliable analysis of ¹³⁵Cs and ¹³⁷Cs with extremely low detection limits (0.002 pg mL^–1, corresponding to 0.006 Bq mL^–1 ¹³⁷Cs). The accuracy and applicability of the APEX-ICPMS/MS method was validated by analysis of seven standard reference materials (soils, sediment, and plants). For the first time, ultratrace determination of ¹³⁵Cs and ¹³⁵Cs/¹³⁷Cs isotope ratio at global fallout source environmental samples was achieved with the ICPMS technique