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, Cs-135 and Cs-137, have entered the environment as a result of anthropogenic nuclear activities. Analytical methods for ultratrace measurement of Cs-135 and Cs-137 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 Cs-135 and Cs-135/Cs-137 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 (Ba-135(+) and Ba-137(+)) and polyatomic interferences ((MoAr+)-Mo-95,97-Ar-40, (SnO+)-Sn-119-O-16, and (SbO+)-Sb-121-O-16). 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 (10(4)-10(7)) 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 Cs-135 and Cs-137 with extremely low detection limits (0.002 pg mL(-1) corresponding to 0.006 Bq mL(-1) Cs-137). 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 Cs-135 and Cs-135/Cs-137 isotope ratio at global fallout source environmental samples was achieved with the ICPMS technique.