In the field of nuclear safety and radiological environmental monitoring, the actinides (An), namely thorium, uranium, plutonium and americium need to be monitored. Given low levels of actinides in the environment, the analysis of those elements involves time consuming and expensive chemical separation techniques, to avoid interferences during measurements, prior to mass spectrometry or alpha spectrometry. In a situation following an accidental release of actinides in the environment, an optimized turnaround time is required to detect and identify actinides. In such cases, routine techniques are not appropriate.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS) equipped with a Collision Reaction Cell (CRC) minimizes the sample preparation steps required for the chemical extraction of target elements. The addition of a reaction gas in the CRC helps to resolve spectral interferences. However, even with the use of the CRC technology, some interferences could persist, primarily due to the recombination of matrix elements with oxygen, hydrogen, argon, and others.

In the last decade, the introduction of tandem quadrupole mass spectrometry (ICP-MS/MS) was a major breakthrough. The addition of a mass filter ahead of the CRC allows a better control of reactions occurring in the CRC, considerably improving the direct resolution of polyatomic and isobaric interferences, thereby minimizing the need of upstream chemical separation steps.

In this work, we present the capabilities of the ICP-MS/MS to resolve spectral interferences during multi-elemental analysis. First, synthetic solutions spiked with actinides of interest and interfering elements were analyzed. The use of a combination of oxygen and helium in the CRC, to mass shift actinides to AnO+ or AnO2+, for spectral interferences removal was studied. Results of the reaction yields will be presented.

Additionally, the analysis of environmental samples (e.g., soil and sediment) prepared by alkaline fusion followed by beads dissolution in nitric acid and direct ICP-MS/MS analysis was performed. Results were compared to alpha spectrometry and ICP-MS/MS measurements, after chemical separation for analyte isolation. This comparison highlighted that the optimization of the CRC parameters is crucial to efficiently remove all interferences during isotopes measurements. Particular attention was paid to the “Wait Time Offset” of the ICP-MS/MS that was found to drastically improve the accuracy in the case of multi-elemental analysis. The increase of the residence time of ions in the collision cell, combined to the short settling time of the two quadrupoles, is thought to be responsible of the overestimation of low-concentration actinides when a major isotope (in this case 238U) is detected just beforehand. These aspects will be presented and discussed.