An Evaluation of Actinide Reactivity with CO2, O2, and O2/He Gases using Inductively Coupled Plasma Tandem Mass Spectrometry: Application to Simultaneous Measurement of 241Am/241Pu Ratios in Unseparated Complex Matrices

Accurate actinide measurements are critical within the field of nuclear science. Traditional methods for actinide quantification require time-consuming sample processing prior to analysis. There is a need for rapid analytical techniques that still maintain a high degree of accuracy. In this work, actinide reactivity was assessed for multiple oxygen-containing reaction gases using quadrupole inductively coupled plasma tandem mass spectrometry (Q-ICP-MS/MS) to evaluate actinide analysis in complex sample matrices without analyte-matrix separation. A novel method was developed to measure ²⁴¹Am/²⁴¹Pu in complex sample matrices using an O₂/He reaction gas with no matrix removal or analyte preconcentration. This inline method reduces matrix-derived polyatomic interferences that complicate traditional ICP-MS analyses by mass-shifting to ²⁴¹Am¹⁶O⁺ and ²⁴¹Pu¹⁶O₂⁺, allowing Am and Pu to be mass separated for simultaneous analysis. While mass shifting is efficient, a small portion of Am⁺ (<1.3%) and Pu⁺ (<1.4%) react to form AmO₂⁺ and PuO⁺, respectively. Therefore, a mass balance approach was used, in combination with reactivity determined from ²⁴²Pu and ²⁴³Am standard solutions, to correct for residual ²⁴¹PuO⁺ and ²⁴¹AmO₂⁺. The method was validated by measuring ²⁴¹Am/²⁴¹Pu in Pu isotope standards CRM-136 and CRM-137 (separated in March/April 1970 and February 2022, respectively) in both neat solutions and complex matrices containing diluted soil (NIST SRM 2711a, > 1000 μg·g^-1). Method detection limits of 15.9 and 9.6 fg·g^-1 were determined for ²⁴¹Am and ²⁴¹Pu, respectively, and ²⁴¹Am/²⁴¹Pu ratios were measured with accuracies within <3.5%. This work presents the first direct analysis of ²⁴¹Am/²⁴¹Pu in unseparated complex matrices, advancing the capabilities for rapid actinide measurements.