Microextraction-TQ-ICP-MS for the Direct Analysis of U and Pu from Cotton Swipes

The microextraction sampling technique was integrated with triple quadrupole─inductively coupled plasma-mass spectrometry (TQ-ICP-MS) to directly sample and measure the isotopic compositions of uranium (U) and plutonium (Pu) from cotton swipes. Once extracted, the U/Pu were directed into the TQ-ICP-MS instrument for isotopic determination. Carbon dioxide (CO₂) and helium (He) gases were delivered to a collision reaction cell within the ICP-MS system for ion separation. The CO₂ reacts with the U⁺ forming UO⁺ which is ultimately separated from the Pu⁺ ions of interest in the third quadrupole. This study demonstrates direct liquid extraction of U/Pu from a solid surface and subsequent measurement by TQ-ICP-MS in <60 s. Flow rates were optimized (0.3 mL min^-1 CO₂ and 5 mL min^-1 He) in the reaction cell of the ICP-MS system to maximize the Pu signal while minimizing U interferences (i.e., ²³⁸U⁺ tail and ²³⁸UH⁺) at m/z 239. Low levels of Pu (∼2 pg) were deposited on a cotton swipe along with U at concentrations ranging from 20 to 200 ng. The ²⁴⁰Pu/²³⁹Pu ratio was measured with <7% relative difference from the certified value at all U concentrations. Major and minor U isotope ratios were also measured with <4% relative difference. This highlights that the microextraction-TQ-ICP-MS method can extract a mixed U/Pu sample directly from a cotton swipe and measure both isotopic systems without chemical separation.