The typical process now for analysts measuring uranium isotope ratios in environmental swipes involves ashing of samples in a furnace, followed by acid digestion and lengthy chemical separations, which can take up to 30 days from ashing to analysis. These tests are crucial for the detection of undeclared nuclear activities or materials and are conducted by several laboratories worldwide through the International Atomic Energy Agency’s Network of Analytical Laboratories (NWAL), which includes the Department of Energy’s Oak Ridge National Laboratory (ORNL). Now, scientists at ORNL’s Ultra-Trace Forensic Science Center have developed a method that could drastically reduce the time it takes to process these samples, allowing solid environmental swipe samples to be rapidly extracted and sent directly to an inductively coupled plasma-mass spectrometer (ICP-MS) with the click of a button.
ORNL's System extracts a solid from a swipe, ionizes it with a plasma torch and measures the mass-to-charge ratio of its ions with a mass spectrometer. Credit: Jaimee Janiga and Michelle Lehman/ORNL, U.S. Department of Energy
The system involves a microextraction device that uses a wet vacuum technology to mobilize the material from the swipe surface into a flowing solvent of 2% nitric acid (HNO3). After extraction, the sample is sent directly to the ICP-MS device for analysis. The researchers tested this method on a series of swip samples with a varying amount of reference standards and found they could detect as little as 50 picograms of 238U. Additionally, the method provided precise and accurate isotope ratio measurements for 235U/238U, 234U/238U and 236U/238U on uranium reference materials. The research could not only aid in the detection of nuclear materials, but also be used to develop systems for other applications that require direct elemental analysis, said Benjamin Manard, who led the proof-of-concept study. This study was published in Analytical Chemistry.
“It truly is an integrated system,” said Manard. “With just a click of a button, you’re going from a solid sample on a swipe to an isotopic measurement.”
Co-author Shalina Metzger is now leading an approach to place a chromatography column between the microextraction probe and the mass spectrometer, allowing uranium to be eluted first while plutonium is retained for later elution. The researchers also hope to optimize solvent conditions to prevent degradation of the microextraction probe head, and are using ORNL’s 3D-printing facilities to fabricate components that could be more resistant to the extraction solvent, according to Manard.