236U/238U analysis of femtogram 236U by MC-ICPMS
A frontier research lead by the assistant professor Huei-Ting (Tina) Lin of the Institute of Oceanography, NTU, collaborating with distinguished chair professor Chuan-Chou (River) Shen, two post-doctoral researchers, Hong-Wei Jiang and Tsai-Lun Yu of the Department of Geology, NTU, and international researchers, was published by the top journal “Analytical Chemistry” on June 9th, 2021. The breakthrough in measuring the difficult-to-get uranium isotope ratio (236U/238U) of extremely small sample size is expected to significantly progress modern oceanography and a new direction in ocean circulation research.
Tracking the ocean circulation and biogeochemical processes with chemical tracers is a major research direction in chemical oceanography. The chemical tracers are analogous to various dyes dropped in the water. By tracking how the dyes move and being diluted, we can obtain information on the water movement and how the dyes are being utilized.
The natural 236U/238U in the ocean is very low. Since the 1940s, thermonuclear hydrogen bomb tests spent nuclear fuel, and nuclear power plant accidents injected large amounts of 236U into the ocean. The so-called “anthropogenic 236U” input to the surface ocean produced high 236U/238U ratios, making 236U/238U a sensitive tracer for ocean circulation and environmental safeguard monitoring. However, the 236U abundance is about one-billion-time smaller than the 238U. To detect the two isotopes simultaneously is challenging and was only possible about 10 years ago using an accelerator mass spectrometer (AMS) with power up to several million volts. The AMS technique requires a large sample size, long analytical time, and high overall cost, limiting its popularity.
Our team developed a novel technique using a piece of commonly available instrumentation, multi-collector high-resolution inductively coupled plasma mass spectrometer (MC-ICPMS). We can determine atomic 236U/238U ratios in samples with only femtograms of 236U. The major interference, the abundance sensitivity of 238U tail at 236 atomic mass unit, is reduced from 10-6 to 10-10 with the deployment of retarding potential quadrupole lens. We also reduced and corrected other interferences, including the polyatomic interferences from hydride, nitride, lead, and plutonium. We carefully evaluate the non-linear detector behavior.
Future application is to use the MC-ICPMS method on small seawater and coral core samples. In addition, we hope the technique will soon be used to analyze samples from the “Sailing to the Blue Sea” cruised funded by the Ministry of Science and Technology. The high-throughput analyses using MC-ICPMS allow us to obtain a coarse look at the environmental variations. Furthermore, we can submit large samples for AMS analysis for the samples near our detection limit or below.
The research is supported by the Ministry of Science and Technology (MOST) funds, Science Vanguard Research Program of the MOST, Geochemical Research & Services MOST, and the Ministry of Education and the National Taiwan University.
The article is published in Analytical Chemistry, 2021, 93, 8442−8449 and accessible via https://pubs.acs.org/doi/full/10.1021/acs.analchem.1c00409.