A preliminary investigation into the feasibility of laser ablation U-Pb isotope ratio measurement via all-faraday cup detection with 1011- and 1013-Ω amplifiers on the Neoma multicollector-inductively coupled plasma-mass spectrometer.

RATIONALE: Signal detection for uranium-lead (U-Pb) dating of zircon is typically performed via ion counters. Here, we develop a preliminary understanding of the strengths and limitations of faraday-cup-based detection. METHODS: A suite of zircon reference materials and the NIST-610 glass were sampled using laser ablation followed by U-Pb isotope ratio measurement on a Neoma multicollector-inductively coupled plasma-mass spectrometer. RESULTS: We were able to produce geologically accurate 207Pb/206Pb, 206Pb/238U, and 207Pb/235U ratios for the NIST-610 glass and the zircon standards, with ages ranging from ~2.5 Ga to ~337 Ma (TanBrown A, Oracle, 91550, Mud Tank, Temora, and Plesovice). Two of the younger zircon standards examined (94-35, ~55.6 Ma, and Fish Canyon, 28.6 Ma) yielded accurate 206Pb/238U but not 207Pb/235U or 207Pb/206Pb ratios, whereas the youngest zircon standard (Penglai, ~4.4 Ma) failed for all three ratios of interest. The accuracy and precision of the all-faraday method are directly tied to signal intensity, with reliable data capable of being produced even when both isotopes in a ratio have signals below ~0.001 V (equivalent to ~62 500 cps on an ion counter). CONCLUSION: The all-faraday cup multicollection method provides sufficient sensitivity to obtain geologically meaningful U-Pb data, with possible advantages being that laser pit depth-dependent changes in the observed interelemental fractionation behavior may be easier to correct using a static collector configuration compared to when the ion beam is swept across a single detector while also removing the need for an interdetector-type calibration. Further work is needed to refine the all-faraday cup method (e.g., application of background subtraction and common Pb corrections, outlier removal, and interelement as well as down-hole fractionation corrections), but our initial results demonstrate that the faraday detector method has sufficient sensitivity to warrant further study.

Uranium Single Particle Analysis for Simultaneous Fluorine and Uranium Isotopic Determinations via Laser-Induced Breakdown Spectroscopy/Laser Ablation-Multicollector-Inductively Coupled Plasma-Mass Spectrometry.

Uranyl fluoride (UO2F2) particles (<20 µm) were subjected to first-of-its-kind analysis via simultaneous laser-induced breakdown spectroscopy (LIBS) and laser ablation multi-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). Briefly, a nanosecond pulsed high-energy laser was focused onto the sample (particle) surface. In a single laser pulse, the UO2F2 particle was excited/ionized within the microplasma volume, and the emission of light was collected via fiber optics such that emission spectroscopy could be employed for the detection of uranium (U) and fluorine (F). The ablated particle was simultaneously transported into the MC-ICP-MS for high precision isotopic (i.e., 234U, 235U, and 238U) analysis. This method, LIBS/LA-MC-ICP-MS was optimized and employed to rapidly measure 80+ UO2F2 particles, which were subjected to different calcination processes, which results in varying degrees of F loss from the individual particles. In measuring the particles, the average F/U ratios for the populations treated at 100 and 500 °C were 2.78 ± 1.28 and 1.01 ± 0.50, respectively, confirming loss of F through the calcination process. The average 235U/238U on the particle populations for the 100 and 500 °C were 0.007262 (22) and 0.007231 (23), which was determined to be <0.2% from the expected value. The 234U/238U ratios on the same particles were 0.000053 (11) and 0.000050 (10) for the 100 and 500 °C, respectively, <10% from the expected value. Notably, each population was analyzed in under 5 min, demonstrating the truly rapid analysis technique presented here.

Mapping of uranium particles on J-type swipes with microextraction-ICP-MS.

A microextraction liquid sampling system coupled to a quadrupole inductively coupled plasma-mass spectrometer (ICP-MS) was utilized to spatially discern uranium particles, isotopically, on a cellulose-based swipe material (i.e., J-type swipe). These types of swipes are often used by the International Atomic Energy Agency (IAEA) as part of their environmental sampling program. A grid was created such that extraction locations covered the center circle (n = 34 without overlapping). Uranium (U) particulates (<20 µm) of varying U isotopic abundance and chemical form (i.e., uranyl fluoride and uranyl nitrate hexahydrate) were mechanically placed on the swipes in random locations and detected via the microextraction-ICP-MS methodology. Heat maps were subsequently generated to show the placement of the particulate with their respective intensity and isotopic determination. This detection of the uranium particulates, via isotopic determination, agreed with reference values for these materials. Additionally, depleted (235U/238U = 0.002) uranium particulates were placed directly within a clay matrix, on the swipe surface, and subjected to analysis by microextraction-ICP-MS. The mapping of the swipe demonstrated, for the first time, the employment of the microextraction-ICP-MS method for extracting sample from a complex matrix, and correctly identifying the uranium isotopic composition. This example ultimately demonstrates the utility of the methodology for detecting particles of interest in complex matrices.

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.

Determination of fluorine distribution in shark teeth by laser-induced breakdown spectroscopy.

Quantifying the chemical composition of fast-growing hard tissues in the environment can shed valuable information in terms of understanding ecosystems both prehistoric and current. Changes in chemical composition can be correlated with environmental conditions and can provide information about the organism's life. Sharks can lose 0.1 to 1.1 teeth/day, depending on species, which offers a unique opportunity to record environmental changes over a short duration of time. Shark teeth contain a biomineral phase that is made up of fluorapatite [Ca5(PO4)3F], and the F distribution within the tooth can be correlated to tooth hardness. Typically, this is determined by bulk acid digestion, energy-dispersive X-ray spectroscopy (EDS), or wavelength-dispersive spectroscopy. Here we present laser-induced breakdown spectroscopy (LIBS) as an alternative and faster approach for determining F distribution within shark teeth. Using a two-volume laser ablation chamber (TwoVol3) with innovative embedded collection optics for LIBS, shark teeth were investigated from sand tiger (Carcharias Taurus), tiger (Galeocerdo Cuvier), and hammerhead sharks (Sphyrnidae). Fluorine distribution was mapped using the CaF 603 nm band (CaF, Β 2Σ+ → X 2Σ+) and quantified using apatite reference materials. In addition, F measurements were cross referenced with EDS analyses to validate the findings. Distributions of F (603 nm), Na (589 nm), and H (656 nm) within the tooth correlate well with the expected biomineral composition and expected tooth hardness. This rapid methodology could transform the current means of determining F distribution, particularly when large sample specimens (350 mm2, presented here) and large quantities of specimens are of interest.

Direct isotopic analysis of solid uranium particulates on cotton swipes by microextraction-ICP-MS.

Direct isotope ratio analysis of solid uranium particulates on cotton swipes was achieved using a solution-based microextraction technique, coupled to a quadrupole inductively coupled plasma - mass spectrometer (ICP-MS). This microextraction-ICP-MS methodology provides rapid isotopic analysis which could be applicable to nuclear safeguards measurements. Particulates of uranyl nitrate hexahydrate (UO2(NO3)2·6H2O) and uranyl fluoride (UO2F2) ranging from 6 µm to 40 µm in length were transferred to cotton swipes with a particle manipulator. The microextraction probe then delivers a 5% nitric acid (HNO3) solvent onto the swipe surface to extract the uranium species. The extracted sample is then delivered to the ICP-MS for isotopic determination. The majority of uranium signal (∼99% and ∼94% for UO2(NO3)2·6H2O and UO2F2, respectively) was detected in the first 15 s extraction, while subsequent extractions on the same location had low or no U signal, suggesting near complete removal of the solid uranium compounds from the swipe surface. Ten samples (for each of the uranium compounds), were analyzed for their isotopic composition. For UO2(NO3)2·6H2O, the determined isotope ratios resulted in a % relative difference (% RD) from the referenced isotope ratios of 0.97, 1.0, and 7.3% for 234U/238U, 235U/238U, and 236U/238U, respectively. The % RD of the UO2F2 isotope ratios were 1.9 and 0.60% for 234U/238U and 235U/238U, respectively. The preliminary limits of detection were determined to be 0.002, 0.4, and 60 pg for 234U, 235U and 238U, respectively This work demonstrates that microextraction ICP-MS is a rapid and sensitive method that could directly determine uranium isotope ratios of UO2(NO3)2·6H2O and UO2F2 particulates on cotton swipes.

Direct Uranium Isotopic Analysis of Swipe Surfaces by Microextraction-ICP-MS.

The ability to directly measure uranium isotope ratios on environmental swipes has been achieved through a solution-based microextraction process and represents a significant advancement toward the development of a rapid method to analyze international nuclear safeguard samples. Here, a microextraction probe is lowered and sealed onto the swipe surface, and analytes within the sampling site (∼8 mm2) are dissolved and extracted into a flowing solvent of 2% nitric acid (HNO3). The mobilized species are subsequently directed into an inductively coupled plasma-mass spectrometer (ICP-MS) for accurate and precise isotope ratio determination. This work highlights the novelty of the sampling mechanism, particularly with the direct coupling of the microextraction probe to the ICP-MS and measurement of uranium isotope ratios. The preliminary method detection limit for the microextraction-ICP-MS method, utilizing a quadrupole-based MS, was determined to be ∼50 pg of 238U. Additionally, precise and accurate isotope ratio measurements were achieved on uranium reference materials for both the major (235U/238U) and minor (234U/238U and 236U/238U) ratios. While the present work is focused on directly measuring uranium isotopic systems on swipe surfaces for nuclear safeguards and verification applications, the benefits would extend across many applications in which direct solid sampling is sought for elemental and isotopic analysis.

Trace Elemental Analysis of Bulk Thorium Using an Automated Separation-Inductively Coupled Plasma Optical Emission Spectroscopy Methodology.

Presented here is a novel automated method for determining the trace element composition of bulk thorium by inductively coupled plasma-optical emission spectroscopy (ICP-OES). ICP-OES is a universal approach for measuring the trace elemental impurities present in actinide-rich materials; however, due to the emission rich spectrum of the actinide, a separation from the trace elements is warranted for spectrochemical analysis. Here, AG MP-1 ion exchange resin was utilized for retention of the Th matrix, while allowing the trace element impurities to be separated prior to subsequent analysis using ICP-OES. After demonstrating the separation on traditional gravity-driven columns, the methodology was transitioned to an automated platform for comparison. This automated platform utilizes syringe-driven sample and solvent flow and can collect the trace element and thorium fractions in separate locations. While reducing the sample size (500 µL, 1.5 mg of Th), maintaining the overall separation efficiency (recoveries >95%), and illustrating the sample throughput ability (n = 10+), this automated methodology could be readily adopted to nuclear facilities in which the determination of trace elemental impurities in Th samples is warranted.

A preliminary investigation into the use of molecular oxide and hydride secondary ion relationships for improvement of the 236U/238U determination on a NanoSIMS 50L.

A NanoSIMS 50L is used to investigate uranium molecular (235U16O, 236U16O, 238U16O, 235U1H, 238U1H, 236U16O1H, and 238U16O1H) and elemental (235U, 236U, and 238U) secondary ion production during sputtering of synthetic UO2 and the NIST-610 standard to determine if: (1) the 236U16O/238U16O molecular oxide ratio performs better than the 236U/238U elemental ratio, and (2) there is co-variance between the molecular hydrides and oxides. Despite an order of magnitude greater abundance of 236U16O secondary ions (compared to 236U), the 236U16O/238U16O ratios are less accurate than the 236U/238U ratios. Further work is needed before the higher count rate of the 236U16O secondary ion can be used to obtain a better 236U/238U ratio. The second objective was undertaken because correction for the interference of 235U1H on the 236U secondary ion species typically utilizes the 238U1H/238U ratio. This becomes problematic in samples containing 239Pu, so our aim was to understand if the hydride formation rate can be constrained independently of having to measure the 238U1H. We document correlations between the hydride (238U1H and 238U16O1H) and oxide (236U16O) secondary ions, suggesting that pursuing an alternative correction regime is worthwhile.