Accurate and precise 230Th/232Th isotope ratio measurement by multi-collector inductively coupled plasma mass spectrometry using a pre-cell mass filter for collision/reaction cell (MC-ICP-MS/MS)

The goal of the study is to demonstrate a high-precision and accurate method for measuring ²³⁰Th/²³²Th isotope ratios using a novel MC-ICP-MS/MS system equipped with a pre-cell mass filter and collision/reaction cell. This configuration significantly reduces argon-based ion interference and eliminates the need for tailing correction, enhancing measurement reliability.

By applying this method to certified reference materials IRMM-035 and IRMM-036, the researchers achieved results in excellent agreement with consensus values but with substantially improved precision. This technique opens new possibilities for high-precision thorium isotope analysis in fields such as geochemistry, environmental monitoring, nuclear safeguards, and forensics.

The original article

Accurate and precise ²³⁰Th/²³²Th isotope ratio measurement by multi-collector inductively coupled plasma mass spectrometry using a pre-cell mass filter for collision/reaction cell (MC-ICP-MS/MS)

Zsolt Varga, Maria Wallenius, Klaus Mayer

Talanta, Volume 292, 2025, 127896

https://doi.org/10.1016/j.talanta.2025.127896

licensed under CC-BY 4.0

Selected sections from the article follow. Formats and hyperlinks were adapted from the original.

Thorium (Th) isotope measurements are of great importance in geology, archeometry, life sciences and nuclear applications. The demand for the precise Th isotope analysis is constantly increasing, especially in geosciences: U and Th isotopes are widely used for the determination of erosion and chemical weathering rates, in climate studies as well as in various fields in marine geochemistry or palaeoclimatology [[1], [2], [3], [4], [5]]. Recently, there is high emphasis on the chronometry of young silicate and carbonate rocks for climate research or dating of fresh corals and speleothems in climate reconstruction, where precise dating of samples with low Th content is of vital importance [1,6,7]. Besides the geological applications, in nuclear forensic investigations the production date (also known as the age of the nuclear material) is one of the most important characteristics of sample provenance [[8], [9], [10], [11]]. For this purpose, the most frequently used chronometer for U materials is ²³⁰Th/²³⁴U, which requires precise ²³⁰Th measurements [[12], [13], [14], [15], [16]]. Moreover, in laser ablation investigations the analyte has to be determined from a U-based sample, where U is the major component, often together with several impurities in the U matrix. The elevated tailing requires the reduction of the adjacent, very high intensity ²³⁸U peak, while the impurities present in the matrix constituents entails the use of higher mass resolution [15,[17], [18], [19]].

The reliability and significance of the obtained information depends on the accuracy of these Th isotope measurements, while the interpretation and the applicability is determined by the precision of the measured values. Therefore, the development of novel methods with improved accuracy and increased precision are indispensable. From the 1980s mass spectrometric techniques widely replaced the α-particle counting methods, allowing the improvement of the precision and, at the same time, the extensive reduction of sample size (e.g. Refs. [1,[20], [21], [22]]). Several types of mass spectrometry are frequently used for U and Th isotope determinations in analytical chemistry: thermal ionization mass spectrometry (TIMS), single collector high-resolution inductively coupled plasma mass spectrometry (ICP-MS), multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and recently, triple quadrupole ICP-MS instruments or inductively coupled plasma time-of-flight mass spectrometry (e.g. Refs. [6,7,[22], [23], [24], [25]]). In the last years, MC-ICP-MS turned out to be the most widely used instrument for the accurate and precise measurements of actinides due to the improved ionization efficiency, high sensitivity, low sample need and high precision [5,7,21]. However, using an MC-ICP-MS also has some disadvantages compared to TIMS, such as a higher background, the occurrence of potential spectral interferences (e.g. instrumental mass biases or molecular interferences) and a lower abundance sensitivity. Such effects need exhaustive evaluation and the measured data must be corrected accordingly.

The overall performance of the MC-ICP-MS/MS technique was already proposed for several fields, including extreme low ²³⁶U/²³⁸U isotope ratio measurement, Ca, K, Si analysis or in situ Rb/Sr dating by laser ablation [30]. The system was also applied effectively in practice for the uranium isotope measurements of certified reference material solutions [32], or for the direct analysis of UO₂F₂ particles [33] and in situ Rb–Sr geochronology of biotite analysis by laser ablation coupled to a MC-ICP-MS/MS instrument [34].

We are currently exploring the capabilities of the MC-ICP-MS/MS Neoma™ instrument installed in JRC-Karlsruhe, predominantly for applications in the areas of nuclear safeguards and forensics. The present paper aims at developing a reliable method for the precise and accurate measurement of the ²³⁰Th/²³²Th isotope ratio, in order to establish the technique for the age dating in nuclear forensics by the ²³⁰Th/²³⁴U chronometer in the future. Beyond that, it can also be applied in other scientific disciplines where the ²³⁰Th/²³²Th isotope ratio plays an essential role (geology, archeometry or chronometry).

2. Experimental

2.1. Instrumentation

The mass spectrometric analysis was carried out using a Thermo Fisher Scientific™ (Bremen, Germany) Neoma™ multi-collector inductively coupled plasma mass spectrometer with the pre-cell mass analyser option (MC-ICP-MS/MS), installed in JRC-Karlsruhe in 2024. The MC-ICP-MS/MS instrument is connected to a glove-box for future nuclear applications. In order to do that, the plasma extraction lens area had to be re-designed by Thermo Fisher Scientific™ and an additional extraction lens had to be inserted due to the physical thickness of the glove-box and the modified ion trajectory. Thus, some parameters here are to some extent different from the stand alone Neoma™ MC-ICP-MS/MS. The analysis was performed with a Ni Jet sample cone in combination with a Ni X skimmer cone. All measurements were carried out in either in low-resolution (R ∼ 2100) or medium-resolution mode (R ∼ 7020): these are the standard (built-in) mass resolutions in this model. The ion transmission in medium-resolution mode was somewhat higher than 25 % compared to the transmission observed in low-resolution mode. No collision/reaction gas was added to the cell during these measurements, only the pre-cell mass filter was used. The samples were introduced in the plasma using an Apex-Ω HF membrane desolvator (Elemental Scientific, Omaha, US) having a low-flow Teflon® microconcentric nebulizer operated in a self-aspirating mode (the flow rate was approximately 100 μL min⁻¹).

The detector configuration of the Neoma™ MC-ICP-MS/MS in JRC-Karlsruhe is shown in Fig. 1. The instrument is equipped with 12 F cups (10 moveable and 2 fixed) and 3 discrete dynode secondary electron multipliers (SEMs) operating in pulse counting mode. The SEMs are placed at the low-mass side of the detection system designed for nuclear applications. Note the detector SEM1 can measure the incoming beam in two ways: either when the ion beam passes between the center cup and L1, or when it passes between L4 and the low fixed cup. The signal can be directed with a potential either to SEM2 or to the low fixed cup in case of a higher signal.

Talanta, Volume 292, 2025, 127896: Fig. 1. Detector configuration of the Neoma™ MC-ICP-MS/MS installed in JRC-Karlsruhe. The red lines show the ion trajectories reaching the SEMs. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

2.5. Data evaluation

Initial calculations (instrument tuning, background subtraction, outlier test, ratioing) was carried out with the built-in Neoma™ software (Qtegra Intelligent Scientific Data Solution, ISDS), while the final calculation (mass bias correction) was performed externally by MS Excel®. For the mass bias calculation, the exponential isotopic fractionation model was applied [36]. The given uncertainties are expanded uncertainties, expressed with a coverage factor of k = 2, (corresponding to a 95 % confidence level) throughout the present work. The final, overall results of the IRMM-035 and IRMM-036 samples were calculated using the Student's t-distribution (N = 10, so degree of freedom is 9) from the replicates. The IRMM-035 and IRMM-036 samples were measured on two separate days.

3. Results and discussion

3.1. Abundance sensitivity of MC-ICP-MS/MS

The abundance sensitivity (tailing effect) was investigated at m-2 mass (i.e. around m/z = 230) by measuring the IRMM-036 standard (its approx. concentration was 60 ppb) and scanning over the mass range of 229.6–231.4 with the SEM1, while monitoring the ²³²Th signal with the L4 Faraday simultaneously. The ²³²Th signal was ca. 3.5 × 10⁹ cps (<60 V), about 60 % of saturation value. It was high enough to maximise to signal-to-noise ratio, but also well below the saturation value. The scanning was performed with a mass step of 0.0176 m/z, measuring each data point for at 0.5 s. The obtained spectra are shown in Fig. 2. The abundance sensitivity at 2 mass units below the high intensity peak (m-2) was found to be well below 1 ppb and indistinguishable from the background. The tailing, which can be actually modelled with a two-component function (exponential function down to m/z = 231.2 and linear until m/z = 230.6) has a negligible effect on the 229.6–230.3 mass region (Fig. 2). Therefore, no correction is needed for the tailing effect and the ²³⁰Th can be measured purely without any overlap from the abundant ²³²Th peak. This observed abundance sensitivity is at least 20 times less compared to a standard MC-ICP-MS instrument [28]. Moreover, due to the high ²³²Th measured signal (∼60 V), the ²³⁰Th intensity can also be measured with elevated intensity: the ²³⁰Th peak for IRMM-036 (which has lower ²³⁰Th/²³²Th ratio) is more than 10⁴ cps. Thus, the signal-to-noise can be highly improved with the current setup without the need for tailing correction.

Talanta, Volume 292, 2025, 127896: Fig. 2. IRMM-036 isotope standard measured at low mass resolution (R = ∼2100). The 230Th peak was scanned using the SEM1 at 229.6–231.4 mass region (A), while the high intensity 232Th peak is measured with L4 at 231.65–232.6 (B).

3.2. Measurement of IRMM-035 and IRMM-036

In order to demonstrate the applicability of the developed method, the final validation was performed by serial measurements of the certified IRMM-035 and IRMM-036 Th isotope reference materials. Ten replicate analyses (N = 10) of the samples were performed on two separate days. The measured results, together with the values from the literature, are collected in Table 3, and shown in Fig. 4 and Fig. 5.

Talanta, Volume 292, 2025, 127896: Fig. 4. Measured 230Th/232Th amount ratio in IRMM-035 isotope reference material (the individual results and the calculated average for the 10 replicates) and the reported values. The red solid line represents the recommended value by Sims et al. (with its uncertainty a dotted line), which is achieved by the re-measurement of the standard and the compilation of literature values (230Th/232Th = 11.380 ± 0.096 ppm) [28]. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Talanta, Volume 292, 2025, 127896: Fig. 5. The measured 230Th/232Th amount ratio in IRMM-036 isotope reference material (the individual results and the calculated average for the 10 replicates) and the literature reported values. The red solid line represents the recommended value by Sims et al. (with its uncertainty a dotted line), which is achieved by the re-measurement of the standard and the compilation of literature values (230Th/232Th = 3.047 ± 0.024 ppm) [28]. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

4. Conclusions

A novel method has been developed for the accurate and precise ²³⁰Th/²³²Th measurement with a novel Neoma™ MC-ICP-MS/MS. It was found that no tailing correction is needed due to the exceptional abundance sensitivity (42 ± 3 ppb at m-1 and less than 1 ppb at m-2), allowing to achieve a high accuracy. Such a low degree of tailing is comparable to that of the TIMS instruments. This technique improves the precision for Th isotope ratio measurement by a factor of ∼10 relative to the already published MC-ICP-MS values. The measurements showed that using a higher mass resolution slightly improves the abundance sensitivity. Moreover, this will remove most molecular interferences, which is of vital importance for coupled techniques, e.g. for laser ablation. It should also be noted that higher mass resolution does not significantly improve the abundance sensitivity arising from the peak tailing effect. Principally, our findings can have significant implications for the ²³⁰Th/²³²Th measurements in the future (in particular age dating studies in geology or nuclear forensics), especially for dating studies of young samples or if only small sample amounts are available. Deviations in tailing corrections can cause a bias at permille level on the isotope ratios, and, by this, also on the corrected ages for young samples [7]. Additionally, the developed method can also be used in other areas of analytical chemistry, where a low-abundant isotope has to be measured in a matrix with an excess of neighbouring nuclides (e.g. in the nuclear field this can be the direct analysis of ²³⁷Np or Pu isotopes in an U matrix). From this point of view, this study demonstrates the potentials of a MC-ICP-MS/MS instrument for advancing the mass spectrometry applications.