Yoshitaka Takagai

The quantitative mapping of radioisotopes in solids is crucial for understanding their microscale localization and migration. However, conventional radiometric and mass spectrometric techniques are only capable of bulk measurements without spatial resolution. Imaging plates provide spatial resolution, but they lack selectivity and cannot quantify radioisotopes that only emit beta radiation, such as ⁹⁰Sr. Here, the first system for isotope-specific quantitative mapping of a pure beta emitter is demonstrated, integrating online isotope dilution (ID) with laser ablation inductively coupled plasma tandem mass spectrometry (LA-ICP-MS/MS). In a case study, ultratrace levels of ⁹⁰Sr were accurately quantified without requiring matrix-matched certified reference materials (CRMs) for calibration or chemical separation. Severe isobaric/polyatomic interference (e.g., ⁹⁰Zr⁺, ⁷⁴Ge¹⁶O⁺, ⁸⁹Y¹H⁺, and ⁵⁸Ni¹⁶O₂⁺) was efficiently suppressed by O₂ reaction in a dynamic reaction cell, which greatly improved the abundance sensitivity and allowed subfemtogram amounts of ⁹⁰Sr to be quantitatively imaged. The background equivalent concentration (BEC) was 0.14 ng g^-1 (equivalent to 7.0 × 10² Bq g^-1) per data point, and the measurement time was within 1 s. Validation with CRMs and ⁹⁰Sr samples confirmed accuracy of 88-116% relative to reference values. The proposed system is a versatile platform that can be applied to the practical mapping of radioactive solids, which has broad implications for nuclear waste disposal, environmental remediation, and radiation protection.

A stepwise hierarchical clustering-assisted materials principal component analysis (SHAMP) method was developed to provide a semi-quantitative estimate of mixture ratios in multicomponent inorganic materials. In this SHAMP approach, hierarchical cluster analysis was first utilized to coarsely classify clusters that encompass the original materials in the training dataset, followed by a stepwise application of principal component analysis (PCA) within each cluster at different hierarchical levels of the dendrogram. This study demonstrated the efficacy of SHAMP by using multi-elemental data obtained from inductively coupled plasma mass spectrometry (ICP-MS) for 2- and 3-component material models—SUS304, brass, and nichrome—allowing for spatial visualization of the mixture ratios in inorganic composite materials. PCA scores facilitated a semi-quantitative estimation of the mixture ratios, while triangular diagrams notably illustrated the mixing proportions of the original components in the 3-component materials. The significance and consistency of the principal components were confirmed through loading analysis and the meticulous treatment of values below the limit of quantification for each element. The SHAMP method proved to be an effective strategy for inferring the original material composition from mixture data derived from multi-elemental analysis such as ICP-MS.

The thermosensitive zwitterionic surfactant, 3-(n-nonyl-N,N-dimethylammonio) propyl sulfate (C₉-APSO₄), in solution displays a liquid-liquid miscibility gap exhibiting an upper critical solution temperature (UCST) in contrast to the lower critical temperature, as is characteristic of nonionic surfactants. In this study, the micellar and aggregational properties of this surfactant in its homogeneous isotropic one phase region above the UCST were examined using surface tension and small-angle neutron scattering (SANS) measurements. The critical micelle concentration (CMC) of C₉-APSO₄ was determined in both H₂O and D₂O at 40 and 70 °C as well as the cloud point (CP) temperature determined over a surfactant concentration range of 0.07-0.70 M. The CMC values were in the range of 48-55 mM (40 °C) and 61-65 mM (70 °C), respectively. The SANS data were analyzed using a core-shell model in which the C₉-APSO₄ is represented by a C-9 nonyl hydrophobic core and polar [(N,N-dimethylammonio)-propylsulfate] headgroup shell. Results revealed that the C₉-APSO₄ micelle aggregates are spherical in shape with a core diameter of 2.56 nm and shell thickness of 2.28 nm (total diameter 4.84 nm) and an aggregation number of 30 ± 1 at 40 °C. The data at 70 °C indicated that the aggregates were slightly larger in size, i.e., core diameter of 2.70 nm and shell thickness of 2.52 nm (total diameter 5.22 nm) with an aggregation number of 35. At a fixed surfactant concentration, the aggregation number and size slightly decreased with a decrease in temperature from 70 to 40 °C (as one approaches the CP and two-phase regime). These results were compared with that reported for other zwitterionic surfactant micellar systems.

The zwitterionic surfactant 3-(nonyldimethylammonio)-propyl sulfate (C9-APSO4) exhibits unique reversible temperature-dependent phase separation properties, which can be used for the extraction of watermiscible negatively charged gold nanoparticles (AuNPs) without any aggregation at high concentrations using only temperature change. This study investigated the extraction behaviors and properties of AuNPs transferred from the water phase to a small volume of the surfactant-rich phase. The AuNPs retained both their original shapes and sizes after temperature-dependent phase separation step. In addition, the concentration of AuNPs was enriched by a factor of 150 in the surfactant-rich phase without any changes in the sizes and shapes of AuNPs. The capacity was 63.7 μg (Au) present in 10 μL of the surfactant-rich phase (i.e., the saturated concentration was 1.6 × 1012 particles/mL for 60 nm AuNPs). Furthermore, the extraction percentage was greater than 99% regardless of the pH (in the pH range of 2 – 12).

Thermal ionization mass spectrometry (TIMS) is a specialized technique that precisely measures isotopic ratios. This paper proposes the direct quantification of ⁹⁰Sr at femtogram per litter (fg L⁻¹) levels using isotope dilution (ID)-TIMS of preconcentrated samples. Although preconcentration is ineffective when measuring isotope ratios in classical TIMS, the magnification afforded by preconcentration improves the limit of detection (LOD) of ⁹⁰Sr in ID-TIMS. After fixing the isotope ratio between stable Sr isotopes and ⁹⁰Sr, the fg L⁻¹-level ⁹⁰Sr concentration was successfully obtained by the ID method (recovery R% = 100 ± 11.1%). The obtained LODs are improved by 10⁵ times from those of conventional radiometric analysis such as oxalic acid precipitation/low background gas flow counters, which depend on the amount of coexisting natural Sr. Under the optimized conditions, the LOD was 9.03 attograms per liter (ag L⁻¹), equivalent to a radioactivity concentration of 46.1 micro becquerels per liter (μBq L⁻¹). The sample volume (10 mL containing 10 μg L⁻¹ of natural Sr) roughly corresponds to a rainwater-level sample with 10⁴-fold preconcentration. Spike and recovery tests were conducted on rain water and related environmental waters (stagnant stock water collected from a swimming pool immediately after the Fukushima Daiichi Nuclear Power Plant accident). The ⁹⁰Sr levels were successfully quantified by the proposed method.

The conventional multivariate analysis method using chemical data obtained by elemental analysis has been still challenging to discriminate materials are included in composite materials. Hierarchical cluster analysis (HCA) and Dempster–Shafer theory, that are series of multivariate analysis method, can evaluate the similarity of each material using the chemical data; however, are still difficult to identify the materials contained in a composite material. In this study, evaluating the similarity of elemental analysis data by HCA was conducted, and then stepwisely evaluating was implemented using degree of belief. In other words, it is a stepwise identification means of composite materials that combines the distance output from HCA with a concept of the degree of belief. Artificial model samples were demonstrated implementing in the result of standard reference materials such as metal alloys and plastics. Consequently, the proposed method consistently results prerequisites, and the degrees of belief were quantitatively calculated for each sample confirming the usefulness of this method.

For quantitative visualization of trace elements, an online isotope dilution (ID) laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) method capable of accurately correcting the detection amount of targets is presented. Two aerosols from an ablated sample and a nebulized isotopically enriched spike solution were mixed online using a dual-port spray chamber. In this paper, transmission efficiency (TE) of each of the two gas streams to the ICP-MS detector is revealed, and the quantification values were corrected by including the ratio of TEs in the calculation of the online ID method. To verify the developed method, Fe and Sr as model elements in five certified reference materials (CRMs, glass: SRM 610, 612, 614, alloy: SS-356, -383) were quantified without the use of matrix-matched CRMs. The resultant values agreed with the certified values of CRMs in the range of 92.7–104.7% and 92.8–109.0% for Fe and Sr, respectively. The LODs (3σ) were 0.54 and 0.17 μg g⁻¹ for Fe and Sr, respectively. In addition, the applicability of this method to quantitative imaging of unknown solid samples was demonstrated for actual biological hard tissues (a mouse incisor, human primary tooth, and fish otolith) using the result of shot-analysis. The results were consistent with the reported concentration range obtained by wet chemical analysis.

To reveal the dynamics of underground water flows, measuring many factors and chemical components are usually required. However, it is difficult for the human sense to find the right answers using the many chemo-data in different factors. Although principal component analysis is one of the useful means in a multivariate analysis (chemometrics) that can reduce the multivariable to only 2-D or 3-D and can successfully categorize quantitative multidata into several groups based on the similarity of water quality, underground water flows are difficult to elucidate owing to the lack of continuous information. This paper presents the dynamics of groundwater around the Goshiki-numa pond community (Goshiki-numa), National park of Japan, using multi-chemical component analysis and elevation-considered principal component analysis. Despite the difficulty of understanding the dynamics of groundwater flows of the pond community using limited factors thus far, an elevation-considered principal component analysis (e-PCA) presented herein has revealed the underground water flows around Goshiki-numa ponds by inputting 19 factors × 102 water (total 1,938 data) collected from 2011 to 2014 and 2016. This e-PCA proved to be an effective chemometrics technique for revealing underground water flows. It is believed to apply not only to analytical sciences but also to environmental sciences, civil engineering, and other fields dealing with multidata of water quality. Graphical abstract

Although thermal ionization mass spectrometry (TIMS) has been employed for the high-precision analysis of isotope ratios, direct quantification of artificial mono-nuclide in the environment is difficult by even using isotope dilution (ID) due to the coexistence of the great magnitude of natural stable nuclides or isobars. In traditional TIMS and ID-TIMS, a sufficient amount of stable Sr doped on a filament is required to realize a stable and adequate ion-beam intensity (i.e., thermally ionized beams). However, the background noise (BGN) at m/z 90, detected by an electron multiplier, disturbs ⁹⁰Sr analysis at low concentration levels due to peak tailing of a significant ⁸⁸Sr ion beam dependent on the ⁸⁸Sr-doping amount. Here, TIMS assisted by quadruple energy filtering was successfully employed for the direct quantification of attogram levels of an artificial monoisotopic radionuclide strontium-90 (⁹⁰Sr) in microscale biosamples. Direct quantification was achieved by integrating the ID quantification of natural Sr and simultaneous ⁹⁰Sr/⁸⁶Sr isotope ratio analysis. Additionally, the measurement amount calculated by the combination of the ID and intercalibration was corrected for the net result amount of ⁹⁰Sr by subtracting dark noise and the detected amount derived from the survived ⁸⁸Sr, which are equivalent with the BGN intensity at m/z 90. Background correction revealed that the detection limits were in the range of 6.15 × 10^-2-3.90 × 10^-1 ag (0.31-1.95 μBq), depending on the concentration of natural Sr in a 1 μL sample, and the quantification of 0.98 ag (5.0 μBq) of ⁹⁰Sr in 0-300 mg/L of natural Sr was successful. This method could analyze small sample quantities (1 μL), and the quantitative results were verified against authorized radiometric analysis techniques. Furthermore, the amount of ⁹⁰Sr in actual teeth was successfully quantified. This method will be a powerful tool for measuring ⁹⁰Sr in the measurement of micro-samples, which are required to assess and understand the degree of internal radiation exposure.