In this study, fluorine and carbon compounds were added to the sample to enhance the CO gas production during thermal decomposition. First, following previous studies ^11, ¹², polytetrafluoroethylene (PTFE) was used as the fluorine source. The following inorganic fluorides were also tested: KF, NaF, LiF, CaF₂, AlF₃ (FUJIFILM Wako Pure Chemical Corporation, 99%, 99%, 98.0%, 99.9%, 31.0–34.0 (as Al) % purity, respectively), and BaF₂ (Kojundo Chemical Laboratory Co., Ltd., 99% purity). Graphite powder (5–11 μm of particle size, AS ONE Corporation) was used as a carbon source. Granular nickelized carbon (50 wt% Ni, Elemental Microanalysis Co., Ltd.) was also tested instead of graphite to enhance the reaction. This carbon has been nickelized and is henceforth referred to as Ni/C. Ni/C is activated carbon doped with metallic Ni nanoparticles; the doped Ni’s catalytic function is thought to speed up the pace at which gas is produced as organic matter breaks down ¹⁷.

2.2. Methods

2.2.1. Pre-treatment for clay samples

Pre-treatment for montmorillonite samples were performed by the following method: To remove impurities such as quartz and feldspar, the clay fraction was extracted from the bentonite ores (TKN-01, -29, and -31). The samples were dispersed in deionized water and placed in an ultrasonic bath to aid in disintegration. After this, the samples were separated into the < 2 μm size fraction by centrifugation based on Stoke’s Law. Finally, the clay fractions were freeze-dried. The appropriate size fraction was determined using powder X-ray diffraction, and no impurities were observed in any fraction.

In order to obtain representative δ¹⁸O values for clay minerals, removing adsorbed water molecules is necessary. The majority of the adsorbed water linked to smectite is bound to the interlayer at ambient temperature and humidity levels ¹⁸. In this study, montmorillonite samples were ion-exchanged with K⁺, which has a low hydration enthalpy. The smectite saturated with K⁺ was the least hygroscopic compared to other exchangeable cations (Ca²⁺, Na⁺) and provided a more accurate H isotope ratio in the structure ¹⁹. In this study, K⁺saturation for the montmorillonite samples was achieved by dispersion in a 2M KCl solution, shaking for 15 min, and solid-liquid separation by centrifugation. This procedure was repeated three times to produce homoionic K⁺-saturated forms of each sample. After saturation, the samples were dispersed in deionized water and shaken for 15 min to remove excess salt before solid-liquid separation. This procedure was repeated three times. The samples were then freeze-dried and gently ground into a fine powder using an agate mortar and pestle.

2.2.2. TC/EA configuration

The TC/EA-IRMS used in this study was in the standard configuration, except for a gas trap between the reactor and GC column (Fig. 1). The samples were inserted into a zero-blank autosampler (Costech), which contained a maximum of 99 samples (Fig. 1(a)). The autosampler was continually purged with He gas to remove atmospheric gases such as N₂, O_2, and H₂O. Thermal decomposition was performed after the samples fell into the TC/EA (Thermo Fischer Scientific) system. The central part of the TC/EA is a high-temperature reactor composed of an outer ceramic tube and an inner glassy carbon tube (Fig. 1(b)). The glassy carbon tube was filled with a layer of silver wool at the bottom and a layer of glassy carbon granule (several millimeters in size). The reaction residue was collected using a graphite crucible that was held up by glassy carbon granules. The crucible was positioned in the hottest zone of the reactor and was set at 1450˚C in this study.

Here, *C represents carbon derived from graphite or Ni/C powder. CO and fluoride gases were generated. Although the chemical species of the fluoride gases produced here have not been identified, it is assumed that they are mainly SiF₄.When PTFE is thermally decomposed, CF₄ gas is produced, which is converted into more stable SiF₄ in the presence of SiO₂^20, ²¹. Similarly, the more stable AlF₃ (solid and gaseous) forms in the presence of Al₂O₃ ²². Silicate minerals contain other cations such as Mg, Fe, as well as Si and Al in their crystal structures, and it is thought that fluoride gases containing these cations will form when they are decomposed in TC/EA.

The gases produced in the reactor were transferred to a gas trap. The gas trap is 30 cm long which is filled with 20 cm of ascarite II® adsorbent and 10 cm of Mg(ClO₄)₂granular (Fig. 1(c)). Mg(ClO₄)₂ was employed as the moisture adsorbent, and Ascarite II was utilized to remove fluoride gasses in order to safeguard the GC column. Unlike a prior publication, which included a liquid N₂-trap between the reactor and the GC column to remove possible fluoride gas remains ²³, we did not use one. Although this point should be examined in the future to prevent possible damage to the GC column, no fluoride species were detected using the following mass spectrometer. After passing through the gas trap, the CO gas flowed into a GC column set at 90˚C and then to an IRMS via an open-slit interface (Con Flo IV, Thermo Fischer Scientific) (Fig. 1(d)). The IRMS model used here was the Delta V Advantage (Thermo Fischer Scientific). This provided the CO peak areas for m /z 28, 29, and 30 (Fig. 1(e), (f)).