11:00 AM - 11:15 AM
[MTT42-07] Analysis of single platinum nanoparticles using high-time resolution multiple collector ICP-MS (HTR-MC-ICP-MS) and application to cosmochemical samples
Keywords:ICP-MS, Nanoparticles, Isotopic ratio, meteorites
Introduction
Heavy elements, which played an important role to define the physicochemical evolution of both the Earth and life, are mainly produced by a neutron capture reaction, including s- and r-processes (Burbidge et al., 1957). The r-process is an explosive reaction process, and thus, the estimating of amount and isotopic composition of the elements produced have been retarded. To overcome this, we are trying to derive the material evidence or constraints from presolar grains in meteorites.
The presolar grains are the small grains of sizes varying from nm to µm (Lewis and Anderson, 1983). Isotopic signatures of the light elements, such as C, N, O, or Si, have been widely used to identify the presolar grains, and very few approaches were made to identify the presolar grains based on heavy elements. To understand both the origin and transport mechanism of the heavy elements, new identification protocol using the heavy elements is highly desired. In this study, the element Pt, which enriched in r-process isotopes is used as a test element to establish the new identification protocol for the presolar grains in meteorites. To do this, we have developed a new analytical technique for both the sensitive and fast Pt isotopic ratio measurements from small particles (i.e., nanoparticles).
With the isotopic analysis of nanoparticles using the ICP-MS technique, great care must be placed on the systematical error in the measured isotopic ratio caused by counting loss due to very high-count rates of the analytes (Obayashi et al., 2017). Faced with this, careful correction for the counting loss due to detector dead time was made to obtain reliable isotopic ratio measurements from the transient signals emanating from nanoparticles.
Another important development is that the sampling technique of the cosmic nanoparticles from meteorites. In this study, a laser ablation in liquid (LAL) technique was applied to extract the cosmic nanoparticles in meteorites. With the LAL technique, sample grains can be collected into the small volume of water without nearly no chemical invasions, and also the particles could be physically dispersed in the liquid by the laser-induced shock wave (Okabayashi et al., 2011). Moreover, since the size of laser pit is about 10 µm, and thus, the sampling from specific area can be made with the LAL technique. In this study, development of both the analytical technique for the Pt isotopes and sampling procedures were conducted.
Experiments and results
In this study, dead time was carefully measured for reliable correction of the counting loss. The MC-ICP-MS instrument is the Nu Plasma 2 equipped with in-house high-time resolution ion counter (e.g., maximum time resolution 10 µs). Dead time of the ion counter was calibrated using the W and Ba mixed solutions of various concentrations (0.24, 0.48, 1.2, 2.4, 4.8, 12, 24 µg/L). To evaluate the reliability of dead time obtained here, the isotope ratios for Pt nanoparticles of various sizes (30 nm, 50 nm, and 70 nm) were measured, and the resulting 196Pt/194Pt was 0.77143±0.02905 (2SE) for 30 nm Pt NPs, 0.76856±0.01597 (2SE) for 50 nm Pt NPs and 0.76889±0.00309 (2SE) for 70 nm Pt NPs. Relative deviations from the natural isotopic ratios (196Pt/194Pt = 0.76566) were 0.74% for 30 nm Pt NPs, 0.38% for 50 nm Pt NPs, and 0.42% for 70 nm Pt NPs, suggesting that the relative deviation of the measured 196Pt/194Pt ratio agreed with the natural value within analytical uncertainties.
This is followed by the optimization of the sample extraction procedures using the LAL. Hence, Nd:YAG laser (wavelength 266 nm and pulse width 1 ns) was used. Laser ablation was carried out with a repetition rate of 1 kHz, laser fluence of 0.7 J cm-2, and laser scanning speed of 20 µm/s.
The resulting nanoparticle-dispersed solutions obtained through LAL sampling on the matrix phase of the carbonaceous chondrite were subsidized to the Pt isotopic analysis using the HTR-MC-ICP-MS technique. From the resulting sample solution, Pt NPs being sizes of 20 – 200 nm were detected, suggesting the effectiveness of both the analytical protocol and sampling procedure using the LAL technique. In this presentation, we will demonstrate the 196Pt/194Pt and 196Pt/194Pt ratios for individual small grains collected from the meteorites, and also discuss the possible sources of these grains.
Heavy elements, which played an important role to define the physicochemical evolution of both the Earth and life, are mainly produced by a neutron capture reaction, including s- and r-processes (Burbidge et al., 1957). The r-process is an explosive reaction process, and thus, the estimating of amount and isotopic composition of the elements produced have been retarded. To overcome this, we are trying to derive the material evidence or constraints from presolar grains in meteorites.
The presolar grains are the small grains of sizes varying from nm to µm (Lewis and Anderson, 1983). Isotopic signatures of the light elements, such as C, N, O, or Si, have been widely used to identify the presolar grains, and very few approaches were made to identify the presolar grains based on heavy elements. To understand both the origin and transport mechanism of the heavy elements, new identification protocol using the heavy elements is highly desired. In this study, the element Pt, which enriched in r-process isotopes is used as a test element to establish the new identification protocol for the presolar grains in meteorites. To do this, we have developed a new analytical technique for both the sensitive and fast Pt isotopic ratio measurements from small particles (i.e., nanoparticles).
With the isotopic analysis of nanoparticles using the ICP-MS technique, great care must be placed on the systematical error in the measured isotopic ratio caused by counting loss due to very high-count rates of the analytes (Obayashi et al., 2017). Faced with this, careful correction for the counting loss due to detector dead time was made to obtain reliable isotopic ratio measurements from the transient signals emanating from nanoparticles.
Another important development is that the sampling technique of the cosmic nanoparticles from meteorites. In this study, a laser ablation in liquid (LAL) technique was applied to extract the cosmic nanoparticles in meteorites. With the LAL technique, sample grains can be collected into the small volume of water without nearly no chemical invasions, and also the particles could be physically dispersed in the liquid by the laser-induced shock wave (Okabayashi et al., 2011). Moreover, since the size of laser pit is about 10 µm, and thus, the sampling from specific area can be made with the LAL technique. In this study, development of both the analytical technique for the Pt isotopes and sampling procedures were conducted.
Experiments and results
In this study, dead time was carefully measured for reliable correction of the counting loss. The MC-ICP-MS instrument is the Nu Plasma 2 equipped with in-house high-time resolution ion counter (e.g., maximum time resolution 10 µs). Dead time of the ion counter was calibrated using the W and Ba mixed solutions of various concentrations (0.24, 0.48, 1.2, 2.4, 4.8, 12, 24 µg/L). To evaluate the reliability of dead time obtained here, the isotope ratios for Pt nanoparticles of various sizes (30 nm, 50 nm, and 70 nm) were measured, and the resulting 196Pt/194Pt was 0.77143±0.02905 (2SE) for 30 nm Pt NPs, 0.76856±0.01597 (2SE) for 50 nm Pt NPs and 0.76889±0.00309 (2SE) for 70 nm Pt NPs. Relative deviations from the natural isotopic ratios (196Pt/194Pt = 0.76566) were 0.74% for 30 nm Pt NPs, 0.38% for 50 nm Pt NPs, and 0.42% for 70 nm Pt NPs, suggesting that the relative deviation of the measured 196Pt/194Pt ratio agreed with the natural value within analytical uncertainties.
This is followed by the optimization of the sample extraction procedures using the LAL. Hence, Nd:YAG laser (wavelength 266 nm and pulse width 1 ns) was used. Laser ablation was carried out with a repetition rate of 1 kHz, laser fluence of 0.7 J cm-2, and laser scanning speed of 20 µm/s.
The resulting nanoparticle-dispersed solutions obtained through LAL sampling on the matrix phase of the carbonaceous chondrite were subsidized to the Pt isotopic analysis using the HTR-MC-ICP-MS technique. From the resulting sample solution, Pt NPs being sizes of 20 – 200 nm were detected, suggesting the effectiveness of both the analytical protocol and sampling procedure using the LAL technique. In this presentation, we will demonstrate the 196Pt/194Pt and 196Pt/194Pt ratios for individual small grains collected from the meteorites, and also discuss the possible sources of these grains.