Physicochemical Gas-Solid Sorption Properties of Geologic Materials Using Inverse Gas Chromatography.

The goal of this study was to determine the physicochemical properties of a variety of geologic materials using inverse gas chromatography (IGC) by varying probe gas selection, temperature, carrier gas flow rate, and humidity. This is accomplished by measuring the level of interaction between the materials of interest and known probe gases. Identifying a material's physicochemical characteristics can help provide a better understanding of the transport of gaseous compounds in different geologic materials or between different geological layers under various conditions. Our research focused on measuring the enthalpy (heat) of adsorption, Henry's constant, and diffusion coefficients of a suite of geologic materials, including two soil types (sandy clay-loam and loam), quartz sand, salt, and bentonite clay, with various particle sizes. The reproducibility of IGC measurements for geologic materials, which are inherently heterogeneous, was also assessed in comparison to the reproducibility for more homogeneous synthetic materials. This involved determining the variability of physicochemical measurements obtained from different IGC approaches, instruments, and researchers. For the investigated IGC-determined parameters, the need for standardization became apparent, including the need for application-relevant reference materials. The inherent physical and chemical heterogeneities of soil and many geologic materials can make the prediction of sorption properties difficult. Characterizing the properties of individual organic and inorganic components can help elucidate the primary factors influencing sorption interactions in more complex mixtures. This research examined the capabilities and potential challenges of characterizing the gas sorption properties of geologic materials using IGC.

Isotopic Heterogeneity Imaged in a Uranium Fuel Pellet with Extreme Ultraviolet Laser Ablation and Ionization Time-of-Flight Mass Spectrometry.

We use extreme ultraviolet laser ablation and ionization time-of-flight mass spectrometry (EUV TOF) to map uranium isotopic heterogeneity at the nanoscale (≤100 nm). Using low-enriched uranium fuel pellets that were made by blending two isotopically distinct feedstocks, we show that EUV TOF can map the 235U/238U content in 100 nm-sized pixels. The two-dimensional (2D) isotope maps reveal U ratio variations in sub-microscale to ≥1 µm areas of the pellet that had not been fully exposed by microscale or bulk mass spectrometry analyses. Compared to the ratio distribution measured in a homogeneous U reference material, the ratios in the enriched pellet follow a ∼3× wider distribution. These results indicate U heterogeneity in the fuel pellet from incomplete blending of the different source materials. EUV TOF results agree well with those obtained on the same enriched pellets by nanoscale secondary ionization mass spectrometry (NanoSIMS), which reveals a comparable U isotope ratio distribution at the same spatial scale. EUV TOF's ability to assess and map isotopic heterogeneity at the nanoscale makes it a promising tool in fields such as nuclear forensics, geochemistry, and biology that could benefit from uncovering sub-microscale sources of chemical modifications.

Collisional dampening for improved quantification in single particle inductively coupled plasma mass spectrometry.

Inductively coupled plasma mass spectrometry (ICP-MS) is a powerful method for detection and quantification of nanoparticles. Unfortunately, the linear dynamic range of single particle analysis is hindered by "unruly" transient signals, momentary pulse pile-ups at the electron multiplier detector. This study seeks to extend the dynamic range of ICP-MS nanoparticle quantification via addition of a collision gas in the collision cell of the ICP-MS. The collision gas temporally broadens the nanoparticle signal resulting in decreased pulse pile-up and increased integrated intensity, up to a point where scattering losses begin to dominate. We tested collisional broadening with a dual mode simultaneous secondary electron multiplier (pulse counting switching to analog) and the same detector configured for pulse counting only operation. With no collision gas and the detector operating in its standard dual mode, the data shows a linear response for gold nanoparticles from 20 nm (smallest measured size) to 150 nm. With the addition of helium as a collision gas in the cell, the linear range extends up to 250 nm. The data collected exclusively from the pulse counting mode shows that with no collision gas there is a linear response for gold nanoparticles from 20 nm to 60 nm. While the signal slightly improves with the addition of a collision gas, the linear range fails to extend up to 80 nm, the next largest nanoparticle size in this study. The addition of a collision gas used together with the dual mode detector shows a promising path forward towards mitigating unruly transient signals, improving the dynamic range of nanoparticle quantification.