Designing Porous Ion Emitters for Thermal Ionization Mass Spectrometry: Evaluating Metal–Organic Frameworks

Abstract

This work describes the first exploration of metal− organic frameworks (MOFs) as "next-generation" ion emitters for thermal ionization mass spectrometry (TIMS). MOFs were identified as promising candidates for this application given the synthetic control over their desired structural properties. This tunability results in well-ordered, high-surface-area, high-porosity frameworks with targeted sorption affinities. Here, we explored an aluminum-based, bipyridine-containing MOF (MOF-253) with and without incorporating a high work function metal, rhenium (Re). After analysis of an Nd-bearing MOF, we hypothesized that the well-dispersed, sponge-like interconnected network of the degraded structure would enhance Nd ionization more than traditional TIMS loading techniques (i.e., phosphoric acid). Compared to filaments loaded with phosphoric acid that require an additional benzene carburization step, the Nd ionization efficiencies (atoms detected relative to atoms loaded) for heated filaments loaded with MOF-253 were similar (∼1%). Electron microscopy after TIMS analysis demonstrated that the MOF was retained on the filament. While these results are preliminary, they demonstrate that MOFs have potential to enhance ionization and exceed the performance of traditional loading techniques by forming nanoporous ion emitters. Thus, further experimentation is likely to exceed this performance through more specific selection of the base MOF structure and modifications to porosity and composition. This work represents a novel application of MOFs and a next step in the pursuit of advanced thermal ionization with potential to expand across the periodic table.

Role of Transition Metals in Metal–Organic Frameworks as Nanoporous Ion Emitters for Thermal Ionization Mass Spectrometry

“…Instead, these averages increase with a decrease in the work function of the incorporated metals. This is similar to the observation in our previous nano-PIE study, where Nd ionization efficiency decreases with rhenium (Re) incorporation in MOF-253 . Without a full understanding of the mechanism of U ionization from a Re filament containing MOF-derived nano-PIE, it is difficult to explain this counterintuitive trend.…”

“…This is similar to the observation in our previous nano-PIE study, where Nd ionization efficiency decreases with rhenium (Re) incorporation in MOF-253. 16 Without a full understanding of the mechanism of U ionization from a Re filament containing MOF-derived nano-PIE, it is difficult to explain this counterintuitive trend. It is likely that the oxidation state of the metal species (within the MOF), the potential for alloying of that metal with the Re filament, thermal insulation properties of the MOF, and chemical interaction/coordination of U within the nano-PIE are all factors contributing to this trend between each metal analogue explored in this study.…”

“…We have previously documented that the use of MOF-derived nano-PIEs has been shown to promote the ionization of analytes such as neodymium with more than a 3fold increase in ionization efficiencies compared to a traditional activator. 16 In this current work, we develop and implement a series of nano-PIEs for actinide ionization based on a subfamily of MOF materials, namely, M-MOF-74, in which the metal constituent (M = Mg, Mn, Co, Ni, Cu, Zn, Cd) can be varied but maintain a consistent starting porous structure. Originally reported by Rosi et al, 17 MOF-74 is an isoreticular MOF in which the metal constituent as a cation interacts and crystallizes 2,5-dihydroxyterephthalic acid (DHTA) to generate one-dimensional, ∼1−2 nm cylindrical pores.…”

“…MOFs have been extensively documented in porous media literature as sorbents for applications ranging from gas separations, filtration, catalysis, sensing, etc. , Here, we utilize MOFs to leverage the modularity and synthetic control in their preparation as well as the extremely high surface area afforded by the ordered, network arrangement of their chemical constituents. We have previously documented that the use of MOF-derived nano-PIEs has been shown to promote the ionization of analytes such as neodymium with more than a 3-fold increase in ionization efficiencies compared to a traditional activator . In this current work, we develop and implement a series of nano-PIEs for actinide ionization based on a subfamily of MOF materials, namely, M-MOF-74, in which the metal constituent (M = Mg, Mn, Co, Ni, Cu, Zn, Cd) can be varied but maintain a consistent starting porous structure.…”

Evaluation of Graphene Oxide as a Thermal Ionization Enhancer for Plutonium in TIMS Measurement

“…However, it still required to take several hours for preparation, vacuum baking, and sintering of the PIE in the filament pretreatment, hindering its application in emergency monitoring for nuclear accident analysis and nuclear forensics. In recent years, carbonaceous nanomaterials, like graphene oxide (GO) and metal–organic frameworks (MOFs), appeared to be the “new generation” of ionization enhancers for TIMS by combining both of the above strategies, together with the outstanding properties such as high specific surface areas, porous structures, well-ordered frameworks, controllable structure and chemical immobilization, and high absorption capacity with target elements. In 2019, our group initially utilized GO as the ion source additive for trace-level uranium measurement .…”