Enhancement of Uranium Ionization Efficiencies Using Zn-MOF-74 Derived Nanoporous Ion Emitters for Thermal Ionization Mass Spectrometry

The recent introduction of nanoporous ion emitters (nano-PIEs) formed from metal-organic frameworks (MOFs) has demonstrated their potential to enhance sensitivity for thermal ionization mass spectrometry (TIMS). Nano-PIEs take advantage of the parent MOF's chemical and structural tunability to form scaffolds for ion emitters. A study by Barpaga et al., 2023 on MOF-74 as the parent material found that with high volatility metals in their framework, uranium sample utilization efficiency (SUE) increases by up to 9-fold (e.g., Zn-MOF-74) compared to that of the analyte on a bare filament (∼0.05%). In this study, we investigate the performance of Zn-MOF-74 to maximize uranium efficiencies at the trace level (= 10^-12 g) by altering the parent MOF morphology and chemistry (i.e., MOF crystal sizes and thermal degradation) and optimizing its integration with TIMS (i.e., MOF mass on a filament and ramp conditions). We observed improvement in SUE up to 20 times (≤1.0%) that of a bare filament load when nano-PIEs derived from nanocrystals of Zn-MOF-74 were heated under a specific current ramp condition. This demonstrates that rates of nano-PIE structural collapse during TIMS analysis and the subsequently formed nanomaterials (and their features) can be tuned to control the analyte ionization.