Advanced Method Optimization for Sampling and Analysis Instrumentation.

This work presents a generalized approach for analytical method optimization that branches the gap between techniques historically employed and accurate modern optimization techniques suitable for various applications. The novelty of the described strategy is the utilization of multivariate, multiobjective optimization with Karush-Kuhn-Tucker conditions to bound the optimization space to solutions within the physical limitations of instrumentation. Briefly, the basic steps outlined in this paper are to (1) determine the objective(s) that should be maximized or minimized based on the goals of the analytical application, (2) conduct a screening experiment, (3) perform ANOVA to determine the parameters which have a statistically significant effect on the objective, (4) conduct an experiment (e.g., Box-Behnken design) to collect data for fitting the objective equation, and (5) determine the physical constraints of the parameters and solve the Lagrangian to determine the optimal method parameters. A broad approach to optimization target selection allows for robust method tuning to develop improved data sets amenable for chemometrics and machine learning algorithm development. Gas chromatography-mass spectrometry was selected as a use case due to its broad use across scientific fields and time-consuming method development involving numerous parameters. This strategy can reduce the cost of research, improve data quality, and enable the rapid development of new analytical techniques.

Matrix-Assisted Ionization of Molecular Uranium Species.

Matrix-assisted ionization (MAI) demonstrates high sensitivity for a variety of organic compounds; however, few studies have reported the application of MAI for the detection and characterization of inorganic analytes. Trace-level uranium analysis is important in the realms of nuclear forensics, nuclear safeguards, and environmental monitoring. Traditional mass spectrometry methods employed in these fields require combinations of extensive laboratory chemistry sample preparation and destructive ionization methods. There has been recent interest in exploring ambient mass spectrometry methods that enable timely sample analysis and higher sensitivity than what is attainable by field-portable radiation detectors. Rapid characterization of uranium at nanogram levels is demonstrated in this study using MAI techniques. Mass spectra were collected on an atmospheric pressure mass spectrometer for solutions of uranyl nitrate, uranyl chloride, uranyl acetate, and uranyl oxalate utilizing 3-nibrobenzonitrile as the ionization matrix. The uranyl complexes investigated were detectable, and the chemical speciation was preserved. Sample analysis was accomplished in a matter of seconds, and limits of detection of 5 ng of uranyl nitrate, 10 ng of uranyl oxalate, 100 ng of uranyl chloride, and 200 ng of uranyl acetate were achieved. The observed gas-phase speciation was similar to negative-ion electrospray ionization of uranyl compounds with notable differences. Six matrix-derived ions were detected in all negative-ion mass spectra, and some of these ions formed adducts with the uranyl analyte. Subsequent analysis of the matrix suggests that these molecules are not matrix contaminants and are instead created during the ionization process.

Rapid paper spray mass spectrometry characterization of uranium and exemplar molecular species.

RATIONALE: The ability to detect and quantify the presence of specific inorganic elements and complexes is essential for environmental monitoring and nuclear safeguards applications. In this work, paper spray ionization mass spectrometry was used for the rapid chemical and isotopic characterization of trace inorganic species collected on cotton swipe substrates. The direct analysis of cotton swipes using this ambient ionization technique led to fast sample analysis that retained original chemical information of the source material with minimal sample preparation. METHODS: Mass spectra were collected with an atmospheric pressure ionization, high-resolution mass spectrometer for solutions containing uranyl acetate, uranyl chloride, uranyl nitrate, and uranyl tri-n-butylphosphate complexes. Gadolinium nitrate was used as an internal standard for the quantitative analysis of uranium. To demonstrate the ability to characterize inorganic contaminants in the presence of uranium, a multi-element inorganic standard containing U, Bi, Pb, Cd, Fe, and Zn was deposited onto cotton substrates and directly analyzed without purification. RESULTS: All elements doped on the cotton substrate were detected with strong signal-to-noise ratios (ca 1000 for UO2 + on multi-element doped swipes) and high integrated intensities (>105 counts) from collection periods of approximately 1 min. Limits of detection were determined to be approximately 94 ng for UO2 + and uranyl acetate through the measurement of ppb level solutions. CONCLUSIONS: The rapid analysis of uranium and other inorganic-containing samples while still retaining original chemical information (e.g. uranyl complexation) was demonstrated. Qualitative detection and speciation were achieved in less than 1 min of analysis. For uranium isotopic quantitation, longer accumulations (>15 min) can be sustained to improve the accuracy of minor 235 U isotopic abundance measurements to approximately 1% error.

Anion-exchange polymer filament coating for ultra-trace isotopic analysis of plutonium by thermal ionization mass spectrometry.

A new sample loading procedure was developed for isotope measurements of ultra-trace amounts of Pu with thermal ionization mass spectrometry (TIMS) that is based on a polymer thin film architecture. The goals were to simplify single filament TIMS sample preparation for Pu, while preserving the sensitivity and accuracy of the resin bead loading method, and to eliminate sample losses experienced with the bead loading method. Rhenium filaments were degassed, dip-coated with a thin (~ 120 nm) hydrophobic base layer of poly(vinylbenzyl chloride) (PVBC), and spotted with an aqueous solution comprising triethylamine-quaternized PVBC and diazabicyclo[2.2.2]octane crosslinker. This procedure formed a toroidal, hydrophilic anion-exchange polymer spot surrounded by the hydrophobic base polymer. The thin film-coated filaments were direct loaded with 10 pg of New Brunswick Laboratory certified reference material (NBL CRM) 128 from a 9 M HCl matrix. Aqueous sample droplets adhered to the anion-exchange polymer spot, facilitating sample loading. Toroidal spots with a thickness of 20-30 µm generated the highest sample utilization, surpassing the sample utilization of the standard bead loading method by 175%. Measured isotopic ratios were in good agreement with the certified value of the 239Pu/242Pu ratio for NBL CRM 128. The use of dimpled filaments further aided sample loading by providing a well-shaped substrate to deposit the sample droplet. No sample losses were experienced with the thin film loading method over 65 sample analyses. Finally, polymer coatings suppressed filament aging under atmospheric conditions, enabling the bulk production of filaments with adequate shelf life for future analyses.

Anion-Exchange Fibers for Improved Sample Loading in Ultra-Trace Analysis of Plutonium by Thermal Ionization Mass Spectrometry.

A new sample loading procedure was developed for isotope ratio measurements of ultratrace amounts of plutonium with thermal ionization mass spectrometry (TIMS). The goal was to determine the efficacy of a polymer fiber architecture for TIMS sample loading by following similar sample loading procedures as those used in bead loading. Fibers with diameter of approximately 100 µm were prepared from triethylamine-quaternized-poly(vinylbenzyl chloride) cross-linked with diazabicyclo[2.2.2]octane. Fiber sections (2.5 mm) were loaded with 10 pg of New Brunswick Laboratory certified reference material (NBL CRM) 128 from an 8 M HNO3 matrix and affixed to rhenium filaments with collodion. A single filament assembly was used for these analyses. Total ion counts (239Pu + 242Pu) and isotope ratios obtained from fiber-loaded filaments were compared to those measured by depositing Pu amended resin beads on the filament. Fiber loading was found to improve sensitivity, accuracy, and precision of isotope ratio measurements of plutonium when compared to the established resin bead loading method, while maintaining its simplicity. The average number of detected Pu+ counts was 180% greater, and there was a 72% reduction in standard deviation of ratio measurements when using fiber loading. An average deviation of 0.0012 (0.117%) from the certified isotope ratio value of NBL CRM Pu128 was measured when fiber loading versus a deviation of 0.0028 (0.284%) when bead loading. The fiber formation method presented in this study can be extended to other anion-exchange polymer chemistries and, therefore, offers a convenient platform to investigate the efficacy of novel polymer chemistries in sample loading for TIMS.

Rhenium filament oxidation: Effect on TIMS performance and the roles of carburization and humidity.

This communication presents findings on the effect of rhenium filament oxidation on thermal ionization mass spectrometry (TIMS) analyses of plutonium. Additionally, the roles of atmospheric humidity and carburization on the oxidation characteristics (i.e. aging) of rhenium filaments were studied. Degassed and carburized filaments were aged for up to 79 days under dry and humid conditions, and the growth of oxo-rhenium crystallites was investigated intermittently by scanning electron microscopy (SEM) to construct growth profiles. SEM images were analyzed to determine average crystallite size, number density, and percent surface coverage. Crystallite growth was found to be suppressed by both filament carburization and dry storage conditions (~13% relative humidity). Under humid conditions (75% relative humidity), crystallite growth progressed steadily over the investigatory period, reaching >2.3% surface coverage within 79 days of aging. Atomic ion production of Pu (Pu+) was suppressed by approximately 20% and the standard deviation of isotope ratio measurements was increased by 170% when filaments with 1% oxide surface coverage were used in sample loading. Measurement sensitivity and reproducibility are imperative for applications involving ultra-trace analysis of plutonium by TIMS. These findings offer validation for observations regarding the detrimental effect of excessive filament aging post-degassing, improve the understanding of conditions that impel the oxidation of rhenium filaments, and provide practical means to suppress the growth of oxides.

Ambient aging of rhenium filaments used in thermal ionization mass spectrometry: Growth of oxo-rhenium crystallites and anti-aging strategies.

Degassing is a common preparation technique for rhenium filaments used for thermal ionization mass spectrometric analysis of actinides, including plutonium. Although optimization studies regarding degassing conditions have been reported, little work has been done to characterize filament aging after degassing. In this study, the effects of filament aging after degassing were explored to determine a "shelf-life" for degassed rhenium filaments, and methods to limit filament aging were investigated. Zone-refined rhenium filaments were degassed by resistance heating under high vacuum before exposure to ambient atmosphere for up to 2 months. After degassing the nucleation and preferential growth of oxo-rhenium crystallites on the surface of polycrystalline rhenium filaments was observed by atomic force microscopy and scanning electron microscopy (SEM). Compositional analysis of the crystallites was conducted using SEM-Raman spectroscopy and SEM energy dispersive X-ray spectroscopy, and grain orientation at the metal surface was investigated by electron back-scatter diffraction mapping. Spectra collected by SEM-Raman suggest crystallites are composed primarily of perrhenic acid. The relative extent of growth and crystallite morphology were found to be grain dependent and affected by the dissolution of carbon into filaments during annealing (often referred to as carbonization or carburization). Crystallites were observed to nucleate in region specific modes and grow over time through transfer of material from the surface. Factors most likely to affect the rates of crystallite growth include rhenium substrate properties such as grain size, orientation, levels of dissolved carbon, and relative abundance of defect sites; as well as environmental factors such as length of exposure to oxygen and relative humidity. Thin (∼180 nm) hydrophobic films of poly(vinylbenzyl chloride) were found to slow the growth of oxo-rhenium crystallites on the filament surfaces and may serve as an alternative carbon source for filament carburization.

Synthesis and self-assembly of polymer and polymer-coated ag nanoparticles by the reprecipitation of binary mixtures of polymers.

Binary polymer nanoparticles were synthesized by the reprecipitation of poly(4-vinylpyridine) in the presence of poly(diallyldimethylammonium chloride) and further used to make polymer-coated Ag nanoparticles. Polymer shells around Ag nanoparticles were formed by two methods: the reduction of Ag(2)O in the presence of the polymer nanoparticles and by mixing the polymer nanoparticles with already-made Ag nanoparticles. The resulting nanoparticles were coated with layers of the two polymers with the hydrophilic polymer on the outside providing their stability in water. The exposure of the polymer-coated Ag nanoparticles to unmodified Ag nanoparticles resulted in spontaneous self-assembly due to the electrostatic attraction. The polymer-coated nanoparticles and the nanoparticle assemblies were characterized by UV-vis, surface-enhanced Raman scattering spectroscopy, and transmission electron microscopy.