Benjamin T. Manard

Liquid sampling-atmospheric pressure glow discharge (LS-APGD) microplasmas for diverse spectrochemical analysis applications

Over the last 15 years there has been a great deal of interest in the potential development of spectrochemical sources that come with lower operational overhead than the inductively-coupled plasma (ICP). There are many driving forces for the development of such devices, even with the likely sacrifices in terms of analytical performance. Some of these devices operate in ambient atmospheres in the electrical regime of glow discharge (GD) plasmas, wherein one of the electrodes is an electrolytic solution that serves as the medium for sample introduction. The basic operational space and analytical performance of these devices has been reviewed in the recent past. We focus in this review on the design and operational attributes of the liquid sampling-atmospheric pressure glow discharge (LS-APGD) microplasma. The rationale for the development and basic source designs are first considered, followed by practical contrasts to the other devices provided. A number of studies have been performed to assess the fundamental plasma characteristics including kinetic and excitation temperatures, as well as charged particle densities. Perhaps the greatest difference between the LS-APGD and the other plasmas is the analytical versatility that has been demonstrated. Analytes can be determined by optical emission spectroscopy or mass spectrometry (OES/MS), with sampling regimes consisting of solution phase introduction, particles produced via laser ablation, or through an ambient desorption mechanism directly from the solid state. Finally, the microplasma can be operated in alternative modes wherein either elemental or molecular-form mass spectra are obtained. It is believed that the simplicity of the LS-APGD design, combined with its analytical versatility, suggest that this singular platform could be implemented to address diverse analytical challenges.

Preliminary Assessment of Potential for Metal–Ligand Speciation in Aqueous Solution via the Liquid Sampling–Atmospheric Pressure Glow Discharge (LS-APGD) Ionization Source: Uranyl Acetate

The determination of metals, including the generation of metal-ligand speciation information, is essential across a myriad of biochemical, environmental, and industrial systems. Metal speciation is generally affected by the combination of some form of chromatographic separation (reflective of the metal-ligand chemistry) with element-specific detection for the quantification of the metal composing the chromatographic eluent. Thus, the identity of the metal-ligand is assigned by inference. Presented here, the liquid sampling-atmospheric pressure glow discharge (LS-APGD) is assessed as an ionization source for metal speciation, with the uranyl ion-acetate system used as a test system. Molecular mass spectra can be obtained from the same source by simple modification of the sustaining electrolyte solution. Specifically, chemical information pertaining to the degree of acetate complexation of uranyl ion (UO2(2+)) is assessed as a function of pH in the spectral abundance of three metallic species: inorganic (nonligated) uranyl, UO2Ac(H2O)n(MeOH)m(+), and UO2Ac2(H2O)n(MeOH)(m)H(+) (n = 1, 2, 3, ...; m = 1, 2, 3, ...). The product mass spectra are different from what are obtained from electrospray ionization sources that have been applied to this system. The resulting relationships between the speciation and pH values have been compared to calculated concentrations of the corresponding uranyl species: UO2(2+), UO2Ac(+), UO2Ac2. The capacity for the LS-APGD to affect both atomic mass spectra and structurally significant spectra for organometallic complexes is a unique and potentially powerful combination.

Liquid sampling-atmospheric pressure glow discharge excitation of atomic and ionic species

The liquid sampling-atmospheric pressure glow discharge (LS-APGD) was characterized with respect to the effects of interrelated operational source parameters on the excitation of atomic (I) and ionic (II) states for expanding the fundamental understanding of this microplasmas characteristics as an excitation source for optical emission spectroscopy (OES) analyses. Parameters that were investigated for identifying the key driving forces for atomic and ionic excitation conditions were discharge current, interelectrode gap, and He sheath and counter gas flows. The addition of the He counter gas flow allowed assessment of the additional parameter relevant when aerosol samples are introduced following laser ablation sampling of solid matrices. The introduction of the analytes (500 μg g−1 copper and zinc in 2% HNO3) in liquid form through the solution capillary permitted the investigation of source parameter effects, without introducing additional influences from solid sampling such as heterogeneous particle populations. Individual driving forces for excitation/ionization conditions and inter-parametric dependencies were assessed by changing the operating conditions according to a design of experiment (DOE) plan and monitoring Zn and Cu atomic and ionic emission lines (Zn I 213.9 nm, Cu I 324.7 nm, Zn I 481.1 nm, and Zn II 202.5 nm). Pareto plots of standardized effects were used for evaluating levels of significance as well as magnitudes of both individual and inter-active parametric effects on emission responses, background emissions and signal-to-background ratios as well as the LS-APGDs tolerance against changes in excitation conditions (i.e. robustness). The results indicate that parameter settings leading to high plasma power density are the key driving forces for enhanced analyte emission, with the inter-electrode distances showing the most pronounced influences for the investigated parameter space.

Liquid Sampling–Atmospheric Pressure Glow Discharge as a Secondary Excitation Source for Laser Ablation-Generated Aerosols: Parametric Dependence and Robustness to Particle Loading

Liquid sampling–atmospheric pressure glow discharge (LS-APGD) microplasma is being developed as a secondary vaporization–excitation source for the optical emission analysis of laser ablation (LA)-generated particle populations. The practicalities of this coupling are evaluated by determining the influence of source parameters on the emission response and the plasmas robustness upon LA introduction of easily ionized elements (EIEs). The influence of discharge current (45–70 mA), LA carrier gas flow rate (0.1–0.8 L min–1), and electrode separation distance (0.5–3.5 mm) was studied by measuring Cu emission lines after ablation of a brass sample. Best emission responses were observed for high-discharge currents, low He carrier gas flow rates, and relatively small (<1.5 mm) electrode gaps. Plasma robustness and spectroscopic matrix effects were studied by monitoring Mg(II): Mg(I) intensity ratios and N2-derived plasma rotational temperatures after the ablation of Sr- and Ca-containing pellets. Plasma robustness investigations showed that the plasma is not appreciably affected by the particle loadings, with the microplasma being slightly more ionizing in the case of Ca introduction. In neither case did the concentration of the concomitant element change the robustness values, implying a high level of robustness. Introduction of the LA particles results in slight increases in the rotational temperatures (~10% relative), with Ca-containing particles having a greater effect than Sr-containing particles. The observed variation of 9% in the plasma rotational temperature is in the same order of magnitude as the short-term reproducibility determined by the proposed LA-LS-APGD system. The determined rotational temperatures ranged from 1047 to 1212 K upon introducing various amounts of Ca and Sr. The relative immunity to LA particle-induced matrix effects is attributed to the relatively long residence times and high power densities (10 W mm–3) of the LS-APGD microplasma.

Solid-phase extraction microfluidic devices for matrix removal in trace element assay of actinide materials

Advances in sample nebulization and injection technology have significantly reduced the volume of solution required for trace impurity analysis in plutonium and uranium materials. Correspondingly, we have designed and tested a novel chip-based microfluidic platform, containing a 100-µL or 20-µL solid-phase microextraction column, packed by centrifugation, which supports nuclear material mass and solution volume reductions of 90% or more compared to standard methods. Quantitative recovery of 28 trace elements in uranium was demonstrated using a UTEVA chromatographic resin column, and trace element recovery from thorium (a surrogate for plutonium) was similarly demonstrated using anion exchange resin AG MP-1. Of nine materials tested, compatibility of polyvinyl chloride (PVC), polypropylene (PP), and polytetrafluoroethylene (PTFE) chips with the strong nitric acid media was highest. The microcolumns can be incorporated into a variety of devices and systems, and can be loaded with other solid-phase resins for trace element assay in high-purity metals.

Novel sample introduction system to reduce ICP-OES sample size for plutonium metal trace impurity determination

A new methodology for trace elemental analysis in plutonium metal samples was developed by interfacing the novel micro-FAST sample introduction system with an ICP-OES instrument. This integrated system, especially when coupled with a low flow rate nebulization technique, reduced the sample volume requirement significantly. Improvements to instrument sensitivity and measurement precision, as well as long term stability, were also achieved by this modified ICP-OES system. The sample size reduction, together with other instrument performance merits, is of great significance, especially to nuclear material analysis.

Laser ablation – inductively couple plasma – mass spectrometry/laser induced break down spectroscopy: a tandem technique for uranium particle characterization

Laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) in tandem with laser induced breakdown spectroscopy (LIBS) was employed to chemically map and characterize uranium particles. The uranium particles were doped in various concentrations (0.01, 0.1, 1.0, and 2.0%) to a 50 : 50 Ni : Fe mixture. There was an excellent correlation in regards to concentration and the LA-ICP-MS measurements. In addition, the isotopic composition of the uranium particles was determined within 10% measurement uncertainty. LIBS measurements also showed strong agreement in the particle mapping when compared to the LA-ICP-MS analysis. Moreover, the total analysis time for a 5 × 5 mm area was only 50 minutes. These data suggest that the tandem LA-ICP-MS/LIBS technique can provide rapid and valuable information for nuclear material safeguards and actinide material characterization.

Optimization of capillary-channeled polymer (C-CP) fiber stationary phase extractions of proteins from MALDI-MS suppressing media

Solid phase extraction (SPE) is an essential component in many proteomic analyses involving matrix-assisted laser desorption/ionization mass spectrometry detection (MALDI-MS). Parametric variables affecting the efficiency of protein extractions from buffer (Tris–HCl) and urine media using a capillary-channeled polymer (C-CP) fiber stationary phase in a tip-based format are evaluated. Proteins are immobilized on to polypropylene (PP) C-CP fibers and eluted in organic solvents conducive to high analytical performance using a simple benchtop centrifuge to drive the fluids. Experimental variables, including elution solvent strength, aqueous wash volume, the interactive aspects of fiber tip length (2.5–10 mm), and loading and elution volumes, were evaluated to gain insights into fundamental processes and to optimize protein recoveries/analytical signals. Based on the MALDI-MS responses of the test proteins cytochrome c, lysozyme, and myoglobin, the optimized elution solvent was determined to be a 60 : 40 ACN–H2O mixture with 0.07% trifluoroacetic acid (TFA). It was determined that different aqueous wash volumes were required to flush test solution remnants, depending on the buffer concentration, concluding that a 100 μL wash was effective at concentrations of up to 1 M. Fiber tip length was explored to determine the limits of loading volume/protein mass for each bed size while efficiently extracting from the buffer environment. Interestingly, the shortest C-CP fiber tips provided the best efficiency, isolating nanogram levels of protein from 1 μL aliquots of sample. After optimization, a proof of practice experiment was performed to extract the three-protein suite (<5 μM of each) from a synthetic urine matrix. The previously undetected proteins could be readily distinguished with high spectral clarity due to the SPE procedure utilizing C-CP fiber packed micropipette tips.

Capillary‐channeled polymer (C‐CP) fibers for the rapid extraction of proteins from urine matrices prior to detection with MALDI‐MS

While MS is a powerful tool for biomarker determinations, the high salt content and the small molecules present in urine poses incredible challenges. Separation/extraction methods must be employed for the isolation of target species at relevant concentrations. Micropipette tips packed with capillary‐channeled polymer (C‐CP) fibers are employed for the SPE of proteins from a synthetic and a certified urine matrix.

An automated micro-separation system for the chromatographic removal of uranium matrix for trace element analysis by ICP-OES

An automated, miniaturized, off-line separation technique is presented here using an Elemental Scientific Inc. microFAST MC system with UTEVA resin to extract the uranium matrix from its trace element impurities in aqueous media. The collected fractions were analyzed for ~ 30 trace elements using inductively coupled plasma - optical emission spectroscopy. Ten replicate samples were processed with a single column resulting in precision ranging from 3.3% to 6.2% relative standard deviation with regards to the trace element recoveries. Accuracy, with respect to trace element concentrations in the U3O8 Certified Reference Material 124-1, resulted in an average of 13.9% relative deviation while accuracy to the Canadian U3O8 reference material, CUP-2, resulted in an average relative deviation of 8.6%. The total separation time of this automated process was reduced to ~ 30 min per sample while employing a 0.5 mL UTEVA chromatographic resin bed and 2.5 mg of uranium.