Liquid Sampling-atmospheric Pressure Glow Research Articles

Preliminary investigation of an uncertainty budget for uranium isotope ratio analysis using a liquid sampling—atmospheric pressure glow discharge—orbitrap mass spectrometer system

Preliminary investigation of an uncertainty budget for uranium isotope ratio analysis using a liquid sampling—atmospheric pressure glow discharge—orbitrap mass spectrometer system

Improved Uranium Isotope Ratio Analysis in Liquid Sampling-Atmospheric Pressure Glow Discharge/Orbitrap FTMS Coupling through the Use of an External Data Acquisition System.

Isotope ratio (IR) analysis of natural abundance uranium presents a formidable challenge for mass spectrometry (MS): the required spectral dynamic range needs to enable the quantitatively accurate measurement of the 234UO2 species present at ∼0.0053% isotopic abundance. We address this by empowering a benchtop Orbitrap Fourier transform mass spectrometer (FTMS) coupled with the liquid sampling-atmospheric pressure glow discharge (LS-APGD) ion source and an external high-performance data acquisition system, FTMS Booster X2. The LS-APGD microplasma has demonstrated impressive capabilities regarding elemental and IR analysis when coupled with Orbitrap FTMS. Despite successes, there are limitations regarding the dynamic range and mass resolution that stem from space charge effects and data acquisition and processing restrictions. To overcome these limitations, the FTMS Booster was externally interfaced to an LS-APGD Q Exactive Focus Orbitrap FTMS to obtain time-domain signals (transients) and to process unreduced data. The unreduced time-domain data acquisition with user-controlled processing permit the evaluation of the effects of in-hardware transient phasing, increased transient lengths, advanced transient coadding, varying the length of a transient to be processed with a user-defined time increment, and the use of absorption-mode FT (aFT) processing methods on IR analysis. The added capabilities extend the spectral dynamic range of the instrument to at least 4-5 orders of magnitude and provide a resolution improvement from ∼70k to 900k m/Δm at 200 m/z. The empowered LS-APGD Orbitrap platform allows for the simultaneous measurement of 234UO2 and the prominent 235UO2 and 238UO2 isotopic species at their natural abundances, ultimately yielding improvements in performance when compared to previous uranium IR results on this same Q Exactive Focus instrument.

Evaluation of the powering modes and geometries of the Liquid Sampling – Atmospheric Pressure Glow Discharge – Orbitrap system for analytical performance and isotope ratio analysis

The liquid sampling atmospheric pressure glow discharge (LS-APGD) microplasma has shown promise in the fields of optical emission spectroscopy and mass spectrometry. In terms of mass spectrometry, it has allowed use of instruments normally applied in “organic” mass spectrometry, to be used for elemental/isotopic applications. The LS-APGD/Orbitrap combination is a particularly attractive alternative to traditional elemental MS systems due to its ability to perform ultra-high-resolution analyses, eliminating isobaric interferences which typically require extensive chemical separation prior to analysis. The LS-APGD has the ability to operate using four different powering modes; solution grounded cathode (SGC), solution grounded anode (SGA), solution powered cathode (SPC), and solution powered anode (SPA). To investigate the utility of each powering mode, the elemental responses and isotope ratio performance were assessed for the pertinent operating parameters (discharge current, solution flow rate, gas flow rate, and inter-electrode displacement). Experiments were performed using a 500 ng mL−1 multielement solution containing Rb, Ag, Ba, Tl, and U. Measurements of 235U/238U were performed using a 200 ng mL−1 solution of CRM-129a. Ultimately, it was determined that the SGC mode showed the best performance in terms of elemental intensity, accuracy, and precision for isotope ratio measurements. The optimal electrode configuration consists of the solution electrode in line with the ion-sampling orifice of the MS, with the counter electrode oriented perpendicular to that axis.

Demonstration of a novel ion-exchange column for pre-concentration of silver ions in optical emission spectroscopy utilizing a liquid-sampling atmospheric pressure glow discharge microplasma

Sulfonated, nylon 6 C-CP fibers hold promise as a simple, low cost phase for cation pre-concentration, here for the LS-APGD-OES.

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 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 plasma's 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.

Femtosecond laser ablation particle introduction to a liquid sampling-atmospheric pressure glow discharge ionization source

This work describes the use of a compact, liquid sampling–atmospheric pressure glow discharge (LS-APGD) ionization source to ionize metal particles within a laser ablation aerosol. Mass analysis was performed with a Thermo Scientific Exactive Mass Spectrometer which utilizes an orbitrap mass analyzer capable of producing mass resolution exceeding m/Δm > 160,000. The LS-APGD source generates a low-power plasma between the surface of an electrolytic solution flowing at several μl min−1 through a fused silica capillary and a counter electrode consisting of a stainless steel capillary employed to deliver the laser ablation particles into the plasma. Sample particles of approximately 100 nm were generated with an Applied Spectra femtosecond laser located remotely and transported through 25 meters of polyurethane tubing by means of argon carrier gas. Samples consisted of an oxygen free copper shard, a disk of solder, and a one-cent U.S. coin. Analyte signal onset was readily detectable relative to the background signal produced by the carrier gas alone. The high mass resolution capability of the orbitrap mass spectrometer was demonstrated on the solder sample with resolution exceeding 90,000 for Pb and 160,000 for Cu. In addition, results from a laser ablation depth-profiling experiment of a one cent coin revealed retention of the relative locations of the ∼10 μm copper cladding and zinc rich bulk layers.

Micro Plasma Generation Using Liquid Sampling-Atmospheric Pressure Glow Discharge

In open air and without any type of inert gas, stable and bright micro plasma was successfully obtained using Liquid Sampling Atmospheric Pressure Glow Discharge (LS-APGD). The discharge current varied between 20 to 80 mA with a maximum voltage 550 V, discharge gap 0.5–2 mm and solution pH of 1. The produced plasma operates in the normal glow discharge region at a low power (11–40 W). For analytical application the linear dynamic range is obtained up to 500 µg mL−1. Limits of detection based on 3σ of the background intensity determined for Ca I (422.673 nm), Cu I (324.754 nm), Fe I (497.5 nm), and Zn I (213.856 nm) are 0.3, 0.65, 0.1, 0.7 µg mL−1 respectively.

Effects of easily ionizable elements on the liquid sampling–atmospheric pressure glow discharge

A series of studies has been undertaken to determine the susceptibility of the liquid sampling–atmospheric pressure glow discharge (LS–APGD) atomic emission source to easily ionizable element (EIE) effects. The initial portions of the study involved monitoring the voltage drop across the plasma as a function of the pH to ascertain whether or not the conductivity of the liquid eluent alters the plasma energetics and subsequently the analyte signal strength. It was found that altering the pH (0.0 to 2.0) in the sample matrix did not significantly change the discharge voltage. The emission signal intensities for Cu(I) 327.4 nm, Mo(I) 344.7 nm, Sc(I) 326.9 nm and Hg(I) 253.6 nm were measured as a function of the easily ionizable element (sodium and calcium) concentration in the injection matrix. A range of 0.0 to 0.1% (w/v) EIE in the sample matrix did not cause a significant change in the Cu, Sc, and Mo signal-to-background ratios, with only a slight change noted for Hg. In addition to this test of analyte response, the plasma energetics as a function of EIE concentration are assessed using the ratio of Mg(II) to Mg(I) (280.2 nm and 285.2 nm, respectively) intensities. The Mg(II)/Mg(I) ratio showed that the plasma energetics did not change significantly over the same range of EIE addition. These results are best explained by the electrolytic nature of the eluent acting as an ionic (and perhaps spectrochemical) buffer.

An atmospheric pressure glow discharge optical emission source for the direct sampling of liquid media

A liquid sampling–atmospheric pressure glow discharge (LS-APGD) optical emission spectroscopy source is described. This device utilizes an electrolytic solution containing the analyte specimen as one of the discharge electrodes. The passage of an electrical current (either electrons or positive ions) across the solution/gas phase interface causes local heating and the volatilization of the analyte species. Collisions in the negative glow region of the plasma result in optical emission, which is characteristic of the analyte elements. Operation of this device with the analyte solution acting as either the cathode or anode is demonstrated. Typical discharge conditions include currents of 80 mA, potentials of up to 1000 V, with the i-V characteristics exhibiting ‘abnormal’ glow discharge behavior. The analyte-containing electrolyte solution is not limited to hydronium ions, as other salts (e.g., Li+ and Na+) also provide sufficient conductivity to sustain the discharge. Spatial maps of analyte emission and spectral background provide insights into the basic operation mechanism of the discharge. The use of a coaxial gas flow around the stainless-steel capillary outlet permits stable plasma operation at low liquid flow rates (0.1–0.3 mL min−1) where pulsation present in the liquid delivery system makes the discharge itself quite unstable. Analytical response curves for Hg, Mg, Na and Pb are demonstrated to have good linearity, with preliminary limits of detection determined to be in the range of 1.1–2.0 ppm (≈10 ng) for 5 µL sample volumes.