Liquid Sampling-atmospheric Pressure Glow Discharge Research Articles

Characterization of arsenic species by liquid sampling-atmospheric pressure glow discharge ionization mass spectrometry

A liquid sampling-atmospheric pressure glow discharge (LS-APGD) ionization source operating at a nominal power of 30 W and solution flow rate of 30 µL min−1 and supported in a He sheath gas flow rate of 500 mL min−1 was interfaced to an Orbitrap mass spectrometer and assessed for use in rapid identification of inorganic and organic arsenic species, including As(III), As(V), monomethylarsonic acid, dimethylarsinic acid, and arsenobetaine in a 2% (v/v) nitric acid medium. Mass spectral acquisition in low-resolution mode, using only the ion trap analyzer, provided detection of protonated molecular ions for AsBet (m/z 179), DMA (m/z 139), MMA (m/z 141), and As(V) (m/z 143). As(III) is oxidized to As(V), likely due to in-source processes. Typical fragmentation of these compounds resulted in the loss of either water or methyl groups, as appropriate, i.e., introducing DMA also generated ions corresponding to MMA and As(V) as dissociation products. Structure assignments were also confirmed by high-resolution Orbitrap measurements. Spectral fingerprint assignments were based on the introduction of solutions containing 5 µg mL−1 of each arsenic compound.

Investigation of potential polyatomic interferences on uranium isotope ratio measurements for the LS-APGD-Orbitrap MS system

The determination of actinide (e.g., U and Pu) content and isotopics is of importance to the nuclear forensics and safeguards communities. However, in the analysis of environmental samples, such as those collected by the International Atomic Energy Agency, uranium measurements can be complicated by isobaric interferences from polyatomic variants of heavy elements (e.g., Pb). This leads to complex, time-consuming sample manipulations (i.e., separations) before determining isotope ratios. An alternative strategy to sample pretreatment is to use high-resolution mass spectrometric platforms during the analysis to fully resolve the uranium isotopes from potential polyatomic interferences, negating the need for prior chemical separation. The liquid sampling - atmospheric pressure glow discharge (LS-APGD) coupled with an Orbitrap mass spectrometer provides a high-resolution (>70,000 m/Δm at m/z 200) inorganic mass spectrometry platform. As a demonstration of the power of this instrumental platform, and indeed, the high-resolution approach in general, uranium isotope ratios were determined in the presence of elemental impurities (e.g., Pb, Pt, Ta, W) commonly encountered with environmental sample swipe analysis, without any prior treatment. Even at elemental impurity concentrations of 1000–5000× relative to uranium, no interference was observed with the 235U or 238U signal. In addition, the 235U/238U isotope ratio for the samples with the concomitants present are within 2 standard deviations of the values obtained without their addition, indicating that these impurities do not impact the determined uranium isotope ratio. These findings represent a significant first step in leveraging the high resolution of the LS-APGD-Orbitrap-MS to overcome isobaric and molecular interferences instead of relying on chemical separations.

Initial Characterization and Optimization of the Liquid Sampling-Atmospheric Pressure Glow Discharge Ionization Source Coupled to an Orbitrap Mass Spectrometer for the Determination of Plutonium.

Plutonium measurements are essential to the nuclear forensics and safeguards community. The liquid sampling-atmospheric pressure glow discharge (LS-APGD) microplasma ionization source coupled with an Orbitrap mass spectrometer is a proven platform for uranium isotope ratio determinations. This work expands the LS-APGD-Orbitrap platform capabilities by reporting the first-ever analysis of plutonium with the LS-APGD and the first-ever measurement of elemental plutonium with an Orbitrap mass spectrometer. This coupling has the potential to dramatically reduce the complex sample manipulations required for traditional analysis techniques employed for actinide isotope ratio determinations. As a first step toward the goal of simultaneous uranium and plutonium isotope ratio determinations, the initial characterization and optimization of the platform for the detection of plutonium are reported. Collision-induced dissociation modality settings were optimized to reduce water-related and other molecular clusters containing plutonium, maximizing 242Pu16O2+ responses. A design of experiments study was conducted to optimize the discharge conditions of the dual-electrode LS-APGD toward the responsivity of 242Pu16O2+. The measurement sensitivity was determined from a Pu response curve, yielding a limit of detection of 10 fg (absolute) of total analyte when data was collected and processed with a Spectroswiss FTMS Booster X2 data acquisition system. Additionally, plutonium and uranium were measured in a simultaneous acquisition, and each analyte remained unaffected by the other. It is believed that the LS-APGD-Orbitrap platform could be a valuable addition to the nuclear forensics' toolbox and, indeed, other scientific disciplines and regulatory communities in which rapid, high-resolution plutonium determinations are paramount.

Coupling of the Liquid Sampling-Atmospheric Pressure Glow Discharge to Orbitrap Mass Analyzers for Uranium Isotope Ratio Analysis: Evolution of the Methodology and Implications to the Field.

Just over a decade ago, a truly outside-of-the-box approach to isotope ratio mass spectrometry (IRMS) was undertaken between research groups at Clemson University and the Pacific Northwest National Laboratory. The original motivation dealt with projections as to whether or not microplasmas could be developed into practical elemental ionization sources, perhaps for transportable analysis applications. In particular, the use of the liquid sampling-atmospheric pressure glow discharge (LS-APGD) was pursued. Its interfacing to an ultra-high resolution Orbitrap platform, proved not only facile, but opened up a wealth of potential applications. Here, we lay out a historical, tutorial description of the interfacing and the evolution of the methodology regarding IRMS of uranium. Practical challenges and opportunities are described, which hopefully provide guidance to further applications in high resolution IRMS. It is hoped that, while detailed and lengthy, the didactic nature of the presentation provides experimental insights and tips, and also serves as an homage to our very good friend, Professor Gary M. Hieftje, whose scientific inspiration and comradery have been immeasurably important in our own careers.

Improved uranium isotopic ratio determinations for the liquid sampling-atmospheric pressure glow discharge orbitrap mass spectrometer by use of moving average processing

Applying moving average processing to isotope ratio determinations collected using an LS-APGD source coupled with an Orbitrap MS increases the precision of the measurements and reduces the number of points needed to collect high precision data.

Combined atomic and molecular (CAM) ionization with the liquid sampling-atmospheric pressure glow discharge microplasma.

In a world where information-rich methods of analysis are often sought over those with superior figures of merit, there is a constant search for ionization methods which can be applied across diverse analytical systems. The liquid sampling-atmospheric pressure glow discharge (LS-APGD) is a microplasma device which has the inherent capabilities to operate as a combined atomic and molecular (CAM) ionization source. The plasma is sustained by placement of a high voltage (~500 V, dc) onto an electrolytic solution through which the analyte is generally delivered to the discharge. Judicious choice of the solvent provides a means of obtaining atomic/elemental and/or molecular mass spectra. Presented here are the diverse modes of sample introduction and mass spectrometer platforms to which the LS-APGD has been interfaced. Likewise, representative spectra and figures of merit are presented towards elemental and isotope ratio measurements, as well as application to small organic molecules, organometallic complexes, and intact proteins. It is believed that the diversity of analytical applications and ready implementation across the entirety of mass spectrometry platforms portends a level of versatility not realized with other ionization sources.

Roles of collisional dissociation modalities on spectral composition and isotope ratio measurement performance of the liquid sampling – atmospheric pressure glow discharge / orbitrap mass spectrometer coupling

Coupling of the liquid sampling – atmospheric pressure glow discharge (LS-APGD) ionization source with Orbitrap mass spectrometers has demonstrated tremendous success in regards to isotope ratio (IR) measurements. Isotope ratios from a wide variety of analytes have been measured, with a primary focus on the 235U/238U measurement, yielding IR measurement accuracy and precision that meets international targets. Orbitrap mass spectrometers are not commonly associated with either elemental analysis or isotope ratio measurements. Understanding sources of IR errors and imprecision which arise from both the LS-APGD ionization source and the Orbitrap system itself can assist with enhancing measurement figures of merit. Because a variety of solvent related species are generated in the LS-APGD, the preponderance of those species in the orbitrap analyzer affects the recoveries and IR qualities for target analytes. In this work, the effects of the collisional dissociation of solvent-related species are investigated as a source of variability in IR measurements. Additionally, the quadrupole band-pass filter used in the ThermoScientific Q Exactive Focus is investigated for its transmission properties along with the effects of the transmission window on IR accuracy and precision. With these considerations, updated IR measurements are presented for a few analytes, with measurement precision never exceeding 0.71% RSD, and going down to a level of 0.05% for diverse species. It is expected that these considerations will inform how IR measurements should be taken on this platform, with lessons that may be applicable to other instruments.

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.

Development of an integrated, single electrode liquid sampling – atmospheric pressure glow discharge microplasma ionization source

The liquid sampling – atmospheric pressure glow discharge (LS-APGD) has been demonstrated as a versatile source for optical emission spectroscopy (OES) and mass spectrometry (MS). The initial LS-APGD design consisted of a solution cathode and a stainless-steel counter electrode. While shown to perform well across various MS applications, efforts to improve upon the initial design of this system have been ongoing. In this study, an integrated, single electrode design is presented in which the plasma is generated between the solution electrode and the sampling orifice of the mass spectrometer. The driving force, in this case, is the reduction of system components and moving parts, thus simplifying the ionization source. This preliminary study includes an optimization of the operating parameters with a focus on maintaining a stable plasma and improving analytical performance. The multi-parametric evaluation completed here included discharge current, solution flow rate, and the electrode/sampling orifice gap. Most significantly, the manner in which the plasma is powered has changed from the solution cathode being held at ground potential to being powered negatively versus the grounded sampling orifice. Comparisons between the formation of solvent related species in the original source design and this version were performed. With optimal conditions determined, initial LODs were established for Rb, Ag, Tl, and U, with values ranging from 0.02–0.17 ng mL−1 for 20 μL injections, demonstrating increased sensitivity (72-1900×) relative to the previous two-electrode design on the same MS system which employed 50 μL injections.

Coupling of Laser Ablation and the Liquid Sampling-Atmospheric Pressure Glow Discharge Plasma for Simultaneous, Comprehensive Mapping: Elemental, Molecular, and Spatial Analysis.

The spatial distributions of elemental and molecular species are vital pieces of information for a broad number of applications such as material development and bio/environmental analysis. There is currently no single analytical method that can simultaneously acquire elemental, molecular, and spatial information from a single sample. This paper presents the coupling of an NWR213 laser ablation (LA) system to the liquid sampling-atmospheric pressure glow discharge (LS-APGD) microplasma for combined atomic and molecular (CAM) analysis. The work demonstrates a fundamental balance that must be considered between the extent of fragmentation of molecules and ionization of atoms for CAM analysis. Detailed studies showed that the interelectrode gap to be a critical parameter for controlling the ionization efficiency of atomic and molecular species. Utilizing Design-of-Experiment (DoE) procedures, the discharge current was also found to be a significant parameter to control. Elemental lead, caffeine, and simultaneous lead and caffeine analysis via LA-LS-APGD-MS was made possible through improved understanding of the influence of plasma parameters on the product mass spectra of laser-ablated particles. Finally, a chemical map of elemental lead and molecular caffeine, from lead nitrate and caffeine residues, was generated, demonstrating the comprehensive mapping capabilities of LA-LS-APGD-MS. The practical relevance of the capabilities is demonstrated by mapping glutamic acid from a cryosectioned chicken breast with a thallium spike deposited within the tissue. It is believed that the LA-LS-APGD-MS could be a valuable methodology for the simultaneous mapping of elemental and molecular species from a variety of samples.

Rapid Determination of Uranium Isotopic Abundance from Cotton Swipes: Direct Extraction via a Planar Surface Reader and Coupling to a Microplasma Ionization Source.

The collection of solid particulates and liquids from surfaces by the use of cloth swipes is fairly ubiquitous. In such methods, there is a continuous concern regarding the ability to locate and quantitatively sample the analyte species from the material. In this effort, we demonstrate the initial coupling of an Advion Plate Express plate reader to a liquid sampling-atmospheric pressure glow discharge (LS-APGD) microplasma ionization source with an Orbitrap mass spectrometer to perform uranium isotopic analyses of solution residues on cotton swipes. The Plate Express employs a sampling probe head to engage and seal against the swipe surface. Subsequentially, the analyte residues are desorbed and transported within a 2% HNO3 electrolyte flow to the ionization source. Quantitative recoveries were observed following a single 30 s extraction step, with the absolute mass sampled per extraction being ∼100 ng. While the intrasample variability in the analytical responses for triplicate sampling of the same swipe yield ∼30% RSD, this lack of precision is offset by the ability to determine isotope ratios for enriched uranium specimens with a precision of better than 10% RSD. Pooled, intersample precision (n = 9) was found to be <5%RSD across the various sample compositions. Finally, 235U/238U determinations (ranging from 0.053 to 1.806) were accurate with errors of <10%, absolute. The 234U- and 236U-inclusive ratios were determined with similar accuracy in enriched samples. While the driving force for the effort is in the realm of nuclear nonproliferation efforts, the ubiquitous use of cloth swipes across many application areas could benefit from this convenient approach, including the use of versatile, reduced-format mass spectrometer systems.

Liquid Sampling-Atmospheric Pressure Glow Discharge Ionization as a Technique for the Characterization of Salt-Containing Organic Samples.

Typical ionization techniques used for mass spectrometry (MS) analysis face challenges when trying to analyze organic species in a high-salt environment. Here, we present results using a recently developed ionization source, liquid sampling-atmospheric pressure glow discharge (LS-APGD), for marine-relevant salt-containing organic samples. Using two representative sample types, a triglyceride mixture and dissolved organic matter, this method is compared to traditional electrospray ionization (ESI) under saline and neat conditions. LS-APGD produced equal or higher (15%+) ion intensities than those of ESI for both salt-containing and neat samples, although important differences linked with adduct formation in high-salt conditions explain the molecular species observed. For all sample types, LS-APGD observed a higher diversity of molecules under optimized settings (0.25 mm electrode spacing at 20 mA) compared to traditional ESI. Furthermore, because the LS-APGD source ionizes molecular species in a ∼1 mm3 volume plasma using a low-power source, there is the potential for this method to be applied in field studies, eliminating desalting procedures, which can be time-consuming and nonideal for low-concentration species.

Ultra-High Resolution Elemental/Isotopic Mass Spectrometry (m/Δm > 1,000,000): Coupling of the Liquid Sampling-Atmospheric Pressure Glow Discharge with an Orbitrap Mass Spectrometer for Applications in Biological Chemistry and Environmental Analysis.

Many fundamental questions of astrophysics, biochemistry, and geology rely on the ability to accurately and precisely measure the mass and abundance of isotopes. Taken a step further, the capacity to perform such measurements on intact molecules provides insights into processes in diverse biological systems. Described here is the coupling of a combined atomic and molecular (CAM) ionization source, the liquid sampling-atmospheric pressure glow discharge (LS-APGD) microplasma, with a commercially available ThermoScientific Fusion Lumos mass spectrometer. Demonstrated for the first time is the ionization and isotopically resolved fingerprinting of a long-postulated, but never mass-spectrometrically observed, bi-metallic complex Hg:Se-cysteine. Such a complex has been implicated as having a role in observations of Hg detoxification by selenoproteins/amino acids. Demonstrated as well is the ability to mass spectrometrically-resolve the geochronologically important isobaric 87Sr and 87Rb species (Δm ~ 0.3mDa, mass resolution m/Δm ≈ 1,700,000). The mass difference in this case reflects the beta-decay of the 87Rb to the stable Sr isotope. These two demonstrations highlight what may be a significant change in bioinorganic and atomic mass spectrometry, with impact expected across a broad spectrum of the physical, biological, and geological sciences. Graphical Abstract "".

Monochromatic spatial imaging of the liquid sampling – Atmospheric pressure glow discharge: Effects of gas flow on spatial profiles of analyte and background species

The liquid sampling – atmospheric pressure glow discharge (LS-APGD) microplasma was paired with a monochromatic imaging spectrometer (MIS) to interrogate relevant plasma species/elements. The coupling of the LS-APGD source to the MIS allows for the generation of spatial emission profiles at specific analyte and background species' wavelengths. A major goal of this work was to utilize these emission profiles to gain an understanding of the species' location in the plasma as well as the effects of plasma gas flow on these emission profiles. It was found that as sheath gas and counter gas flow rates were increased, there were visible differences in intensity and distribution of emission from the monitored species in the plasma. Silver was used as the representative (test) analyte for this work due to its good sensitivity, wavelength positioning, and ability to provide continuity for comparison to previous works. The majority of analyte emission was found to occur at the tip of the solution electrode, where solutes are introduced to the plasma, whereas various background species emitted throughout the plasma. In addition to aiding in the understanding of plasma operation based on spatial emission characteristics, these studies also aided in optimizing the gas flow parameters for analyte emission intensity, signal-to-background (S/B), and signal-to-noise (S/N) values. The optimized gas flow parameters were found to be 0.7 and 0.1 L min−1 for the sheath and counter gas respectively.

Parametric optimization and spectral line selection for liquid sampling-atmospheric pressure glow discharge – optical emission spectroscopy

A design of experiment parameterization and line selection study was successful in improving LS-APGD-OES analytical performance.

Initial Benchmarking of the Liquid Sampling-Atmospheric Pressure Glow Discharge-Orbitrap System Against Traditional Atomic Mass Spectrometry Techniques for Nuclear Applications.

The integration of the liquid sampling-atmospheric pressure glow discharge (LS-APGD) ion source with Orbitrap mass spectrometers has resulted in new opportunities in the field of isotope ratio mass spectrometry. In a field that has been dominated by thermal ionization mass spectrometry (TIMS) and inductively coupled plasma mass spectrometry (ICP-MS) on quadrupole and scanning-mode sector field analyzer platforms for highly accurate and precise measurements, the LS-APGD-Orbitrap system offers a benchtop instrument capable of meeting the rigorous International Target Values for measurement uncertainty for uranium (U). In order to benchmark the LS-APGD-Orbitrap, a series of U certified reference materials with increasing 235U isotopic composition were analyzed. By using U samples ranging in enrichment from 1 to 80%, the ability of the system to measure isotope ratios over a wide range is demonstrated. This analysis represents the first time that the LS-APGD-Orbitrap system has been used to analyze highly enriched U samples, allowing for the measurement of each of the U isotopes, including 234U and 236U-related species, which had not been achieved previously. Ultimately, the LS-APGD-Orbitrap system was able to measure CRM U-800 (assayed as 235U / 238U = 4.265622) as 4.266922, with a combined uncertainty, (uc), of 0.040%. These results are compared to those obtained using traditional elemental mass spectrometers including TIMS and ICP-MS-based instruments. The effectiveness of the LS-APGD-Orbitrap MS system for measuring U isotopes shows excellent promise in nuclear forensics, safeguards, and other nuclear weapon-based applications. Graphical Abstract ᅟ.

Proof-of-concept: Interfacing the liquid sampling-atmospheric pressure glow discharge ion source with a miniature quadrupole mass spectrometer towards trace metal analysis in cell culture media

The facile interfacing of the LS-APGD to a commercial, small-format quadrupole MS holds promise for at-bioreactor elemental analysis.

Investigation of hydrophobic substrates for solution residue analysis utilizing an ambient desorption liquid sampling-atmospheric pressure glow discharge microplasma.

A practical method for preparation of solution residue samples for analysis utilizing the ambient desorption liquid sampling-atmospheric pressure glow discharge optical emission spectroscopy (AD-LS-APGD-OES) microplasma is described. Initial efforts involving placement of solution aliquots in wells drilled into copper substrates, proved unsuccessful. A design-of-experiment (DOE) approach was carried out to determine influential factors during sample deposition including solution volume, solute concentration, number of droplets deposited, and the solution matrix. These various aspects are manifested in the mass of analyte deposited as well as the size/shape of the product residue. Statistical analysis demonstrated that only those initial attributes were significant factors towards the emission response of the analyte. Various approaches were investigated to better control the location/uniformity of the deposited sample. Three alternative substrates, a glass slide, a poly(tetrafluoro)ethylene (PTFE) sheet, and a polydimethylsiloxane (PDMS)-coated glass slide, were evaluated towards the microplasma analytical performance. Co-deposition with simple organic dyes provided an accurate means of determining the location of the analyte with only minor influence on emission responses. The PDMS-coated glass provided the best performance by virtue of its providing a uniform spatial distribution of the residue material. This uniformity yielded an improved limits of detection by approximately 22× for 20 μL and 4 x for 2 μL over the other two substrates. While they operate by fundamentally different processes, this choice of substrate is not restricted to the LS-APGD, but may also be applicable to other AD methods such as DESI, DART, or LIBS. Further developments will be directed towards a field-deployable ambient desorption OES source for quantitative analysis of microvolume solution residues of nuclear forensics importance.

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

The liquid sampling-atmospheric pressure glow discharge (LS-APGD) microplasma provides great diversity in sampling strategies and analytical information content.

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.