Atmospheric-pressure solution-cathode glow discharge: A versatile ion source for atomic and molecular mass spectrometry

Solution Cathode Glow Discharge Coupled to Atmospheric Pressure Chemical Ionization for Elemental Detection of S and P in Organic Compounds

We report a post-plasma chemical ionization approach to evaluate solution cathode glow discharge (SCGD) for S and P elemental analysis. Here, the SCGD serves as a reactor to produce chemical vapors for S and P from organic compounds containing these elements, while a corona discharge operated in negative mode is used to ionize the products. The approach creates long-lived ions in atmospheric pressure, enabling direct investigation of chemical vapor products via mass spectrometric and ion mobility separations. The investigations indicate that SCGD converts S and P to H2SO4 and H3PO4, respectively. These species are then ionized as HSO4HNO3 - and H3PO4NO3HNO3- via reactions with NO3HNO3- produced by corona discharge. The response factors for P among several small molecules varies within 10% of the average response from the compounds, suggesting a reasonable species-independent characteristic. The response factors for S show larger variations among compounds, indicating a higher dependence of chemical vapor generation efficiency on analytes' chemical structures. Detection limits of 15 and 29 ng/mL are achieved for P and S detection, respectively. These figures are limited by background equivalent concentrations and low ion flux in the utilized ion mobility-time of flight mass spectrometer, indicating potential for significant improvements. In particular, the specificity of clustering for S and P-containing ions produced in this approach suggest facile analysis of S and P using quadrupole-based mass spectrometers for improved analytical performance.

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.

MODERN PLASMA-BASED DESORPTION/IONIZATION: FROM ATOMS AND MOLECULES TO CHEMICAL SYNTHESIS

Since the first mass spectrometry (MS) experiments were conducted by Thomson and Aston, plasmas have been used as ionization sources. Historically, plasma ion sources were used for these experiments because they were one of the few known sources of gas-phase ions at the time and they were relatively simple to setup and operate. Since then, developments in plasma ionization have continued to inform and motivate advances in other areas of MS. For example, plasma-desorption MS demonstrated ionization of large peptides and polymers more than 10 years before the first descriptions of electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). As a result, significant effort was placed on development of ionization approaches, mass analysis, and detection approaches for very large molecules: even before the advent of ESI and MALDI. Since then, new analytical challenges and opportunities in plasma ionization have arisen. In this review, the emerging trends in plasma-based ionization for several areas of MS will be discussed, including molecular ionization, elemental ionization, hybrid elemental and molecular ion sources, and unique chemical transformations. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Resolving Severe Elemental Isobaric Interferences with a Combined Atomic and Molecular Ionization Source-Orbitrap Mass Spectrometry Approach: The 87Sr and 87Rb Geochronology Pair

Many fields of basic and applied sciences, including geochronology, astronomy, metabolism, etc., rely on the ability of mass spectrometry to obtain isotope ratio measurements having a high degree of certainty. The inability to resolve difficult isobaric interferences plagues certain measurements. A combined atomic and molecular (CAM) ionization source has been interfaced to a high-field Orbitrap mass spectrometer to alleviate severe atomic, isobaric interferences. This work examines the geochronologically significant ⁸⁷Sr and ⁸⁷Rb isotope pair. The mass difference between ⁸⁷Sr and ⁸⁷Rb is approximately 0.3 mDa, requiring a minimum resolving power (R = m/Δm) of ∼290,000, a value ∼30× higher than available with sector-field elemental mass spectrometers. Under ultrahigh-resolution conditions, Sr isotope ratio accuracy and precision were evaluated using NIST Sr SRM 987, yielding precision values of <0.1% relative standard deviation (RSD) for the major isotopes and a calculated LOD of 2 pg mL^-1 (120 fg of Sr for a 60 μL injection). In addition to manipulating the signal transient length, the total number of ions in the electrostatic trap and the ⁸⁷Sr/⁸⁷Rb concentration ratio were found to influence resolution. Ultimately, the isotopes were baseline-resolved with a calculated mass resolution of >1.7M. At equal ⁸⁷Sr and ⁸⁷Rb intensities, ⁸⁷Sr/⁸⁶Sr was measured as 0.71294 (a relative error of only 0.37%) with a precision of 0.097% RSD, clearly reflecting the alleviation of the isobaric interference.

Five years of innovations in development of glow discharges generated in contact with liquids for spectrochemical elemental analysis by optical emission spectrometry

The newest achievements in the field of glow microdischarges generated in contact with a flowing liquid cathode (FLC) and a flowing liquid anode (FLA), used as the excitation sources for optical emission spectrometry (OES), were summarized herein. The design of recently reported discharge systems was compared and comprehensively discussed. A lot of effort was devoted to evaluate the effect of selected operating parameters, i.e., discharge voltage and current, sample flow rate, sample pH, jet-supporting gas flow rate, and discharge gap, on the microplasma stability and the intensity of measurable analytical signals. Furthermore, the influence of chemical modifiers, i.e., organic acids, alcohols, and surfactants, aimed at improving the sensitivity and reducing matrix effects, was referred to as well. Finally, the analytical performance and the application of these promising excitation sources for the elemental analysis of different-matrix samples were presented.

Interface-free integration of electrothermal vaporizer and point discharge microplasma for miniaturized optical emission spectrometer

A tungsten coil (W-coil) as an electrothermal vaporizer (ETV) was interface-free integrated with a point discharge (PD) microplasma as an excitation source for a miniaturized optical emission spectrometer (OES). The PD microplasma and the W-coil ETV were vertically arranged in one quartz tube, and the W-coil was directly placed just under the PD without any physical interface. Working gas flow could sweep them successively to carry analytes released from the W-coil to the PD microplasma, and exhaust out of the quartz tube. The W-coil firstly acted as an ETV for sampling, on which pipetted with a tiny amount of sample solution (typically 10 μL), followed by a heating program for eliminating sample moisture and matrix. Vapor of analytes was subsequently released from the W-coil at a high temperature and immediately swept into the PD microplasma for excitation of atoms to obtain their optical emission spectra. Due to the high temperature of the W-coil, the released analyte species from the W-coil probably had been already atomized/excited partly and partially maintained prior to entering into the PD microplasma, thus saving the energy in the PD for sample evaporation and dissociation. In other words, the W-coil indirectly provided extra energy to the PD microplasma, thus its excitation capability was intensified. Under optimal experimental conditions, simultaneous determination of Ag, As, Bi, Cd, Cu, In, Pb, Sb and Zn was achieved, with LODs of 0.6, 45, 40, 0.08, 15, 8, 8, 41 and 5 μg L^-1, respectively, and RSDs all less than 4.5% (n = 3, at corresponding concentrations of 5, 250, 250, 0.5, 100, 50, 50, 250 and 25 μg L^-1). The accuracy validation of the proposed technique was demonstrated by successfully analyzing Certified Reference Materials (CRMs, including water, soil, stream sediment and biological samples), and preliminarily analyzing one CRM with direct slurry injection, both with satisfactory results, which had no significant difference with the certificated values at a confidence level of 95% by t-test.

Unsupervised Reconstruction of Analyte-Specific Mass Spectra Based on Time-Domain Morphology with a Modified Cross-Correlation Approach

Concomitant species that appear at the same or very similar times in a mass-spectral analysis can clutter a spectrum because of the coexistence of many analyte-related ions (e.g., molecular ions, adducts, fragments). One method to extract ions stemming from the same origin is to exploit the chemical information encoded in the time domain, where the individual temporal appearances inside the complex structures of chronograms or chromatograms differ with respect to analytes. By grouping ions with very similar or identical time-domain structures, single-component mass spectra can be reconstructed, which are much easier to interpret and are library-searchable. While many other approaches address similar objectives through the Pearson's correlation coefficient, we explore an alternative method based on a modified cross-correlation algorithm to compute a metric that describes the degree of similarity between features inside any two ion chronograms. Furthermore, an automatic workflow was devised to be capable of categorizing thousands of mass-spectral peaks into different groups within a few seconds. This approach was tested with direct mass-spectrometric analyses as well as with a simple, fast, and poorly resolved LC-MS analysis. Single-component mass spectra were extracted in both cases and were identified based on accurate mass and a mass-spectral library search.

Highly sensitive determination of mercury by improved liquid cathode glow discharge with the addition of chemical modifiers

The sample introduction system of early miniaturized liquid cathode glow discharge (LCGD) was improved, and then LCGD was used as an excitation source of atomic emission spectrometry (AES) for the detection of mercury in water samples. The effects of chemical modifiers, such as ionic surfactants and low molecular weight organic substances, on emission intensities of Hg were investigated. The results showed that the addition of 4% methanol and 0.15% hexadecyltrimethylammonium bromide (CTAB) can enhance the net intensity of Hg about 15.5-fold and 7.7-fold, and the sensitivity (S) of Hg about 15.2-fold and 5.6-fold, respectively. Adding chemical modifiers markedly reduce the interferences from Fe³⁺, Co²⁺, Cl^-, Br^-, and I^- ions. The limit of detection (LOD) is reduced from 0.35 mg L^-1 for no chemical modifier to 0.03 mg L^-1 for 4% methanol and 0.05 mg L^-1 for 0.15% CTAB. The relative standard deviation (RSD) of Hg with adding 4% methanol, 0.15% CTAB and no chemical modifier is 2.38%, 1.17% and 3.00%, respectively, and the power consumption is below 75 W. All results indicated that the determination of Hg using improved LCGD with the addition of chemical modifiers has high sensitivity, low LOD, well precision and low power consumption. Water samples containing high mercury (10-20 mg L^-1) and low mercury (0.2-5 mg L^-1) can be determined by improved LCGD-AES with no chemical modifier and 4% methanol, respectively. Adding 4% methanol significantly reduces the matrix effects from real water samples. The measurement results of spiked samples using LCGD-AES are largely consistent with the spiked value. In addition, the recoveries of Hg are ranged from 95.7% to 114.8%, suggesting that the measurement results of Hg by LCGD-AES are accurate and reliable. Overall, the improved LCGD-AES with adding chemical modifiers is a promising technique for on-site and real-time monitoring of Hg in water samples because of its portability, lower cost and speed.

Chemical Analysis of Secondary Electron Emission from a Water Cathode at the Interface with a Nonthermal Plasma

When a nonthermal plasma and a liquid form part of the same circuit, the liquid may function as a cathode, in which case electrons are emitted from the liquid into the gas to sustain the plasma. As opposed to solid electrodes, the mechanism of this emission has not been established for a liquid, even though various theories have attempted to explain it via chemical processes in the liquid phase. In this work, we tested the effects of the interfacial chemistry on electron emission from water, including the role of pH as well as the hydroxyl radical, the hydrogen atom, the solvated electron, and the presolvated electron; it was found that none of these species are critical to sustain the plasma. We propose an emission mechanism where electrons, generated from ionized water molecules in the uppermost monolayers of solution, are emitted into the plasma directly from the conduction band of the water.

Considerations for uranium isotope ratio analysis by atmospheric pressure ionization mass spectrometry

The accurate measurement of uranium isotope ratios from trace samples lies at the foundation of achieving nuclear nonproliferation. These challenging measurements necessitate both the continued characterization and evaluation of evolving mass spectrometric technologies as well as the propagation of sound measurement approaches. For the first time in this work, we present the analysis of uranium isotope ratio measurements from discrete liquid injections with an ultra-high-resolution hybrid quadrupole time-of-flight mass spectrometer. Also presented are important measurement considerations for evaluating the performance of this type and other atmospheric pressure and ambient ionization mass spectrometers for uranium isotope analysis. Specifically, as the goal of achieving isotope ratios from as little as a single picogram of solid material is approached, factors such as mass spectral sampling rate, collision induced dissociation (CID) potentials, and mass resolution can dramatically alter the measured isotope ratio as a function of mass loading. We present the ability to accurately measure 235UO2+/238UO2+ down to 10s of picograms of solubilized uranium oxide through a proper consideration of mass spectral parameters while identifying limitations and opportunities for pushing this limit further.