Mechanistic Understanding Leads to Increased Ionization Efficiency and Selectivity in Dielectric Barrier Discharge Ionization Mass Spectrometry: A Case Study with Perfluorinated Compounds

Perfluorinated compounds have unique properties and many practical applications, but are difficult to ionize efficiently with soft ionization methods. An active capillary plasma ionization source based on dielectric barrier discharge ionization (DBDI) coupled with mass spectrometry was used to study the ionization pathway of perfluorinated compounds (PFCs), with the aim of both increasing the ionization efficiency and influencing the selectivity for generating product ions in negative ion mode. Cyclic and linear perfluorinated alkanes were found to mainly form [M – F]⁻ and [M – F + O]⁻ ions, respectively; the [M]^−• ion was only obtained at low discharge voltage. Additionally, fluorine attachment [M + F]⁻ was observed mostly for perfluorinated alkenes. An isotope labeling experiment with ¹⁸O₂ showed that the primary source of oxygen in the substitution reaction is molecular oxygen, reacting with the analyte in the form of O^–• ions. The abundance of [M – F + O]⁻ ions can thus be enhanced by increasing the plasma voltage to produce a higher O^–• ion density. The loss of the fluorine (without substitution by oxygen) was mainly observed at high frequency, a fact which can be exploited for tuning the ionization toward specific product ions. Overall, the mechanistic understanding of the ionization of PFCs allowed to increase the selectivity of the product ions, resulting in increased ionization efficiency

Atmospheric Helium Capillary Dielectric Barrier Discharge for Soft Ionization: Determination of Atom Number Densities in the Lowest Excited and Metastable States

The populations of the lowest excited helium states 2s ³S₁, 2s ¹S, 2p ³P⁰_J, and 2p ¹P⁰ created in an atmospheric helium capillary dielectric barrier discharge were determined by means of optical emission spectroscopy. The emitted intensities of 388, 501, 587, and 667 nm lines were measured side-on and end-on with respect to the discharge axis. The comparison of optically thin side-on spectra with end-on spectra, which exhibited the absorption effects in the line kernels, enabled the determination of the average values of the number densities <i>n</i>₁ in the considered He states along the plasma length <i>L</i>. The field of the theoretical profiles for a series of the <i>n</i>₁<i>L</i> parameters pertinent to the experimental conditions was calculated for each line. By introducing the experimental data into the field of calculated curves, <i>n</i>₁<i>L</i> corresponding to the particular state could be obtained. The measurements of the emission profiles were done as a function of the discharge voltage in the range covering homogeneous as well as filamentary DBD operation mode. Due to nonuniformity of the excited atom density distribution along the plasma, the values of <i>n</i>₁ could be obtained only in the homogeneous operation mode where the nonuniformity was small. The following maximum values were found for the number densities in the investigated states: <i>n</i>₁^av (2s ³S₁) = (2.9 ± 1.1) × 10¹³ cm^–3, <i>n</i>₁^av (2s ¹S) = (1.4 ± 0.5) × 10¹³ cm^–3, <i>n</i>₁^av (2p ³P⁰_J) = (1.1 ± 0.4) × 10¹³ cm^–3, <i>n</i>₁^av (2p ¹P⁰) = (4.2 ± 1.6) × 10¹² cm^–3, and they represent the average populations along the plasma column in the capillary

Investigations into Modeling and Further Estimation of Detection Limits of the Liquid Electrode Dielectric Barrier Discharge

The liquid electrode dielectric barrier discharge (LE-DBD) is a miniaturized atmospheric pressure plasma as emission excitation source for elemental determination with pulsed behavior. Metals dissolved in liquids are detectable in flow systems with low flow rates of 20 μL min^–1 by means of optical emission spectrometry using a simple portable spectrometer. Time-resolved determination of the hydrogen excitation temperature <i>T</i>_αβ indicates that the LE-DBD does not reach a stable state during a burning phase, whereat the maximum and minimum <i>T</i>_αβ is independent of the flow rate. Adding dissolved metals to the liquid electrode does not influence the minimum <i>T</i>_αβ at the end of a burning phase. With the help of measured doubly charged lanthanum lines and spatially resolved measurements, the mechanism of the liquid transfer into the plasma will be clarified. Emissions from metal oxides indicate a thermal evaporation transfer mechanism, but only an additional electrospray-like transfer mechanism can explain the observed La III emissions and nonhomogeneous spatial distribution of exited species. The reaction pathways for electrosprayed hydrated metal ions are discussed for triply and doubly charged ions. The analytical performance is evaluated for 23 elements from the categories of alkali, alkaline earth, transition, and poor metals. The achieved detection limits are between 0.016 mg L^–1 for Li and 41 mg L^–1 for Bi

Atmospheric Helium Capillary Dielectric Barrier Discharge for Soft Ionization: Determination of Atom Number Densities in the Lowest Excited and Metastable States

The populations of the lowest excited helium states 2s ³S₁, 2s ¹S, 2p ³P⁰_J, and 2p ¹P⁰ created in an atmospheric helium capillary dielectric barrier discharge were determined by means of optical emission spectroscopy. The emitted intensities of 388, 501, 587, and 667 nm lines were measured side-on and end-on with respect to the discharge axis. The comparison of optically thin side-on spectra with end-on spectra, which exhibited the absorption effects in the line kernels, enabled the determination of the average values of the number densities <i>n</i>₁ in the considered He states along the plasma length <i>L</i>. The field of the theoretical profiles for a series of the <i>n</i>₁<i>L</i> parameters pertinent to the experimental conditions was calculated for each line. By introducing the experimental data into the field of calculated curves, <i>n</i>₁<i>L</i> corresponding to the particular state could be obtained. The measurements of the emission profiles were done as a function of the discharge voltage in the range covering homogeneous as well as filamentary DBD operation mode. Due to nonuniformity of the excited atom density distribution along the plasma, the values of <i>n</i>₁ could be obtained only in the homogeneous operation mode where the nonuniformity was small. The following maximum values were found for the number densities in the investigated states: <i>n</i>₁^av (2s ³S₁) = (2.9 ± 1.1) × 10¹³ cm^–3, <i>n</i>₁^av (2s ¹S) = (1.4 ± 0.5) × 10¹³ cm^–3, <i>n</i>₁^av (2p ³P⁰_J) = (1.1 ± 0.4) × 10¹³ cm^–3, <i>n</i>₁^av (2p ¹P⁰) = (4.2 ± 1.6) × 10¹² cm^–3, and they represent the average populations along the plasma column in the capillary

Atmospheric Helium Capillary Dielectric Barrier Discharge for Soft Ionization: Determination of Atom Number Densities in the Lowest Excited and Metastable States

The populations of the lowest excited helium states 2s ³S₁, 2s ¹S, 2p ³P⁰_J, and 2p ¹P⁰ created in an atmospheric helium capillary dielectric barrier discharge were determined by means of optical emission spectroscopy. The emitted intensities of 388, 501, 587, and 667 nm lines were measured side-on and end-on with respect to the discharge axis. The comparison of optically thin side-on spectra with end-on spectra, which exhibited the absorption effects in the line kernels, enabled the determination of the average values of the number densities <i>n</i>₁ in the considered He states along the plasma length <i>L</i>. The field of the theoretical profiles for a series of the <i>n</i>₁<i>L</i> parameters pertinent to the experimental conditions was calculated for each line. By introducing the experimental data into the field of calculated curves, <i>n</i>₁<i>L</i> corresponding to the particular state could be obtained. The measurements of the emission profiles were done as a function of the discharge voltage in the range covering homogeneous as well as filamentary DBD operation mode. Due to nonuniformity of the excited atom density distribution along the plasma, the values of <i>n</i>₁ could be obtained only in the homogeneous operation mode where the nonuniformity was small. The following maximum values were found for the number densities in the investigated states: <i>n</i>₁^av (2s ³S₁) = (2.9 ± 1.1) × 10¹³ cm^–3, <i>n</i>₁^av (2s ¹S) = (1.4 ± 0.5) × 10¹³ cm^–3, <i>n</i>₁^av (2p ³P⁰_J) = (1.1 ± 0.4) × 10¹³ cm^–3, <i>n</i>₁^av (2p ¹P⁰) = (4.2 ± 1.6) × 10¹² cm^–3, and they represent the average populations along the plasma column in the capillary

Atmospheric Helium Capillary Dielectric Barrier Discharge for Soft Ionization: Determination of Atom Number Densities in the Lowest Excited and Metastable States

The populations of the lowest excited helium states 2s ³S₁, 2s ¹S, 2p ³P⁰_J, and 2p ¹P⁰ created in an atmospheric helium capillary dielectric barrier discharge were determined by means of optical emission spectroscopy. The emitted intensities of 388, 501, 587, and 667 nm lines were measured side-on and end-on with respect to the discharge axis. The comparison of optically thin side-on spectra with end-on spectra, which exhibited the absorption effects in the line kernels, enabled the determination of the average values of the number densities <i>n</i>₁ in the considered He states along the plasma length <i>L</i>. The field of the theoretical profiles for a series of the <i>n</i>₁<i>L</i> parameters pertinent to the experimental conditions was calculated for each line. By introducing the experimental data into the field of calculated curves, <i>n</i>₁<i>L</i> corresponding to the particular state could be obtained. The measurements of the emission profiles were done as a function of the discharge voltage in the range covering homogeneous as well as filamentary DBD operation mode. Due to nonuniformity of the excited atom density distribution along the plasma, the values of <i>n</i>₁ could be obtained only in the homogeneous operation mode where the nonuniformity was small. The following maximum values were found for the number densities in the investigated states: <i>n</i>₁^av (2s ³S₁) = (2.9 ± 1.1) × 10¹³ cm^–3, <i>n</i>₁^av (2s ¹S) = (1.4 ± 0.5) × 10¹³ cm^–3, <i>n</i>₁^av (2p ³P⁰_J) = (1.1 ± 0.4) × 10¹³ cm^–3, <i>n</i>₁^av (2p ¹P⁰) = (4.2 ± 1.6) × 10¹² cm^–3, and they represent the average populations along the plasma column in the capillary

Atmospheric Helium Capillary Dielectric Barrier Discharge for Soft Ionization: Determination of Atom Number Densities in the Lowest Excited and Metastable States

The populations of the lowest excited helium states 2s ³S₁, 2s ¹S, 2p ³P⁰_J, and 2p ¹P⁰ created in an atmospheric helium capillary dielectric barrier discharge were determined by means of optical emission spectroscopy. The emitted intensities of 388, 501, 587, and 667 nm lines were measured side-on and end-on with respect to the discharge axis. The comparison of optically thin side-on spectra with end-on spectra, which exhibited the absorption effects in the line kernels, enabled the determination of the average values of the number densities <i>n</i>₁ in the considered He states along the plasma length <i>L</i>. The field of the theoretical profiles for a series of the <i>n</i>₁<i>L</i> parameters pertinent to the experimental conditions was calculated for each line. By introducing the experimental data into the field of calculated curves, <i>n</i>₁<i>L</i> corresponding to the particular state could be obtained. The measurements of the emission profiles were done as a function of the discharge voltage in the range covering homogeneous as well as filamentary DBD operation mode. Due to nonuniformity of the excited atom density distribution along the plasma, the values of <i>n</i>₁ could be obtained only in the homogeneous operation mode where the nonuniformity was small. The following maximum values were found for the number densities in the investigated states: <i>n</i>₁^av (2s ³S₁) = (2.9 ± 1.1) × 10¹³ cm^–3, <i>n</i>₁^av (2s ¹S) = (1.4 ± 0.5) × 10¹³ cm^–3, <i>n</i>₁^av (2p ³P⁰_J) = (1.1 ± 0.4) × 10¹³ cm^–3, <i>n</i>₁^av (2p ¹P⁰) = (4.2 ± 1.6) × 10¹² cm^–3, and they represent the average populations along the plasma column in the capillary

Atmospheric Helium Capillary Dielectric Barrier Discharge for Soft Ionization: Determination of Atom Number Densities in the Lowest Excited and Metastable States

The populations of the lowest excited helium states 2s ³S₁, 2s ¹S, 2p ³P⁰_J, and 2p ¹P⁰ created in an atmospheric helium capillary dielectric barrier discharge were determined by means of optical emission spectroscopy. The emitted intensities of 388, 501, 587, and 667 nm lines were measured side-on and end-on with respect to the discharge axis. The comparison of optically thin side-on spectra with end-on spectra, which exhibited the absorption effects in the line kernels, enabled the determination of the average values of the number densities <i>n</i>₁ in the considered He states along the plasma length <i>L</i>. The field of the theoretical profiles for a series of the <i>n</i>₁<i>L</i> parameters pertinent to the experimental conditions was calculated for each line. By introducing the experimental data into the field of calculated curves, <i>n</i>₁<i>L</i> corresponding to the particular state could be obtained. The measurements of the emission profiles were done as a function of the discharge voltage in the range covering homogeneous as well as filamentary DBD operation mode. Due to nonuniformity of the excited atom density distribution along the plasma, the values of <i>n</i>₁ could be obtained only in the homogeneous operation mode where the nonuniformity was small. The following maximum values were found for the number densities in the investigated states: <i>n</i>₁^av (2s ³S₁) = (2.9 ± 1.1) × 10¹³ cm^–3, <i>n</i>₁^av (2s ¹S) = (1.4 ± 0.5) × 10¹³ cm^–3, <i>n</i>₁^av (2p ³P⁰_J) = (1.1 ± 0.4) × 10¹³ cm^–3, <i>n</i>₁^av (2p ¹P⁰) = (4.2 ± 1.6) × 10¹² cm^–3, and they represent the average populations along the plasma column in the capillary

Systematic Comparison between Half and Full Dielectric Barrier Discharges Based on the Low Temperature Plasma Probe (LTP) and Dielectric Barrier Discharge for Soft Ionization (DBDI) Configurations

Dielectric barrier discharge (DBD)-based analytical applications have experienced rapid development in recent years. DBD designs and parameters and the application they are used for can vary considerably. This leads to a diverse field with many apparently unique systems that are all based on the same physical principle. The most significant changes among DBDs used for chemical analysis are in how the discharge electrodes are separated from the ignited discharge gas. While the official definition of a DBD states that at least one electrode has to be covered by a dielectric to be considered a DBD, configurations with both electrodes covered by dielectric layers can also be realized. The electrode surface plays a major role in several plasma-related technical fields, surface treatment or sputtering processes, for example, and has hence been studied in great detail. Analytical DBDs are often operated at low power and atmospheric pressure, making a direct transfer of insight and know-how gained from the aforementioned well-studied fields complicated. This work focuses on comparing two DBD configurations: the low temperature plasma probe (LTP) and the dielectric barrier discharge for soft ionization (DBDI). The LTP is representative of a DBD with one covered electrode and the DBDI of a design in which both electrodes are covered. These two configurations are well suited for a systematic comparison due to their similar geometric designs based on a dielectric capillary

Soft Argon–Propane Dielectric Barrier Discharge Ionization

Dielectric barrier discharges (DBDs) have been used as soft ionization sources (DBDI) for organic mass spectrometry (DBDI-MS) for approximately ten years. Helium-based DBDI is often used because of its good ionization efficiency, low ignition voltage, and homogeneous plasma conditions. Argon needs much higher ignition voltages than helium when the same discharge geometry is used. A filamentary plasma, which is not suitable for soft ionization, may be produced instead of a homogeneous plasma. This difference results in N₂, present in helium and argon as an impurity, being Penning-ionized by helium but not by metastable argon atoms. In this study, a mixture of argon and propane (C₃H₈) was used as an ignition aid to decrease the ignition and working voltages, because propane can be Penning-ionized by argon metastables. This approach leads to homogeneous argon-based DBDI. Furthermore, operating DBDI in an open environment assumes that many uncharged analyte molecules do not interact with the reactant ions. To overcome this disadvantage, we present a novel approach, where the analyte is introduced in an enclosed system through the discharge capillary itself. This nonambient DBDI-MS arrangement is presented and characterized and could advance the novel connection of DBDI with analytical separation techniques such as gas chromatography (GC) and high-pressure liquid chromatography (HPLC) in the near future