Spectroscopic characterization of a microplasma used as ionization source for ion mobility spectrometry

We report a miniaturized excitation source for soft ionization of molecules based on a dielectric barrier discharge. An atmospheric plasma is established at the end of a 500 μm diameter capillary using He as buffer gas. The plasma jet which comes out of the capillary is dependent on the gas flow rate. The mechanism of the production of N₂⁺ outside the capillary, which is relevant for the protonation of molecules and sustains the production of primary ions, is investigated by spatially resolved spectroscopic measurements throughout the plasma. Possible application of such miniaturized plasmas is the ionization of gaseous compounds under atmospheric pressure as an alternative to traditional APCI (atmospheric pressure chemical ionization). The miniaturized plasma was applied as ionization source for ion mobility spectrometry where the common sources are radioactive, thus limiting the place of installation. First measurements of gaseous compounds with such a plasma ion mobility spectrometer with promising results showed detection limits comparable or even better than those obtained using common radioactive ionization sources.     

Spatially resolved spectroscopic measurements of a dielectric barrier discharge plasma jet applicable for soft ionization

An atmospheric pressure microplasma ionization source based on a dielectric barrier discharge with a helium plasma cone outside the electrode region has been developed for liquid chromatography/mass spectrometry and as ionization source for ion mobility spectrometry. It turned out that dielectric barrier discharge ionization could be regarded as a soft ionization technique characterized by only minor fragmentation similar to atmospheric pressure chemical ionization (APCI). Mainly protonated molecules were detected. In order to characterize the soft ionization mechanism spatially resolved optical emission spectrometry (OES) measurements were performed on plasma jets burning either in He or in Ar. Besides to spatial intensity distributions of noble gas spectral lines, in both cases a special attention was paid to lines of N₂⁺ and N₂. The obtained mapping of the plasma jet shows very different number density distributions of relevant excited species. In the case of helium plasma jet, strong N₂⁺ lines were observed. In contrast to that, the intensities of N₂ lines in Ar were below the present detection limit. The positions of N₂⁺ and N₂ distribution maxima in helium indicate the regions where the highest efficiency of the water ionization and the protonation process is expected.     

Operation modes of the helium dielectric barrier discharge for soft ionization

Among different applications of dielectric barrier discharge (DBD) plasma, the soft ionization ability is certainly one of the most interesting. In this paper the helium plasma jet, produced by a capillary DBD, penetrating in the ambient atmosphere, has been spectroscopically investigated in dependence on applied voltage and helium flow. It was found that the change of the applied voltage leads to different discharge modes. Based on the measurements of the emission spectra of atomic He and N₂⁺ and N₂ molecules in the capillary and in the plasma jet with high spatial resolution, it can be assessed in which mode, i.e. under which conditions the plasma jet is expected to be most effective for soft ionization of molecules.     

The behavior of molecules in microwave-induced plasmas studied by optical emission spectroscopy. 1. Plasmas at atmospheric pressure

The behavior of molecules in different atmospheric microwave-induced plasmas (MIPs) has been studied by means of optical emission spectroscopy. This is in order to obtain more insight into molecular processes in plasmas and to investigate the feasibility of emission spectroscopy for the analysis of molecular compounds in gases, e.g. flue gases. Various molecular species (i.e. N₂, CO₂, H₂O, SF₆ and SO₂) have been introduced into discharges in argon or in molecular gases such as carbon dioxide or nitrogen. The plasmas were created and sustained by a guide-surfatron or a torch in the power range of 150 W to 2 kW. Only nitrogen sometimes yielded observable emission from the non-dissociated molecule (first and second positive system). Using other molecular gases, only dissociation and association products were observed (i.e. atomic species and diatomic molecules such as CN, C₂, CO, OH, NH and N₂⁺). The intensities of these products have been studied as a function of the concentration of introduced molecules, the position in the plasma and the composition of the plasma environment. Since in most cases the same diatomic association products are seen, observed associated molecules can only to some extend be related to the molecules originally present in the plasma gas. Therefore, it will be difficult to use atmospheric microwave discharges for the analysis of gas mixtures under the experimental conditions studied.     

Fermi-shuttle processes in the electron emission by ion impact: Contribution to radiation damages

Secondary electrons, formed in biological tissues by high-energy particle impact, significantly contribute to the fragmentation of small molecules and to single- and double-strand brakes in DNA. Differential spectra of electrons emitted in the collisions of decelerating swift ions are of vital importance for estimating ion impact radiation damages. We demonstrate that the so-called Fermi-shuttle-type acceleration mechanism can produce a significant enhancement in the emission of high-energy secondary electrons. Double differential cross-sections for electron emission, measured in the collisions of N⁺ and N₂⁺ ions with Ar targets at 750 keV/u impact energy, clearly show this effect. The measured cross-sections are in good agreement with the theoretical results of CTMC calculations. Multiple scattering contribution to the Ar spectra above 300 eV is proved to be significant.     

Electron excitation coefficients of neutral and ionic levels of krypton in Townsend discharges

In this paper, we present experimental results for excitation coefficients of krypton atoms to several Kr and Kr⁺ excited levels for E/N (electric field to gas particle number density ratio usually in units of Townsend, 1 Td = 10^− 21 V m²) values from 7 × 10^− 20 V m² to above 1 × 10^− 17 V m². The data have been obtained in two different parallel plate self-sustained Townsend discharge drift tubes. The spatial distribution of the emission intensities were recorded and then normalized to give excitation coefficients at the anode, by using the electron flux at this point. The values of these coefficients are placed on an absolute scale by using a standard tungsten ribbon lamp calibrated against a primary blackbody radiation standard. The ionization rates at different E/N are obtained from the spatial emission profiles.The data for atomic krypton levels 2p₂, 2p₃, 2p₅, 2p₆, 2p₇, 2p₈, 3p₅ and 3p₆ (in Paschen notation) were converted to excitation coefficients by using quenching coefficients from the literature. The emission coefficients of eight 4s²4p⁴ (³P)5p levels of Kr⁺ have also been measured for E/N values from about 1 × 10^− 18 V m² up to nearly 8 × 10^− 18 V m².     

A binder-free thin film anode composed of Co2 +-intercalated buserite grown on carbon cloth for oxygen evolution reaction

We present a binder-free catalytic anode for highly efficient and stable oxygen evolution reaction in alkaline media. The catalyst consists of a thin film of buserite-type layered manganese dioxide (MnO₂) intercalated with Co^2 + ions, resulting from electrodeposition of the layered MnO₂ film with tetrabutylammonium (Bu₄N⁺) ions on a carbon cloth, followed by ion-exchange of the initially incorporated Bu₄N⁺ with Co^2 + in solution. The electrode is capable to produce a current density of 10 mA cm^− 2 at an overpotential (η) of 377 mV with a Tafel slope of 48 mV dec^− 1, much superior to the layered MnO₂ without Co^2 +.     

Gas temperature determination in microwave discharges at atmospheric pressure by using different Optical Emission Spectroscopy techniques

Non-thermal plasmas sustained at atmospheric pressure are considered as a very promising technology for different purposes, in which the knowledge of the gas temperature is an important issue. In this paper, the gas temperatures of different argon microwave (2.45 GHz) plasma torches were determined by using different Optical Emission Spectroscopy techniques. Thus, they were estimated through the analysis of N₂⁺(B-X) and OH(A-X) molecular spectra. On the other hand, a method based on the measurement of the van der Waals broadening of 588.99 nm Na I line was employed, and the temperatures obtained from it were compared to the rotational temperatures derived from N₂⁺(B-X) and OH(A-X) rotational bands. A reasonable good agreement was found between the values of temperatures obtained by using the 588.99 nm Na I line and those obtained from N₂⁺ rotational band.     

Modeling of thermal and chemical non-equilibrium in a laser-induced aluminum plasma by means of a Collisional-Radiative model

A 0D numerical approach including a Collisional-Radiative model is elaborated in the purpose of describing the behavior of the nascent plasma resulting from the interaction between a 4 ns/65 mJ/532 nm Q-switched Nd:YAG laser pulse and an aluminum sample in vacuum. The heavy species considered are Al, Al⁺, Al²⁺ and Al³⁺ on their different excited states and free electrons. The translation temperatures of free electrons and heavy species are assumed different (T_e and T_A respectively). Numerous elementary processes are accounted for as electron impact induced excitation and ionization, elastic collisions, multiphoton ionization and inverse Bremsstrahlung. Atoms passing from the sample to gas phase are described by using classical vaporization theory so that the surface temperature is arbitrarily limited to values less than the critical point one at 6700 K. The laser flux density considered in the study is therefore moderate with a fluence lower than 7 J cm^? 2.This model puts forward the major influence of multiphoton ionization in the plasma formation, whereas inverse Bremsstrahlung turns out to be quasi negligible. The increase of electron temperature is mainly due to multiphoton ionization and T_e does not exceed 10,000 K. The electron induced collisions play an important role during the subsequent phase which corresponds to the relaxation of the excited states toward Boltzmann equilibrium. The electron density reaches its maximum during the laser pulse with a value ≈ 10²², 10²³ m^? 3 depending highly on the sample temperature. The ionization degree is of some percents in our conditions.     

Electrospray ionization mass spectrometry of a cerium(III) phosphomolybdate complex: Condensed and gas-phase cluster chemistry

Electrospray ionization quadrupole ion trap mass spectrometry (ESI-QIT/MS) of the ammonium cerium(III) phosphomolybdate complex (NH₄)₁₁[Ce(III)(PMo₁₁O₃₉)₂] in aqueous media has revealed a concentration-dependent behavior. Under fixed instrumental parameters, the Ce-containing polyoxomolybdate complexes H₂Ce(III)P₂Mo₂₂O₇₅^3? and Ce(III)PMo₁₁O₃₈^2? are the primary species present at 11 mM (pH = 4.3); at 0.7 mM (pH = 3.6), Ce(III)PMo₁₀O₃₅^2? is the predominant species, Ce(III)PMo₁₁O₃₈^2? is quite diminished, and H₂Ce(III)P₂Mo₂₂O₇₅^3? is absent. As a result of the complex isotopic fingerprints from multiple molybdenums, compositions of such ions are difficult to assign—successive collision induced dissociation (CID) of large ions produced smaller ions for which calculated and experimental isotopic patterns could be compared. The oxidation state of Ce and the number of counter cations on negative complexes was discerned from spectra of ions containing ¹H⁺ and ⁷Li⁺. The overall result is an ESI method applicable to phosphomolybdate complexes containing redox sensitive f-block metal ions such as Ce(IV) and Pu(III/IV). Dissociation studies also gave insight into favored fragmentation pathways, and generated gas ions with empirical formulae similar to known condensed-phase ions. Deconvolution of concentration- and pH-dependent solution behavior via ESI/MS and ³¹P NMR spectroscopy showed speciation dependent on solution concentration, not on pH.     

Effects of N-heteroaromatic ligands on highly luminescent mononuclear copper(I)–halide complexes

A series of mononuclear Cu(I)–halide complexes, [CuX(PPh₃)₂(L)] (X = Cl⁻, Br⁻, I⁻; PPh₃ = triphenylphosphine; L = pyridine (py), isoquinoline (iq), 1,6-naphthyridine (nap)), were synthesized. The emission color of [CuX(PPh₃)₂(L)] varies from blue to red by changing the L ligands and the halide ions, and all the complexes exhibit high emission quantum yields (0.16–0.99) in the crystals. The emission studies revealed that the emissive states of [CuX(PPh₃)₂(L)] differ depending on the L ligand. Complexes [CuX(PPh₃)₂(py)] and [CuX(PPh₃)₂(nap)] mainly emit from the singlet metal-to-ligand charge transfer mixed with the halide-to-ligand charge transfer (¹(M + X)LCT) state at room temperature. In contrast, emissions from [CuX(PPh₃)₂(iq)] at room temperature originate from both ³(M + X)LCT and ³ππ* states. These results indicate that N-heteroaromatic ligands play an important role in the emission properties of mononuclear Cu(I)–halide complexes.     

Characterization of laser induced breakdown plasmas used for measurements of arsenic,antimony and selenium hydrides

Studies have been performed to characterize laser induced breakdown spectroscopy (LIBS) plasmas formed in Ar/H₂ gas mixtures that are used for hydride generation (HG) LIBS measurements of arsenic (As), antimony (Sb) and selenium (Se) hydrides. The plasma electron density and plasma excitation temperature have been determined through hydrogen, argon and arsenic emission measurements. The electron density ranges from 4.5 × 10¹⁷ to 8.3 × 10¹⁵ cm^?3 over time delays of 0.2 to 15 μs. The plasma temperatures range from 8800 to 7700 K for Ar and from 8800 to 6500 K for As in the HG LIBS plasmas. Evaluation of the plasma properties leads to the conclusion that partial local thermodynamic equilibrium conditions are present in the HG LIBS plasmas. Comparison measurements in LIBS plasmas formed in Ar gas only indicate that the temperatures are similar in both plasmas. However it is also observed that the electron density is higher in the Ar only plasmas and that the emission intensities of Ar are higher and decay more slowly in the Ar only plasmas. These differences are attributed to the presence of H₂ which has a higher thermal conductivity and provides additional dissociation, excitation and ionization processes in the HG LIBS plasma environment. Based on the observed results, it is anticipated that changes to the HG conditions that change the amount of H₂ in the plasma will have a significant effect on analyte emission in the HG LIBS plasmas that is independent of changes in the HG efficiency. The HG LIBS plasmas have been evaluated for measurements of elements hydrides using a constant set of HG LIBS plasma conditions. Linear responses are observed and limits of detection of 0.7, 0.2 and 0.6 mg/L are reported for As, Sb and Se, respectively.     

Synthesis of silicon–nitrogen derivatives in quasi-interstellar conditions

At 2–4·10^–5 T, a silicon wafer is the target of a 5–10 keV molecular beam of dinitrogen. The products are extracted by an electric field, and analysed by mass. The ions of this primary spectrum are dissociated in a Kr collision chamber. From the fragments thus obtained, one deduces compositions for the secondary ions, and therefore for the primary products. This is helped by the presence, in silicon, of the isotopes ²⁸Si, ²⁹Si and ³⁰Si. Beside the clusters Si_n (n = 1–7), complex molecular species are thus obtained, such as Si₅N₄⁺, Si₅N₄H⁺, Si₅N₄H₂⁺. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASinterstellar dust / interstellar molecules / atomic impact / silicon / nitrogen derivatives     

Time-of-flight mass spectroscopy of femtosecond and nanosecond laser ablated TeO2 crystals

Single-pulse femtosecond (fs) (pulse duration ～200 fs, wavelength 398 nm) and nanosecond (ns) (pulse duration 4 ns, wavelength 355 nm) laser ablation have been applied in combination with time-of-flight mass spectrometer (TOFMS) to analyze the elemental composition of the plasma plume of single-crystalline telluria (c-TeO₂, grown by the balance controlled Czochralski growth method). Due to the three-order difference of the peak intensities of the ns and fs-laser pulses, significant differences were observed regarding the laser-induced species in the plasma plume. Positive singly, doubly and triply charged Te ions (Te⁺, Te₂⁺, Te₃⁺) in the form of their isotopes were observed in case of both irradiations. In case of the ns-laser ablation the TeO⁺ formation was negligible compared to the fs case and there was no Te trimer (Te₃⁺) formation observed. It was found that the amplitude of Te ion signals strongly depended on the applied laser pulse energy. Singly charged oxygen ions (O⁺) are always present as a byproduct in both kinds of laser ablation.     

Temporal evolution study of the plasma induced by CO2 pulsed laser on targets of titanium oxides

This paper reports studies on time-resolved laser induced breakdown spectroscopy (LIBS) of plasmas induced by IR nanosecond laser pulses on the titanium oxides TiO and TiO₂ (anatase). LIBS excitation was performed using a CO₂ laser. The laser-induced plasma was found strongly ionized yielding Ti⁺, O⁺, Ti^2 +, O^2 +, Ti^3 +, and Ti^4 + species and rich in neutral titanium and oxygen atoms. The temporal behavior of specific emission lines of Ti, Ti⁺, Ti^2 + and Ti^3 + was characterized. The results show a faster decay of Ti^3 + and Ti^2 + ionic species than that of Ti⁺ and neutral Ti atoms. Spectroscopic diagnostics were used to determine the time-resolved electron density and excitation temperatures. Laser irradiation of TiO₂-anatase induces on the surface sample the polymorphic transformation to TiO₂-rutile. The dependence on fluence and number of irradiation pulses of this transformation was studied by micro-Raman spectroscopy.     

Ionic fragmentation processes of core-excited α-alanine in gas phase

Ionic fragmentation of core-excited α-alanine in gas phase was observed. The most dominant ionic species is COOH⁺ for all core-ionizations at C 1s, N 1s, and O 1s. An increase in COO⁺ and a decrease in COOH⁺, which were observed as core-hole atom selectivity for the O 1s ionization, are explained by the enhancement of O–H bond scission. Further state-selective O–H bond scission, observed at the O 1s second peak, is attributed to the O_OH1s → 3s/σ* transition.     

Isopiestic determination of the osmotic and activity coefficients of dilute aqueous solutions of symmetrical and unsymmetrical quaternary ammonium bromides with a new isopiestic cell at 298.15 K

The osmotic coefficients of Bu₄NBr, sec-Bu₄NBr, iso-Bu₄NBr, Bu₂Et₂NBr and Bu₃EtNBr were determined by isopiestic method at 298.15 K in dilute aqueous solutions. A branched isopiestic cell was used. The osmotic coefficients of tetra-alkyl-ammonium solutions were analyzed comparing these with the Debye–Hückel limiting law. The order of the osmotic coefficient variation is Bu²Et₂N⁺ > BuEt₃N⁺ > sec-Bu₄N⁺ > iso-Bu₄N⁺ > n-Bu₄N⁺. The results were fitted to the Pitzer equation and the parameters β₀ and β₁ were calculated. The results are discussed.     

Complex formation between manganese(II), cobalt(II), nickel(II), copper(II) and a series of new ligands derived from N,N′-O-phenylenebis(salicylideneimine)

A series of new ligands derived from N,N′-O-phenylenebis(salicylideneimine) have been synthesized and characterized by spectrometric methods. Their protonation constants and the stability constants of their complexes with Mn²⁺, Co²⁺, Ni²⁺ et Cu²⁺ have been determined by potentiometric methods in a water–ethanol (90:10 v/v) mixture at a 0.2 mol l⁻¹ ionic strength (NaCl) and at 25.0 ± 0.1 °C. The Sirko program was used to determine the protonation constants as well as the binding constants of both species [M(HL)]⁺ and [ML]. The stability order obtained is in agreement with Irving–Williams series.     

Determination of 90Sr / 238U ratio by double isotope dilution inductively coupled plasma mass spectrometer with multiple collection in spent nuclear fuel samples with in situ 90Sr / 90Zr separation in a collision-reaction cell

Strontium-90 is one of the most important fission products generated in nuclear industry. In the research field concerning nuclear waste disposal in deep geological environment, it is necessary to quantify accurately and precisely its concentration (or the ⁹⁰Sr / ²³⁸U atomic ratio) in irradiated fuels. To obtain accurate analysis of radioactive ⁹⁰Sr, mass spectrometry associated with isotope dilution is the most appropriated method. But, in nuclear fuel samples the interference with ⁹⁰Zr must be previously eliminated. An inductively coupled plasma mass spectrometer with multiple collection, equipped with an hexapole collision cell, has been used to eliminate the ⁹⁰Sr / ⁹⁰Zr interference by addition of oxygen in the collision cell as a reactant gas. Zr⁺ ions are converted into ZrO⁺, whereas Sr⁺ ions are not reactive.A mixed solution, prepared from a solution of enriched ⁸⁴Sr and a solution of enriched ²³⁵U was then used to quantify the ⁹⁰Sr / ²³⁸U ratio in spent fuel sample solutions using the double isotope dilution method. This paper shows the results, the reproducibility and the uncertainties that can be obtained with this method to quantify the ⁹⁰Sr / ²³⁸U atomic ratio in an UOX (uranium oxide) and a MOX (mixed oxide) spent fuel samples using the collision cell of an inductively coupled plasma mass spectrometer with multiple collection to perform the ⁹⁰Sr / ⁹⁰Zr separation. A comparison with the results obtained by inductively coupled plasma mass spectrometer with multiple collection after a chemical separation of strontium from zirconium using a Sr spec resin (Eichrom) has been performed. Finally, to validate the analytical procedure developed, measurements of the same samples have been performed by thermal ionization mass spectrometry, used as an independent technique, after chemical separation of Sr.     

A study of the photodeposition over Ti/TiO2 electrode for electrochemical detection of heavy metal ions

Here we reported that UV light irradiation can significantly enhance sensitivity of Ti/TiO₂ electrode for determination of trace heavy metal ions (such as Cu^2 +, Pb^2 + and Cd^2 +) owing to the photodeposition of metal ions on the surface of electrodes. The sensitivity of heavy metal ions can be selectively enhanced over the Ti/TiO₂ electrode, which is attributed to matching between potential of heavy metal ions and the position of the conduction band of TiO₂.