Microsecond Pulsed Glow Discharge Research Articles

Time-of-flight mass spectrometry with pulsed glow discharge for direct determination of volatile organic compounds in air, nitrogen and argon. Volatile organic compounds ionization processes

Determination of volatile organic compounds (VOCs) in various gases, including atmospheric and exhaled human air, is required to solve a wide range of environmental problems, control the composition of various gases and is increasingly used for diagnosis of various diseases. Lately, methods of soft ionization with minimal fragmentation of the components have been rapidly developed. In particular, our research group is developing an approach to direct analysis of mixtures of VOCs using time-of-flight pulsed glow discharge mass spectrometry. Previously, the effects of different gases and gas mixtures on ionization processes were not compared. Therefore, the ionization mechanisms of VOCs in argon, nitrogen, and air were investigated in the present work. Toluene, p-xylene, chlorobenzene and 1,2,4-trimethylbenzene were chosen as the model VOCs. Optimization of microsecond pulsed glow discharge parameters (period and duration of the discharge pulse, repelling pulse delay time and pressure in the discharge cell) for each compound and a gas mixture of several VOCs was carried out. The predominant ionization mechanisms are the formation of a VOC molecular ion by Penning ionization and the proton transfer reaction; their influence being different for various gases. It is shown that the use of argon even with a small addition of water leads to the predominance of the proton transfer reaction, whereas in nitrogen and air the Penning ionization predominates. The maximum VOC intensities were achieved in air, and the developed approach

Microsecond pulsed glow discharge in copper hollow cathode reveals a new approach to ionization and determination of volatile organic compounds

Recently glow discharge (GD) mass spectrometry, which is conventionally used to analyze solid samples, was successfully applied for ionization and the subsequent determination of volatile organic compounds (VOCs). In the present study, the possibility of direct determination of VOCs in ambient air using time-of-flight mass spectrometry with microsecond pulsed discharge in a copper hollow cathode was considered for the first time. The discharge cell of the mass spectrometer was modified for the direct analysis of gaseous samples by the introduction of a quartz capillary into the argon input channel. Various classes of compounds (aromatic and aliphatic hydrocarbons and derivatives of carboxylic acids) were studied. Particular attention was paid to the ionization mechanisms of VOCs. New mechanisms of chemical and electron ionization, resulting in the formation of associate ions with cathode material (copper) – CuM+, were demonstrated and implemented. Alternative mechanisms of organic compounds ionization in GD may be related to Penning process and proton transfer reactions. These processes are mainly responsible for the formation of molecular and protonated molecular ions. The relative contribution of all these mechanisms can be adjusted by operating parameters. In general, an extremely low degree of fragmentation of molecular ions or associates was observed. Due to its isotopic structure (only two isotopes 63Cu and 65Cu with comparable abundances), the use of copper as cathode material additionally increases the reliability of VOC identification in the form of their CuM+ associates. Such association shifts the detected ions to the region of large masses with fewer interferences and noise related to scattered ions.

A similar-matrix-matched calibration strategy by using microsecond pulsed glow discharge mass spectrometry in the application of purity analysis of high purity lanthanum oxide

Quantitative analysis method of impurities in high purity lanthanum oxide (La2O3) by microsecond pulsed glow discharge mass spectrometry (Pulsed-GD-MS) was established. A similar-matrix-matched calibration strategy, i.e., calibrating the GD-MS instrument with the relative sensitivity factors (RSFs) derived from a similar matrix (100% Cu) to the matrix (80% Cu + 20% La2O3) to be analyzed was proposed. The feasibility of using the similar-matrix-matched calibration strategy was demonstrated by investigating the matrix effect in Pulsed-GD-MS. The accuracy of the method was validated by using La2O3 RMs. Finally, 71 impurities in La2O3 were quantitatively measured by Pulsed-GD-MS by using both mixing method and modified secondary cathode technique, and the results of main impurities were validated by HR-ICP-MS. In comparison to HR-ICP-MS, it had more advantages for Pulsed-GD-MS to determine some elements, which was difficult to dissolve such as Pt and Au, easy to be contaminated such as K, and difficult to be ionized in the ICP ion source like Cl. Compared with the mixing method, the modified secondary cathode technique showed some advantages, such as simpler sample pre-treatment, less sample amount, lower risks of contamination and lower limits of quantification. The purity of La2O3 by subtracting 71 impurities from 100% was then measured by Pulsed-GD-MS. The results showed that the measurement uncertainty of purity using the similar-matrix-calibration strategy was almost one order magnitude lower than that using the calibration of commonly used StdRSFs due to the significantly reduced expanded uncertainty (cover factor k = 2) of impurities analysis to about 30%. [Display omitted]

Approaching Standardless Quantitative Elemental Analysis of Solids: Microsecond Pulsed Glow Discharge and Buffer-Gas-Assisted Laser Ionization Time-of-Flight Mass Spectrometry.

Among various ionization sources for mass spectrometry, microsecond pulsed glow discharge (MP-GD) and buffer-gas-assisted laser ionization (BGA-LI) sources have the potential for direct quantitative elemental analysis of solids without the requirement of standard reference materials. The analytical potential of these two ionization sources has been evaluated by coupling them to orthogonal time-of-flight mass spectrometry (MS). A straightforward method was proposed to achieve the quantitative result: if a spectrum contains little interference and elemental peak currents are proportional to their concentrations, then the molar concentration of each element is equal to its ion current proportion in the total ion current. Two series of metal standards were applied for the evaluation. Explicit spectra with little interference can be acquired by both techniques. The interferences contribute only a very small portion to the total ion current for MP-GD-MS and BGA-LI-MS; therefore, their influence on the quantitative result can be ignored. All metal elements can be determined quite accurately by the proposed quantitation method, while gaps exist for nonmetal elements due to the high ionization potentials or gas species interference. Between the two techniques, BGA-LI-MS offers a more accurate quantitative result, primarily due to its higher plasma temperature.

Application of microsecond pulsed glow discharge to modern commercially available optical emission spectrometers for bulk elemental analysis

Microsecond Pulsed Glow Discharge (μs-PGD) was applied to commercially available spectrometers specified for OES elemental analysis with Grimm type sources, in order to have a look at the benefits which PGD can bring for practical bulk analysis.

Sputter crater formation in the case of microsecond pulsed glow discharge in a Grimm-type source. Comparison of direct current and radio frequency modes

Depth profiling with pulsed glow discharge is a promising technique. The application of pulsed voltage for sputtering reduces the sputtering rate and thermal stress and hereby improves the analysis of thin layered and thermally fragile samples. However pulsed glow discharge is not well studied and this limits its practical use. The current work deals with the questions which usually arise when the pulsed mode is applied: Which duty cycle, frequency and pulse length must be chosen to get the optimal sputtering rate and crater shape? Are the well-known sputtering effects of the continuous mode valid also for the pulsed regime? Is there any difference between dc and rf pulsing in terms of sputtering? It is found that the pulse length is a crucial parameter for the crater shape and thermal effects. Sputtering with pulsed dc and rf modes is found to be similar. The observed sputtering effects at various pulsing parameters helped to interpret and optimize the depth resolution of GD OES depth profiles.

Thermal mechanism for formation of electrical prepeak and pressure waves in a microsecond direct current pulsed glow discharge with a Grimm-type source: a modeling investigation

A Monte-Carlo based model is developed for simulating the evolution of Ar concentration, temperature and flow in time and space. Potentially, the model could be used for gas simulations in a wide range of different applications. Here, the model is incorporated into an existing but modified model of a microsecond pulsed glow discharge (μs PGD) in a Grimm-type plasma excitation source. Results of the simulations reveal that a thermal mechanism is responsible for the formation of the electrical prepeak and pressure waves, two phenomena that take place at the leading edge of the μs pulse in this source.

Elemental ratio determinations and compound-independent calibration using microsecond pulsed glow discharge time-of-flight mass spectrometry

A new prototype based on a microsecond pulsed glow discharge ion source coupled to a time-of-flight mass spectrometer was recently designed, constructed and analytically evaluated in our laboratory for simultaneous collection of elemental and molecular information, and as a gas-chromatographic detector of compounds of environmental concern. To investigate further the analytical capabilities of such a new setup, its capability for the determination of element ratios in volatile organic halogenated compounds has been explored. Moreover, compound-independent calibration has been carried out with the prototype as well. The results demonstrated that the intensity ratios (analyte to internal standard) were linear with the corresponding ratio of concentrations. Both for chlorine and bromine (measured in the prepeak) and for BrC (measured in the plateau) the slope was 1 and the intercept was 0. Moreover, detection limits were improved by more than 1 order of magnitude as compared with using external calibration. The applicability of the proposed approach has been demonstrated for the straightforward determination of chloroform in drinking and river waters.

Microsecond pulsed glow discharge in fast flow Grimm type sources for mass spectrometry

The application of microsecond Pulsed Glow Discharge (PGD) in direct current mode to existing commercial mass spectrometers with the Grimm type fast flow sources is described. Significant improvement of analytical properties in PGD, together with time gated detection of ions, in comparison with continuous direct current discharge is demonstrated. Successful application of microsecond pulsed glow discharge with a sector field mass spectrometer for depth profile analysis is shown. Processes of the sample ion transport in PGD are investigated experimentally and theoretically.

A spectroscopic investigation of the afterglow and recombination process in a microsecond pulsed glow discharge

The emission characteristics of afterglow in a microsecond pulsed glow discharge atomic source are studied to obtain insight into the excitation and recombination processes of analytes and fill gases. Each emission line features an initial intense peak at the beginning of the discharge pulse. The afterglow, which has been observed in the temporal behavior for some emission temporal profiles of analytes and discharge gases, exhibits an intense post-pulse signal maximized at 25–35 μs after plasma termination, with a broader profile than their initial peaks. The afterglow of Ar I and Cu I spectra last approximately 100 μs and 300 μs, respectively, after the argon discharge pulse at 2.0 torr pressure. The intensity of the afterpeak increases with the pulse width between 2 μs to 200 μs. A study of fourteen Ar I and twenty-seven Cu I lines show that transitions from high energy levels (5p [14.5–14.7 eV] for Ar and above 6 eV for Cu) are relatively stronger in the afterglow. The highly excited argon and copper atoms are thought to be generated via the recombination of ions and electrons. The results are explained primarily in terms of an electron-electron-ion three-body recombination process.

Gas chromatography coupled to tunable pulsed glow discharge time-of-flight mass spectrometry for environmental analysis

A tuneable microsecond pulsed direct current glow discharge (GD)-time-of-flight mass spectrometer MS(TOF) developed in our laboratory was coupled to a gas chromatograph (GC) to obtain sequential collection of the mass spectra, at different temporal regimes occurring in the GD pulses, during elution of the analytes. The capabilities of this set-up were explored using a mixture of volatile organic compounds of environmental concern: BrClCH, Cl(3)CH, Cl(4)C, BrCl(2)CH, Br(2)ClCH, Br(3)CH. The experimental parameters of the GC-pulsed GD-MS(TOF) prototype were optimized in order to separate appropriately and analyze the six selected organic compounds, and two GC carrier gases, helium and nitrogen, were evaluated. Mass spectra for all analytes were obtained in the prepeak, plateau and afterpeak temporal regimes of the pulsed GD. Results showed that helium offered the best elemental sensitivity, while nitrogen provided higher signal intensities for fragments and molecular peaks. The analytical performance characteristics were also worked out for each analyte. Absolute detection limits obtained were in the order of ng. In a second step, headspace solid phase microextraction (HS SPME), as sample preparation and preconcentration technique, was evaluated for the quantification of the compounds under study, in order to achieve the required analytical sensitivity for trihalomethanes European Union (EU) environmental legislation. The analytical figures of merit obtained using the proposed methodology showed rather good detection limits (between 2 and 13 microg L(-1) depending on the analyte). In fact, the developed methodology met the EU legislation requirements (the maximum level permitted in tap water for the "total trihalomethanes" is set at 100 microg L(-1)). Real analysis of drinking water and river water were successfully carried out. To our knowledge this is the first application of GC-pulsed GD-MS(TOF) for the analysis of real samples. Its ability to provide elemental, fragments and molecular information of the organic compounds is demonstrated.

Optical emission analysis of solid samples by decoupled sputtering/excitation using a low fluence laser synchronized with a pulsed glow discharge

The use of a microsecond-pulsed glow discharge, synchronized with an ablation laser operating under breakdown fluence, assists in the excitation of the ablated material generated during the analysis. With the glow discharge operating in pulsed mode, it is possible to synchronize accurately both sources to maximize the signal. Additionally, both sources can be independently adjusted to optimize the sample removal and its excitation. By means of the decoupling approach, typical laser energies required for detectable emission signals can effectively be lowered below the excitation and ionization thresholds of the analyte to those required solely for ablation of the sample surface. Under these conditions, the laser represents an advantageous mode to transform solid samples into gas phase that will be efficiently excited by the glow discharge. The advantage of the combination of the techniques has been demonstrated in alloyed samples (Cr/Fe and brass) where a net enhancement in the analytes signals even under sub-threshold and sub-sputtering conditions occur.

Model of microsecond pulsed glow discharge in hollow cathode for mass spectrometry

A new model for microsecond pulsed glow discharge in a hollow cathode and its afterglow is described. The model is based on the Monte-Carlo method together with a new method for electrical field calculation, which is based on some phenomenological laws of plasma behavior. The afterglow model uses continuity and Poisson equations. A qualitative agreement between the model results and results published in experimental and theoretical works is demonstrated. Some processes in the microsecond pulsed discharge in the hollow cathode, such as sputtering, ionization and transfer of sample, are investigated. The model is successfully used for the optimization of the operational parameters of the time-of-flight mass spectrometer with ionization by microsecond pulsed glow discharge in a hollow cathode.

Depth profile analysis of layered samples using glow discharge assisted Laser-induced Breakdown Spectrometry (GD-LIBS)

A microsecond-pulsed glow discharge is used to excite ablate material generated by the ablation laser in Laser-induced Breakdown Spectrometry (LIBS). The coupled system provides a simple means to excite the material ablated by the incident laser pulse by taking advantage of enhanced collisional excitation. In this way, one can effectively reduce laser energies below the excitation and ionization thresholds to those needed solely for ablation of the sample surface, in which the excitation of the material is performed by a high voltage discharge. Given this development, there exists the potential of improving upon depth resolutions in LIBS material characterization. This article presents the results of a comparative depth profile study conducted on various galvanized steels and layered brass standards by LIBS and GD-LIBS; demonstrating the advantages of coupling a pulsed glow discharge with a typical LIBS set-up used in materials analysis.

Microsecond pulsed glow discharge applied to a sector-field mass-spectrometer

A microsecond pulsed glow discharge (PGD) is applied to an existing commercial sector-field mass spectrometer to develop a system for routine analysis with PGD. The detection limits obtained are of the same order of magnitude as in DC discharge, which is explained by the geometry of the existing discharge cell. Additionally, the dynamics of ions extracted from PGD are measured for further development of pulsed ion detection.

Microsecond pulsed versus direct current glow discharge as ion sources for analytical glow discharge-time of flight mass spectrometry

Investigations have been carried out into coupling a microsecond pulsed direct current glow discharge to a time-of-flight mass analyser (μs-GD-TOFMS), aiming at studying the effects of the pulse parameters on mass spectra for analytical applications, including signal intensities, crater shapes and sputtering rates of conducting samples. By comparing the results obtained in four model materials (stainless steel, zinc, brass and an aluminium matrix) using pulsed and continuous dc voltages, a strong reduction (3–6 times, depending of the matrix) of the sputtering processes for the pulsed mode has been observed. This can be associated to the low average power available in each period. Conversely, the high voltage reached in a pulse seems to enhance ion generation, allowing for a notable sensitivity increase (about 50 fold) compared with dc continuous voltage operation. Concerning the effect of operating parameters typical of pulsed sources, the repeller delay has been demonstrated to be a critical parameter in achieving good analytical signals.

Energy assistance in laser induced plasma spectrometry (LIPS) by a synchronized microsecond-pulsed glow discharge secondary excitation

The design and preliminary experimental results are presented of a new LIPS chamber permitting the creation of a sub-sputtering threshold glow discharge (GD) environment that is pulsed synchronously with an ablation laser. The new dual GD-LIPS synchronous scheme provides significant signal enhancements (up to 75×) when compared to LIPS or GD under identical conditions. The possible contributions of the gas pressure, discharge gas and secondary excitation on the commented enhancement are discussed. Our experiments suggest that all of the material ablated by the ablation laser is efficiently excited and used to its full potential. Under the described conditions, it is possible to lower the laser energies needed for the excitation/ionization thresholds of materials to those required solely for ablation of the sample surface, thus creating the possibility of improving lateral and depth resolutions in the spatial characterization of material.

Determination of Ca+and K+by pulsed glow discharge sector-field mass spectrometry

A microsecond pulsed glow discharge (PGD) has been employed in conjunction with a miniature Mattauch–Herzog double-focusing mass spectrograph. Several advantages have been realized through use of the PGD, including a higher instantaneous power, reduced sample heating, and the ability to temporally resolve spectra of analyte and support-gas ions. The latter advantage has allowed the determination of 40Ca+ and 39K+ concentrations in an argon discharge. Detection limits are in the tens of μg g−1 regime and are limited by the availability of suitable analytical standards.

Advancing the capabilities of a glow-discharge sector-field mass spectrometer

The improved analytical capabilities of a glow-discharge Mattauch–Herzog mass spectrograph are presented. The glow discharge is a desirable analytical source because of its simplicity and direct solids analysis capabilities. The Mattauch–Herzog double-focusing sector geometry produces a linear focal plane, in contrast to the curved focal region common to most sector instruments. Placing an array detector along this focal plane allows simultaneous and continuous acquisition of data over an entire mass range, whereas an ordinary sector instrument must scan this same range. In this paper, the present capabilities of the mass spectrograph with a glow-discharge source are presented. Due to present-day array-detector limitations, mass spectral acquisition was performed with an electron multiplier by scanning the magnetic field. Ultimately, a suitable array detector will be incorporated and the data shown here will serve as a reference for future array-detector data. Also considered here is the compatibility between the mass spectrograph and a microsecond pulsed glow discharge source. The pulsed source produces analyte and bulk plasma ions at different periods within the pulse cycle. This feature, coupled with gated ion optics, enhances the determination of species in close mass spectral proximity to bulk plasma ions. This capability will be particularly useful when an array detector is used because of the inherent susceptibility of such detectors to accidental overexposure from sample-matrix and plasma ions.

Modeling of a microsecond pulsed glow discharge: behavior of the argon excited levels and of the sputtered copper atoms and ions

A set of models is developed for a microsecond pulsed glow discharge, to describe the behavior of argon excited levels, including the metastables, as well as the cathodic sputtering and the behavior of sputtered copper atoms and ions. These models are coupled to a hybrid Monte Carlo–fluid model for electrons, argon ions and atoms, which was developed previously, to obtain an overall picture of the pulsed glow discharge. Typical results of the present model are the densities of copper atoms and ions, the level populations of argon and copper excited levels, the erosion rate due to cathode sputtering and the contributions due to argon ion, atom and copper ion bombardment, the rates of various collision processes in the plasma, as well as the optical emission intensities. The results are presented as a function of distance from the cathode and as a function of time during and after the pulse.