Plasma diagnostics of an analytical Grimm-type glow discharge in argon and in neon: Langmuir probe and optical emission spectrometry measurements

Comparative investigations were performed on a Grimm-type glow discharge source by Langmuir probe measurements and by optical emission spectrometry. The Langmuir probe measurements yielded electron temperatures and number densities of electrons, whereas the optical emission spectrometry measurements resulted in data for excitation and ionization temperatures of different species. The results confirm that there is no local thermal equilibrium in the discharge plasma. The operating conditions of the glow discharge source and also the working gas and the cathode material were varied to investigate their influence on the plasma parameters. The outcome of the plasma diagnostics will be used to improve the modelling of relevant excitation and ionization processes by computer simulation. The major physical processes in the low pressure glow discharge plasma should be better understood if the analytical capability of this spectrochemical excitation and ionization source has to be further enhanced.     

Design and properties of a microwave boosted glow discharge lamp

The operation of a glow discharge lamp with integrated microwave resonator for the analysis of electrically conducting solid samples by atomic emission spectrometry is described. While the glow discharge in argon at a pressure of 300 Pa mainly serves for the production of free sample atoms by cathodic sputtering, a 40 W microwave discharge is applied for additional excitation of the ablated material. The construction of the lamp and the optimization of the working conditions are described. The intensities as well as the signal-to-background ratios of many analytical lines were found to be improved as compared to a conventional glow discharge lamp. The analytical performance is demonstrated by analysis results for steel samples. Detection limits for 13 elements in steel are between 0.05 and 1 μg/g. Because of the optically thin plasma the new lamp shows a large linear dynamic range.     

Calculation of elemental columnar density from self-absorbed lines in laser-induced breakdown spectroscopy: A resource for quantitative analysis

The presence of self-absorption of emission lines is usually an undesired effect in laser-induced breakdown spectroscopy because it introduces non linear effects in the growth of line intensity versus the concentration of the emitting species. Several methods have been proposed in recent years for identifying and quantifying self-absorption in the emission spectra. After this diagnostic stage, the lines affected by self-absorption are usually disregarded; otherwise, appropriate corrective factors are applied to their intensity before the utilization for analytical purposes. Changing the point of view, this paper remarks as self-absorption can provide useful information for analyzing the composition of laser-induced plasmas and for their characterization. Whenever the extent of self-absorption is quantified, in fact, the optical depth of the line can be rapidly calculated; then, for plasmas in local thermodynamic equilibrium conditions, the columnar density of the emitting species can be derived. Assuming the plasma homogeneity, the concentration ratio between different elements can be obtained. Moreover, in particular cases, the columnar densities can be used to calculate the plasma temperature and the absolute number densities of plasma species. Some applications of the method are reported in the paper and potentialities and limitations are discussed.     

Application of a modulated Grimm-type glow discharge plasma as primary light source for simultaneous reading atomic absorption spectrometry

Summary The Grimm-type glow discharge lamp (GDL) working in a modulated way can be used as primary light source for atomic absorption measurements. The number of element radiations is given by the composition of the target (sample on GDL) which becomes sputtered. Its composition can be adopted to the analytical problem to be solved. It is easy to change the target.The glow discharge source generated at relatively low power (10–24 W) is burning stable for >20 min on the same spot. This is time enough to operate atomic absorption measurements of 10 samples simultaneously, for example, by using the normal flame technique or the graphite tube furnace or the atomsource sputter method to generate atoms of the sample material. The monochromator device of an AA spectrometer has to be replaced by a polychromator one.The spectral behaviour of the glow discharge source compared to that of the hollow cathode lamps of the elements studied is described here by using a double beam two channel AA spectrometer for simultaneous reading of both the signals. In most cases the glow discharge source is the better one. Home-made targets are used to measure first analytical results.

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Einsatz einer modulierten Glimmentladungslampe als Primärlichtquelle zur simultan messenden Atomabsorptionsspektrometrie

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We have to thank the Spectruma company and Bernhard Bogdain especially for supporting this work.     

A glow discharge lamp with supplementary excitation by a radio—frequency discharge-preliminary measurements

The emission radiant output of an ordinary glow discharge plasma was increased by several factors through secondary inductively coupled RF excitation produced by an external coil and a 136.2MHz oscillator. The gain factor was determined at several glow discharge currents and voltages in copper alloys and cast iron samples. Improved linear calibration curves were obtained because the RF-boosted glow discharge source decreased the effect of self-absorption.     

Asymmetric charge transfer with hydrogen ions — An important factor in the “hydrogen effect” in glow discharge optical emission spectroscopy

It is now well known that traces of hydrogen or nitrogen in the argon plasma gas in glow discharge optical emission spectrometry (GD-OES) may affect the sputtering rate. More seriously, such traces can also selectively affect the absolute and relative intensities of individual lines, and thereby have a major effect on the accuracy of analytical results. This problem is becoming more severe as the potential of GD-OES as an analytical tool is steadily increasing, and the technique is now used for the analysis of more complex samples.The results presented form part of an extensive study of the effects of hydrogen and nitrogen on the spectra of a number of elements — Fe, Ti, V, Ni, Zn etc. Two systems have been used to record the spectra for the present results — high resolution Fourier transform spectroscopy at Imperial College, London, and the LECO GDS500A, with CCD spectrometer at LECO Instrumente, Plzeň. The two approaches have yielded consistent results.During the analysis of the data for Fe II and Ti II spectra, it has become clear that asymmetric charge transfer involving hydrogen ions (H-ACT) is a very important selective excitation mechanism for spectral lines with a total excitation energy close to 13.6 eV, the ionisation potential of hydrogen. Detailed evidence for this mechanism is presented. The magnitude of the effect varies for different elements and spectral lines but great care must be taken before choosing ionic lines with a total excitation energy of between 12.5–14 eV for analytical use.     

New hardware for radio frequency powered glow discharge spectroscopies and its capabilities for analytical applications

A new radio frequency (rf) hardware is developed for glow discharge spectroscopic methods. The resulting features and its capabilities for analytical applications are discussed. The electrical equipment developed allows to work as quickly, stably, reliably and easily as known from the direct current (dc) mode. Moreover, the rf power measurement has been improved. The hardware has been developed, optimised and tested for glow discharge optical emission spectroscopy (GDOES), but nevertheless it is possible to use it for all procedures applying glow discharge sources.     

New hardware for radio frequency powered glow discharge spectroscopies and its capabilities for analytical applications

A new radio frequency (rf) hardware is developed for glow discharge spectroscopic methods. The resulting features and its capabilities for analytical applications are discussed. The electrical equipment developed allows to work as quickly, stably, reliably and easily as known from the direct current (dc) mode. Moreover, the rf power measurement has been improved. The hardware has been developed, optimised and tested for glow discharge optical emission spectroscopy (GDOES), but nevertheless it is possible to use it for all procedures applying glow discharge sources.     

Self-absorption and Doppler temperatures of emission lines excited in a glow discharge lamp

The shapes of the emission lines of calcium and chromium emanating from a Grimm type glow discharge lamp have been determined by use of a pressure-scanning Fabry—Perot interferometer using various excitation conditions for different concentrations of these metals in standard matrices. Assuming a certain model for the light source, theoretical line profiles were calculated.The Doppler temperature as a function of current at constant voltage was determined from a comparison of the experimental and theoretical profiles. Also determined was the degree of absorption in emitting and non-emitting layers of the model.It was found that considerable self-absorption and reversal occur in the lamp at higher currents and concentrations.     

Factors affecting the formation of the radiation pre-peak at the operation of a Grimm-type source in pulsed DC mode

The plasma emission pre-peaks of many atomic and ionic spectral lines of Cu and Ar were systematically investigated in a Grimm-type pulsed glow discharge (PGD). To register the pre-peaks with sufficient time resolution, a monochromator with photomultiplier detection was used. When the applied power exceeded a specific threshold, pre-peaks were found in all spectral lines investigated, and it was revealed that the electrical pre-peak was the cause of the atomic emission pre-peak. The form and intensity of the pre-peak radiation were, however, found to be different for different atomic emission lines. The excitation energy of the upper energy level of the atomic line transition, and factors related to recombination and self-absorption, were found to affect the emission pre-peak. Pre-peaks observed when using pulsed DC and pulsed radio-frequency power were compared. This investigation provides insight into best practice when selecting spectral lines most suitable for analytical spectrometry using PGD.     

Relaxation Processes in Polymer Surface Layers

A recently built and patented device Nanoluminograph is used for the investigation of the relaxation processes in the surface layers of HDPE films, produced at various conditions. The work of the device is based on the phenomenon of radiothermoluminescence. The use of high-frequency low-power, low-temperature plasma as an ionizing source, and a reduction of the consumed energy down to several orders of magnitude (as compared to that used in similar devices) provides excitation of a surface layer as thin as 100–200 nm. The high sensitivity of the device enables one to reduce the excitation time to 1–2 seconds for obtaining a sufficient intensity of glow curves even from ultrathin layers 20–30 nm thick. All of that minimizes the modifying plasma action on the samples under investigation and provides well reproducible and reliable data. It is found that the intensity, number and positions of peaks on the glow curves are strongly influenced by the preparation conditions of polymer films. The complicated profile of glow curve peaks allows one to assume the overlapping of multiple relaxation processes. Decomposing and fitting peak profiles with the help of a PEAKFIT computer program result in revealing at least 4 relaxation transitions in the temperature region from 109 to 213 K. The temperatures and activation energies of relaxation transitions in surface layers appeared to be lower than those inferred from the DSC data for the bulk polymer. The activation energies of trap depletion upon heating are calculated. The nature of traps is discussed in terms of molecular conformations, morphology and structural defects, as well as the attribution of the observed relaxation transitions to defreezing mobility of different kinetic units.     

Emission spectral characteristics of Cu,Ag, Zn,and Cd neutral atoms in a glow discharge

Detailed spectra highlighting the neutral atom emission characteristics (i.e. I lines) for Cu, Zn, Ag and Cd in a glow discharge device are presented in this study. A particular focus is the presentation of spectra that document the many high excitation energy neutral atom lines that are observed in these spectra. For Cu, several spectral lines originating from levels close to the ionization potential of copper are observed including lines from the so-called autoionizing levels which are actually just above the ionization potential for copper. Generally similar results are seen for Ag, Zn and Cd, including the observation of many high excitation energy neutral atom lines of Zn originating from the upper levels on the triplet side of the energy level diagram. The spectral data point to ion–electron recombination processes followed by stepwise de-excitation and radiative decay as a key mechanism in setting the spectral character of neutral atom emission in a glow discharge device. Unambiguous identification of spectral lines for specific transitions was facilitated by the acquisition of all spectral data utilizing a UV–visible Fourier transform spectrometer. This spectrometer provided complete and continuous coverage of the spectral region from 200 to 650 nm and allowed spectral lines to be identified with an accuracy of 1–2 pm.     

Fundamental aspects and applications of glow discharge spectrometric techniques

Glow discharges are kind of plasmas which are used in many fields of application, including analytical spectrometry. This review addresses both the fundamental aspects and analytical applications of glow discharges. In the first part, a systematic overview of the most important plasma processes is presented. To obtain better insight into the complexity of the glow discharge, both mathematical modeling and experimental plasma diagnostics can be carried out. Therefore, the models that were developed for a glow discharge are presented and typical results (e.g. three-dimensional density profiles, fluxes and energy distributions of the various plasma species, the electric field and potential distributions, information about collision processes in the plasma and about sputtering at the cathode, etc.) are summarized. Moreover, the most important plasma diagnostic techniques for glow discharges are discussed. In the second part, an overview is given of the various analytical applications of glow discharges.     

Modeling of glow discharge sources with flat and pin cathodes and implications for mass spectrometric analysis

A set of three-dimensional models for a direct current glow discharge in argon, developed previously in our group, is applied to analytical glow discharge cells with either flat or pin-type cathodes. Among other quantities, the densities of the plasma species are calculated and compared for these two cathode shapes. A comparison is also made for the computed argon and sputtered cathode (copper) ion currents leaving the glow discharge cell and entering the mass spectrometer, because this is of major interest from the analytical point of view. Finally, for the pin cell, the influence of sampling distance (i.e., distance between cathode pin and exit slit to the mass spectrometer) on the calculated plasma quantities is investigated.     

Precise determination of manganese in steel by dc glow discharge optical emission spectrometry associated with voltage modulation technique

A voltage modulation technique was applied for the precise and accurate determination of manganese in steels in dc glow discharge optical emission spectrometry. Emission signals from the glow discharge plasma are modulated by a cyclic variation of the discharge voltage so that only the desired signals can be detected at very low noise levels by using a lock-in amplifier. Mn determination in low-alloyed steels was performed to estimate the repeatability of the analytical result. For each measurement, the relative standard deviation (R.S.D.) of the intensities of the Mn I 403.08-nm line was ca 0.1% when a steel sample containing 0.329 mass% Mn was employed. Averaging all analytical values, which are obtained in various discharge conditions, yields statistical information on the precision and accuracy of the analytical values obtained. The relative error from the recommended value is calculated to be 0.5%. The R.S.D. over the analytical values is 1.5%, representing the overall analytical performance of this technique.     

Hydrogen effects on copper,zinc and nickel atomic emission lines in argon radiofrequency glow discharge optical emission spectrometry

The effect of hydrogen (0.5%, 1% and 10% v/v) added to the argon plasma gas on the emission spectra of selected atomic lines for copper, zinc and nickel has been studied by radiofrequency glow discharge optical emission spectrometry (rf-GD-OES). Conductive homogeneous samples containing different concentrations of the elements under study in different matrices have been investigated. Results show different trends of the emission intensity lines with increasing hydrogen concentration in the rf-GD, depending on the line characteristics. In most cases, the emission yields of the lines under study did not change or increased when hydrogen was added to the discharge (no decreases were observed). The emission yields of certain lines showed much higher increases than other lines of the same element (for example, lines 213.86 nm of Zn and 231.10 nm of Ni). Our experiments indicate that such notorious increases could be related with the possible decrease of the self-absorption when hydrogen is added to the discharge. Overall, the results obtained for the emission yield changes of certain lines of a given element in different matrices (with different analyte content) showed that while for resonance emission lines very notorious increases are observed, the values for non-resonance lines do not change significantly (specially if the matrices employed are similar).     

Alignments of a Microparticle Pair in a Glow Discharge

Stability of a vertically aligned microparticle pair in a stratified glow DC discharge is experimentally investigated. Using laser perturbations, it is shown that, for the same discharge parameters, a pair of microparticles can be suspended in two stable configurations: vertical and horizontal. The interparticle interaction and the electric field of the stratum in the region of particle levitation are quantitatively investigated for the first time. The decharging effect of the lower (downstream) particle by the ion flow wake is also observed for the first time in a glow discharge. The obtained experimental data made it possible to check the analytical criteria for the configurational stability of the system.     

On the influence of nitrogen and oxygen on the GD-OES analysis results

Summary The effect of nitrogen and oxygen impurities in argon on the analytical line intensity of various elements and other discharge parameters of a dc glow discharge has been studied. The two gaseous impurities were introduced into the discharge gas argon as molecular gaseous additions as well as sputtering products from the samples. Both elements may initiate chemical reactions in the plasma leading to a reduction of the number of free atoms of the determined elements available for excitation in the discharge plasma. Considering the main components of the investigated sample, nitrogen or oxygen contents exceeding the critical threshold of about 0.1 mass-% may alter the analytical line intensities of the elements of interest significantly. The effect of smaller gaseous impurity contents could be detected only by the determination of the emission yield. The line intensity may increase or decrease depending on the elements present in the discharge plasma.     

The real-time analysis of gases by direct sampling–mass spectrometry: elemental and molecular applications

Real-time analysis of gases for volatile organic compounds or elements is required for a number of applications. Direct sampling-mass spectrometry (DS-MS) is one approach to solve these analytical problems. This article reviews various instrumental configurations and applications of DS-MS. Inlet systems employed for DS-MS include membranes, microtrap interfaces, atmospheric sampling glow discharge ionization, atmospheric pressure ionization, microwave plasma ionization, and capillary restrictors. The use of laser-based ionization methods for DS-MS is described, including resonance-enhanced multiphoton ionization and single photon ionization.     

Modeling the effect of the cathode geometry in a DC glow discharge ion source for mass spectrometry

A self-consistent, two-dimensional hybrid fluid-particle model is used to study the effect of cathode geometry on the plasma produced in an argon glow discharge for conditions typically of the commercially available glow discharge mass spectrometer system (VG9000 spectrometer and Megacell source). For a given power supply voltage and gas pressure, we show that the spatial distribution of the plasma in the discharge volume is strongly dependent on the cathode geometry. The plasma created in a discharge with a pin cathode tends to form a ring around the cathode, while the plasma in a discharge with a larger diameter, disk cathode is centered on-axis between the cathode face and the anode. The ion current arriving at the entry plane of the mass spectrometer thus depends strongly on the cathode geometry. This suggests that analytical performance can be enhanced by optimization of the cathode (sample) geometry.