Millisecond Pulsed Glow Discharge Research Articles

Improving pulsed radiofrequency glow discharge for time-of-flight mass spectrometry simultaneous elemental and molecular analysis

The performance of radiofrequency (rf) millisecond pulsed glow discharge (PGD) coupled to a fast orthogonal time-of-flight mass spectrometer (TOFMS) for chemical characterization and quantification of organic volatile compounds was investigated by using two different GD chamber designs. The designs investigated had substantial differences in the way that the volatile organic compound is introduced into the GD and the distance between the cathode and the sampling cone of the mass spectrometer. Bromochloromethane was selected as the model analyte because of the practical interest of determining trihalomethanes at low concentrations, and also because of both its low boiling point (to avoid problems associated with condensations in the interface) and the fact that it has two different heteroatoms, making the fragmentation patterns easier to follow. Pulse shapes of element, fragment, and molecular parent ions obtained by using the two GD chambers under investigation were critically compared. Results revealed the critical effect of the GD chamber geometry in obtaining the three types of chemical information, temporally discriminated. The spectra of the gaseous samples and of a polymer containing TBBPA (solid sample) were also compared. Detection limits for bromochloromethane in the order of low ng L(-1), and the required high tolerance of the plasmas to the introduction of organic vapours, were achieved using one of the proposed GD designs. The capability of the designed system for the analysis of other volatile compounds, for example dimethyl disulfide and dimethyl selenide, was also successfully evaluated, making use of the analytical potential of the information obtained from the different pulse time regions.

RF-pulsed glow discharge time-of-flight mass spectrometry for glass analysis: Investigation of the ion source design

Radiofrequency (RF) millisecond pulsed glow discharge (PGD) coupled to time-of-flight mass spectrometry (TOFMS) was investigated for direct elemental analysis of glass samples. Aiming at achieving highest elemental sensitivity, appropriate discrimination from polyatomics, and good crater shapes on glasses, a new Grimm-type GD chamber (termed from now “UNIOVI GD”, designed and constructed in our laboratory) was coupled to TOFMS, and the results compared with those obtained with the former GD design (here denominated as “GD.1”) of the initial RF-PGD-TOFMS prototype. The critical differences distinguishing the two GDs under scrutiny are the GD chamber thickness (15.5mm for the GD.1 and 7mm for the UNIOVI GD) and the “flow tube” which is inserted in the GD.1 and inexistent in UNIOVI GD.A pulse period of 4ms and a duty cycle of 50% were selected for the PGD studies. In order to characterizing the UNIOVI GD, the effect of RF-PGD experimental conditions (pressure and power) on signal shapes along the pulse was first investigated. Then, the analytical performance achieved was compared with results obtained using the GD.1 chamber. Results show that the UNIOVI GD source induced less thermal stress on the glass specimens. Consequently higher GD power can be applied to the UNIOVI GD (up to at least 130W), as compared to the GD.1 (maximum of 60W), without glass sample breakdown. Also, better crater shapes on samples were obtained using the UNIOVI GD. Moreover, the detection limits attained with the UNIOVI GD (in the range of μgg−1) were in most cases about one order of magnitude better for the analytes investigated (B, Na, Al, Si, Mn, Ni, Co, Cu, Zn, As, Sr, Ag, In, Sn, Sb, Ba, Pb and Bi), than those observed using the GD.1.

Direct chromium speciation in solid state materials—a GDMS approach

Among the various elemental mass spectrometry techniques, glow discharge mass spectrometry is recognized for its ability to provide direct determination of trace elements present in solid state samples. In the present work, a pulsed glow discharge time-of-flight mass spectrometry method is developed for the direct speciation of chromium in solid state samples. The millisecond pulsed glow discharge operated with radio frequency power is a versatile ion source that provides elemental, structural and molecular information. Careful tuning of the operating parameters yields the plasma chemistry that favors cluster ion formation. Cluster ions unique to specific species permit differentiation between the trivalent and hexavalent forms of chromium, (Cr(III) and (Cr(VI)), respectively, in chromium oxide samples. Specifically, signals at 104 and 120 m/z corresponding to the Cr2+ and Cr2O+ cluster ions arise from the presence of Cr(III) in the sample, whereas signal at 100 m/z corresponding to the CrO3+cluster ion arises from the presence of Cr(VI). The impact of glow discharge operating conditions on the appearance of these characteristic cluster ions is discussed.

Electronic perturbation investigations into excitation and ionization in the millisecond pulsed glow discharge plasma

This study employed a power perturbation method to examine the energy transfer processes at different locations within the afterpeak regime of a millisecond pulsed glow discharge plasma. Brief power perturbation pulses were applied during the afterpeak regime altering the environment of the collapsing plasma. Responses of several transitions to the power perturbations were measured via atomic emission and absorption spectroscopic methods at various distances from the surface of the cathode. The experimental data provide further insight into the energy transfer processes that occur at different spatial locations and in different temporal regimes of these pulsed glow discharge plasmas. Although the enhancement of the large population of metastable argon atoms is again confirmed, the mechanism responsible for this enhancement remains unclear. The most likely possibility involves some form of ion–electron recombination followed by radiative relaxation of the resulting species. The metastable argon atoms subsequently Penning ionize sputtered copper atoms which then appear to undergo a similar ion–electron recombination process yielding variable degrees of observable afterpeak emission for copper atom transitions. The kinetic information of these processes was approximated from the corresponding relaxation time. The electron thermalization time allowing for recombination with ions was found to be ∼25 μs after the discharge power termination.

Quantification of Aromatic and Halogenated Hydrocarbons and Alcohol Mixtures at the Elemental, Structural, and Parent Molecular Ion Level

The capabilities of a millisecond pulsed glow discharge time-of-flight mass spectrometer for the quantitative analysis of organic molecules were investigated. Mixtures of analytes were separated by gas chromatography, and mass spectra were collected at three different time regimes during the pulse cycle-the prepeak, plateau, and afterpeak time regimes. Elemental information was collected in the prepeak, structural information in the plateau, and molecular ion information in the afterpeak. A sample mixture containing toluene, o-xylene, o-dichlorobenzene, and a binary mixture of methanol and sec-butanol were considered. Calibration curves were constructed for each time regime based on the intensities of the elemental, fragment, and molecular ions. Optimum linearity (r2 = 0.999) was achieved during the plateau time regime, although calibration in the prepeak was also demonstrated, albeit with slightly poorer correlation coefficients (r2 > 0.959). The minimum limits of detection (MDL) were 392, 422, and 557 ng, for toluene, o-xylene, and o-dichlorobenzene, respectively, using a 3-microL injection and a split ratio of 68:1. For the binary alcohol mixture, MDLs of 1.87 and 2.44 microg were determined for methanol and sec-butanol, respectively, based on the intensity of the 16O+ ion during the prepeak and using a split ratio of 58:1.

Time-gated pulsed glow discharge: real-time chemical speciation at the elemental, structural, and molecular level for gas chromatography time-of-flight mass spectrometry.

A millisecond pulsed glow discharge is used as a versatile ion source for time-gated generation of elemental, structural, and molecular ions. The utility of this ion source for comprehensive chemical analysis of a series of aromatic and halogenated hydrocarbons is illustrated in this manuscript. To highlight the analytical utility of this transient ion source, it was connected to a gas chromatograph for the mass spectrometric determination of mixtures containing benzene, toluene, o-xylene, cymene, tert-butylbenzene, carbon tetrachloride, chloroform, chlorobenzene, tetrachlorethane, and dichlorobenzene. Explicit chemical analysis was accomplished by introducing the GC eluent into a pulsed glow discharge operating at a rate of 100 Hz with a 50% duty cycle. Using three independent digitizers for time-gated acquisition in three separate time regimes, nearly concurrent collection of elemental, structural, and molecular information was accomplished. In general, elemental information was obtained during the first 0.015 ms after the plasma onset; structural information, as ascertained from molecular fragmentation, was obtained during the plateau time regime when the plasma pulse is at a steady state, whereas molecular M(+) and MH(+) ions were obtained during the afterpeak time regime, that is, after the cessation of the plasma power pulse.

Probing excitation/ionization processes in millisecond-pulsed glow discharges in argon through the addition of nitrogen

The addition of N 2 to a millisecond-pulsed glow discharge (PGD) allows diagnostic measurements of the PGD but is found to drastically influence the transient signals arising from the argon and sputtered analyte atoms. Penning excitation between metastable argon atoms and ground state nitrogen molecules and charge transfer between argon ions and the added nitrogen reduce the degree of ionization of sputtered atoms during the power-on, plateau, period by a factor of ∼10 (at 1% N 2 by vol.). The added nitrogen affects sputtered atom emission signals less at this time because electron excitation dominates the excitation of these species. Upon power termination, afterpeak, the added nitrogen prevents plasma recombination in two major mechanisms: (i) the nitrogen reduces the number of argon ions available for recombination in the afterpeak; and (ii) vibrationally excited states of nitrogen slow the thermalization of electrons thereby decreasing recombination efficiency. The argon ion population contributes significantly to the afterpeak increase in the number of metastable argon atoms. These atoms are essential for the afterpeak ionization of sputtered atoms. Judicious selection of the nitrogen partial pressure can tune the delay time of afterpeak ionization/recombination by up to 200 μs. This could be particularly beneficial for time-resolved optical or mass spectrometric analyses.

Modeling of a millisecond pulsed glow discharge: Investigation of the afterpeak

We have developed a comprehensive modeling network for a millisecond pulsed glow discharge in argon (Ar) with copper (Cu) cathode, to describe the behavior of the various plasma species. Typical results of the model are shown, such as the potential distribution during and after the pulse, the densities of Ar+ ions, Ar atoms in the metastable and various other excited levels, Cu atoms and Cu+ ions, in ground state and excited levels, as well as optical emission intensities as a function of time during and after the pulse. Special attention is paid to the mechanisms giving rise to the so-called “afterpeak”, i.e., the peak in Ar and Cu excited level populations and optical emission intensities, which is experimentally observed in the afterglow of pulsed discharges. This afterpeak is attributed to electron-ion recombination to the highest excited levels, followed by radiative decay to lower levels. It is expected that the electron temperature decreases drastically upon pulse termination, resulting in a significant rise in electron density, making electron-ion recombination more plausible. Because we were not yet able to model these mechanisms, we worked in reversed order, to find out which recombination mechanisms account for the experimentally observed afterpeaks. Collisional-radiative recombination (i.e., three-body recombination with an electron as the third body) is the most plausible candidate, both for Ar and Cu, but it requires a rise in electron density in the afterglow, estimated to be about two orders of magnitude relative to the steady state, or voltage-on period. Therefore, as an alternative, we think that dissociative recombination of Ar2+ ions in high vibrational levels cannot yet completely be ruled out.

Influence of discharge parameters on real-time chemical speciation for gas chromatography pulsed glow discharge plasma time-of-flight mass spectrometry

A millisecond pulsed glow discharge ionization source was employed for the determination of aromatic and chlorinated hydrocarbons. Chemical speciation employed four separate dimensions that incorporate analyte volatility, elemental constituents, chemical structure, and molecular weight. Analyses were performed in real time using a gas chromatograph that was interfaced to a pulsed glow discharge time-of-flight mass spectrometer. This instrument allowed for the sequential collection of the complete mass spectrum at three different time regimes occurring during the glow discharge pulse cycle. This report focuses on discharge parameters such as pressure and sampling distance that enhance or suppress different ionization processes. Increasing the ion source pressure resulted in a decrease in analyte ionization and sampling efficiency. Ions sampled in close proximity to the cathode surface demonstrated the best elemental sensitivity. Sampling ions at increasing distance, in the negative glow, produced structural information in the form of fragmentation patterns similar to conventional 70 eV electron impact spectra. Molecular weight information obtained after power termination was ascertained from M+ and MH+ ions. At very small distances, the most dominant molecular peak was the M+ ion produced by the Penning process. At increased sampling distances molecular peaks were dominated by MH+ ions; the ionization mechanism is attributed to proton transfer reactions from background species such as ArH+.

Spectral, spatial and temporal characteristics of a millisecond pulsed glow discharge: metastable argon atom production

Time resolved atomic emission, atomic absorbance, and laser-induced atomic fluorescence measurements of a millisecond pulsed glow discharge, made perpendicular to the insertion probe, provide temporal profiles of 1s5 (3P2) and 1s3 (3P0) metastable argon atom populations. Acquisition of these profiles at different spatial positions in the plasma provides data from which two-dimensional spatial plots of relative populations are constructed. Each map, the result of 368 individual pulse profiles, provides insight into the production of metastable argon atoms as a function of time and position within the plasma. During power application, intensities plateau after 3 ms as the plasma reaches a steady state condition. Metastable argon atoms are most abundant 1–2 mm above the cathode surface during this time. Excitation mechanisms such as electron excitation and fast atom/ion impact appear to dominate in this temporal regime. In contrast, argon ion–electron recombination dominates metastable formation after pulse termination. The relative population maximum for metastable argon atoms in the afterpeak shifts to 5–9 mm above the cathode surface. This shift should impact signals for analyte species generated by Penning processes in the plasma. Absorption and fluorescence measurements of the 3P2 (11.55 eV) and the 3P0 (11.72 eV) metastable argon atom states indicate possible differences in the populations of these two states between the plateau and afterpeak time regimes.

Consideration of a millisecond pulsed glow discharge time-of-flight mass spectrometer for concurrent elemental and molecular analysis

The glow discharge ionization source operated in the pulsed (or modulated) power mode offers unique characteristics not available from its steady state counterpart. It has been well established that higher instantaneous power densities are obtainable without compromising the sample integrity when pulsed plasmas are implemented. This operating parameter affords higher sputter yields and lower limits of detection relative to the steady state plasmas. Of special interest are the discrete temporal regions associated with the modulated plasma. The presence of these time regimes offers temporal selectivity, allowing the collection of analytical data in a region where the contribution from background and contaminant species is minimized. These regions are characterized by strikingly different ionization mechanisms. Acquisition of data during each of these temporal regimes provides both molecular and elemental information. In this work the potential use of the pulsed glow discharge for collecting concurrent molecular and elemental information was explored. This task was accomplished using time-of-flight mass spectrometry (TOFMS). TOFMS has a significantly high throughput and duty cycle, making it ideally suited for rapid acquisition of spectra. This characteristic allows data acquisition during each of these temporal regions for each discharge pulse power cycle, affording concurrent elemental and molecular detection. p-Xylene was used as a test molecule for these studies.