Feasibility of applying microsecond-pulse glow discharge time of flight mass spectrometry in surface depth analysis

This work evaluates the possibilities of applying microsecond-pulse glow discharge time of flight mass spectrometry (μs-pulse GD-TOFMS) in surface depth analysis. Investigations have been done for effects of discharge pressure on sputtered depth profiles as well as on topographies under μs-pulse GD mode; and also for influences of discharge current and discharge frequency on characteristics of sputtered surface. Sputtering rates of several pure metals under μs-pulse GD and dc-GD modes were studied and compared. The estimated erosion rates are 1.27, 2.90 and 5.18 nm s⁻¹ for pure Fe, Cu and Zn layer, respectively. Depth profiling were conducted for a technical Zn–Fe layer (about 10 μm) and for a Fe–Cu layer (about 1 μm) by μs-pulse GD. A simple model was developed and utilized to convert ion intensity into element concentration, and the experimental results were presented and discussed. Preliminary results show that μs-pulse GD-TOFMS has a promising future in the area of surface depth analysis, especially in the depth analysis of thin layers and of their cross-sections.     

Langmuir probe study of the charged particle characteristics in an analytical radio frequency-glow discharge. Roles of discharge conditions and sample conductivity

The application of a tuned Langmuir probe to the measurement of the charged particle characteristics of electron number density, ion number density, electron energy distribution function, average electron energy and electron temperature, in an analytical radio frequency (r.f.)-glow discharge is described. Studies focus on the roles of discharge operating conditions and plasma sampling position for conductive (copper) and nonconductive (Macor) samples. Based on the data obtained here, apparent differences in plasma characteristics between conductive and nonconductive samples can be reasonably explained. For example, the sputtering of conductive samples results in plasmas with obviously higher electron and ion number densities than the sputtering of nonconductive samples (e.g. n_i = 1.8 × 10¹⁰ cm⁻³ and n_e = 1.5 × 10⁹ cm⁻³ for copper, and n_i = 8 × 10⁹ cm⁻³ and n_e = 5 × 10⁸ cm⁻³ for Macor under the conditions of argon pressure = 4 Torr, r.f. power = 30 W and sampling distance = 4.5 mm). Conversely, nonconductive samples yield electrons with higher energies (average electron energies of 15 and 7.5 eV and temperatures of 6.5 and 3.5 eV respectively for the Macor and copper samples). Lower d.c. bias potentials for the case of sputtering nonconductive samples yield reduced sputtering rates and charged particle densities, though the electrons in the latter case have higher energies and thus improved excitation capabilities. The differences between r.f.- and d.c.-glow discharge optical emission spectra are also discussed relative to reported electron energy characteristics. Studies such as these will lay the ground-work for extensive evaluation of inter-matrix type standardization for r.f.-glow discharge atomic emission spectrometry.     

Microsecond pulsed glow discharge time-of-flight mass spectrometer

A linear time-of-flight mass spectrometer (TOFMS) has been designed, constructed, and coupled with a glow discharge source in microsecond pulsed mode (MSPGD). Orthogonal acceleration, a DC quadrupole and deflecting pulse techniques are used to diminish kinetic distribution and the spatial distribution of ions, and for deflecting Ar⁺ ions in their flight path. Comparison was made in the same discharge source between MSPGD and DC discharge. The continuous ion current is only 0.2 nA in the DC discharge mode, while the peak ion current reaches over 100 nA in the MSPGD mode. In addition, the ratio of the repelled ions to total ions is much higher in MSPGD than with a DC discharge in TOFMS. The mechanism of MSPGD is discussed. A resolving power of 500 was achieved, which is excellent for elemental analysis. To the authors' knowledge, this is the first time that a MSPGD-TOFMS combination has been described. The system is now being further optimized to improve its performance.     

Capillary Gas Chromatography Coupled with Microplasma Mass Spectrometry – Improved Ion Source Design Compatible with Bench-Top Mass Spectrometric Instrumentation

Capillary gas chromatography was performed with mass spectrometric detection using a novel microplasma ion source for operation in an element-selective mode. The ion source was a 350 kHz radio frequency helium plasma, which was sustained inside the 4 cm end of a 0.32 mm i.d. fused silica capillary column, and located inside the high vacuum chamber of the quadrupole mass spectrometer. Due to the low volume of the ion source, a stable low pressure discharge was produced utilizing only the 2.25 mL min⁻¹ of GC carrier gas (helium) for plasma support. Small amounts of oxygen (0.1–0.2% v/v) were added to the plasma gas in order to prevent carbon deposits and to enhance signal-to-noise ratios. Chlorine and bromine were selectively detected at the 5–20 pg s⁻¹ level (S/N = 2), and both produced a response that was linear within 3 orders of magnitude.     

H2/Ar direct current glow discharge mass spectrometry at constant voltage and pressure

The addition of hydrogen to a direct current (dc) - argon glow discharge (GD) coupled to a time of flight mass spectrometer has been studied using a fixed voltage between the electrodes and a fixed discharge pressure. Hydrogen contents investigated were 0.5%, 1% and 10% v/v in the argon discharge and the samples under study consisted of a copper-base, a nickel-base and an iron-base homogeneous materials. Also, the in-depth profile analysis of a tin plate was investigated. Results have shown that hydrogen addition gives rise to significant changes in the slope of the linear relationship between the electrical current and the discharge voltage. Clearly, the electrical resistance of the discharge at the typical operation voltages in the interval 600–1000 V increases with hydrogen added to pure argon.A decrease of the sputtering rates was observed the higher the hydrogen concentrations. Besides, the “reduced sputtering rates”, i.e. the sputtering rates divided by the corresponding electrical current, were also lower for the H₂/Ar discharges than for pure argon. However, the analytical ion signals observed using discharge voltages higher than 900 V turned out to be higher in a 0.5% H₂/Ar discharge than in pure argon for the copper and nickel materials. Besides, for the three samples investigated the ion yields were from 1.5 up to 3 times higher in 0.5% H₂/Ar discharges as compared to the pure argon.Finally, the effect of 0.5% H₂ addition to the Ar discharge on the in-depth profile of a tin plate has also been investigated. As compared to the use of a pure Ar GD, higher sensitivity for major and minor components of the coating were observed without loss of the relative depth resolution achieved.     

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. Figure

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Effect of driving frequency on the operation of a radiofrequency glow discharge emission source

Several characteristics of the r.f. glow discharge were examined to determine their dependence on driving frequency. The cathode potential exhibited a strong dependence on frequency, as would be expected, because of the capacitive nature of the discharge. As the frequency was dropped from 13 to 6 MHz there was a dramatic rise in the r.f. voltage of the discharge (with a conductive sample), attributed to a change in the mode of power coupling. The sputtering rates of a conductive sample were dramatically greater at the lower frequencies, in part due to higher energy of the sputtering ions. The emission characteristics of the source also changed as the frequency was varied from 6 to 13 MHz. At the higher operating frequencies, atomic emission peaked at a particular r.f. power level, whereas at lower frequencies the neutral-atom signal generally increased monotonically with power. The highest signal levels were found at 20 MHz, the highest frequency studied. Detection limits were determined for both conductors and insulators; in both cases they are detector-noise-limited because of the low throughput of the spectrometer. Detection limits for a conducting sample ranged from 0.1 μ g⁻¹ at 20 MHz to 20 μ g⁻¹ at 3 MHz. The emission from an insulating sample showed the same trends as those from a conducting sample but required higher r.f. power; the greatest signals were found at 6 and 13 MHz because not enough power was available from the r.f. amplifier to reach the optimum power for the 20 MHz discharge. Detection limits for elements in a Macor^® ceramic sample ranged from 30 to 110 μg g⁻¹.     

Internal glow discharge-fourier transform ion cyclotron resonance mass spectrometry

A glow discharge (GD) ion source has been developed to work within the high magnetic field of a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. Characterization of this source revealed that the optimum operating voltage, pressure, and current are significantly lower than those for normal glow discharges. The sputter rate was lowered to 1/30th of that found with a normal glow discharge source operated external to the high magnetic field region. Operation of the GD source closer to the FTICR analyzer cell than with previous experimental designs resulted in improved ion transport efficiency. Preliminary results from this internal GD source have established detection limits in the low parts per million range for selected elemental species.     

Arsenic and antimony determination by on-line flow hydride generation–glow discharge–optical emission detection

Hollow cathode (HC) and conventional flat cathode (FC) glow discharge (GD) optical emission spectrometry (OES) were used as detectors for the determination of arsenic and antimony by on-line hydride generation (HG) in a flow system. Both radiofrequency (rf) and direct current (dc) sources were investigated to produce the discharge. The design of the HC and FC and also the parameters governing the discharge (pressure, He flow rate, voltage, current and delivered power) and the HG (sodium borohydride concentration and reagent flow rates) were investigated using both cathodes. The analytical performance characteristics of HG–GD–OES with HC and FC were evaluated for some emission lines of arsenic (193.7, 200.3, 228.8 and 234.9 nm). The best detection limit (0.2 μg l⁻¹) was obtained when the emission line of 228.8 nm was used with FC. Under the same arsenic optimized experimental conditions, the system was evaluated to determine antimony at 259.7, 252.7 and 231.1 nm, 252.7 nm being the emission line which produced the best detection limit (0.7 μg l⁻¹). The rf-HC–GD–OES system was applied successfully to the determination of arsenic in freeze-dried urine in the standard reference material 2670 from NIST. Finally, a flow injection system was assayed to determine arsenic at 228.8 nm, using a dc-GD with both FC and HC. The results indicated that for low volumes of sample, the HC discharge allows better analytical signals than the FC.     

Radio-frequency glow discharge ion source for high resolution mass spectrometry

A radio-frequency powered glow discharge ion source has been developed for a double-focusing mass spectrometer. The sputtering and ionization of conducting, semiconducting and insulating materials have been realized using a 13.56 MHz generator to supply the discharge operating potential. The glow discharge ion source operates stably at argon pressures of 0.1–1 hPa and radio frequency powers of 10–50 W. The influence of discharge parameters and gas inlet system on sputtering rates and ion signal intensities for semi-insulating gallium arsenide wafers has been investigated.     

大气采样辉光放电质谱仪的设计与应用

针对某些弱极性类物质难以通过大气压离子源直接电离的问题,提出基于大气采样辉光放电电离方式实现弱极性物质在大气压下直接进样、电离和质谱分析的方法.通过在大气压接口-四极质谱仪的第一级真空中的离子透镜上施加交流高压产生放电,简化了辉光放电离子源的设计,能直接离子化大气压接口吸人的物质,离子在离子透镜的传输下进入四极杆质量分析器实现质谱分析.实验表明,本方法能电离电喷雾电离离子源和大气压化学电离离子源未能电离的弱极性物质——艾试剂,并且负离子工作模式比正离子工作模式的信号至少强40倍.     

双电离源(辉光放电/激光溅射电离)四极杆质谱仪的研制

本研究将辉光电离源与激光溅射电离源巧妙地结合在同一台仪器中,使固体样品在离子源腔体中既能辉光电离,也能激光电离;并且使用同一质量分析器,两种离子源的结果可以相互比对,进而得到更为准确的分析结果.此仪器主要由真空系统、离子源、离子传输系统、四极杆质量分析器及检测与数据采集系统等组成.实验中分别用两种离子源测试了标准样品SRM 1262b,并获得了半定量结果.结果表明,仪器具有定性能力强,分析速度快,检测灵敏度高等优点,对固体样品元素分析的检出限可达μg/g量级.实验表明,激光溅射电离质谱的性能优于辉光放电质谱.     

Pulsed rf-GD-TOFMS for depth profile analysis of ultrathin layers using the analyte prepeak region

Direct solid analysis of ultrathin layers is investigated using pulsed radiofrequency (rf) glow discharge (GD) time-of-flight mass spectrometry (TOFMS). In particular, previous studies have always integrated the detected ion signals in the afterglow region of the rf-GD pulse, which is known to be the most sensitive one. Nevertheless, the analytical capabilities of other pulse time regions have not been evaluated in detail. Therefore, in this work, we investigate the analyte prepeak region, which is the pulse region where the analyte ions peak after the initial sputtering process of each GD pulse, aiming at obtaining improved depth profile analysis with high depth resolution and with minimum polyatomic spectral interferences. To perform these studies, challenging ultrathin Si-Co bilayers deposited on a Si substrate were investigated. The thickness of the external Si layer was 30 nm for all the samples, whilst the internal Co layer thicknesses were 30, 10, 5, 2 and 1 nm, respectively. It should be remarked that the top layer and the substrate have the same matrix composition (Si > 99.99%). Therefore, the selected samples are suitable to evaluate the response of the Si ion signal in the presence of an ultrathin Co layer as well as the possible oxygen contaminations or its reactions. Additionally, these samples have been evaluated using time-of-flight secondary ion mass spectrometry, and the results compare well to those obtained by our pulsed rf-GD time-of-flight mass spectrometry results.     

The pulsed glow discharge as an elemental ion source

A pulsed glow discharge, rather than a conventional constant dc voltage discharge, is used as an ion source for a quadrupole mass spectrometer. Both sputter yield and ion signal are enhanced by using the pulsed system because of an increase in the voltage necessary to maintain a constant average current at the cathode over the pulse period. Irregularities are seen in the pulse spectrum that appear as rapid surges in the ion signal for both sputtered and contaminant gas species. These peaks appear at the beginning of the pulse for gaseous species but are limited to the postpulse period for sputtered species. Differences in the signal forms allow for the discrimination against selected types of ion signals by using narrow data collection gates placed over different portions of the pulse period.     

Electrothermal vaporization coupled with inductively coupled plasma array–detector mass spectrometry for the multielement analysis of Al2O3 ceramic powders

An electrothermal vaporization (ETV) system useful for the analysis of solutions and slurries has been coupled with a sector-field inductively coupled plasma mass spectrometer (ICP–MS) equipped with an array detector. The ability of this instrument to record the transient signals produced for a number of analytes in ETV–ICP–MS is demonstrated. Detection limits for Mn, Fe, Co, Ni, Cu, Zn and Ga are in the range of 4–60 pg μL^− 1 for aqueous solutions and in the low μg g^− 1 range for the analysis of 10 mg mL^− 1 slurries of Al₂O₃ powders. The dynamic ranges measured for Fe, Cu and Ga spanned 3–5 orders of magnitude when the detector was operated in the low-gain mode and appear to be limited by the ETV system. Trace amounts of Fe, Cu and Ga could be directly determined in Al₂O₃ powders at the 2–270 μg g^− 1 level without the use of thermochemical reagents. The results well agree with literature values for Fe and Cu, whereas deviations of 50% at the 90 μg g^− 1 level for Ga were found.     

Super‐atmospheric pressure chemical ionization mass spectrometry

Super‐atmospheric pressure chemical ionization (APCI) mass spectrometry was performed using a commercial mass spectrometer by pressurizing the ion source with compressed air up to 7 atm. Similar to typical APCI source, reactant ions in the experiment were generated with corona discharge using a needle electrode. Although a higher needle potential was necessary to initiate the corona discharge, discharge current and detected ion signal were stable at all tested pressures. A Roots booster pump with variable pumping speed was installed between the evacuation port of the mass spectrometer and the original rough pumps to maintain a same pressure in the first pumping stage of the mass spectrometer regardless of ion source pressure. Measurement of gaseous methamphetamine and research department explosive showed an increase in ion intensity with the ion source pressure until an optimum pressure at around 4–5 atm. Beyond 5 atm, the ion intensity decreased with further increase of pressure, likely due to greater ion losses inside the ion transport capillary. For benzene, it was found that besides molecular ion and protonated species, ion due to [M + 2H]⁺ which was not so common in APCI, was also observed with high ion abundance under super‐atmospheric pressure condition. Copyright © 2013 John Wiley & Sons, Ltd.     

Development and fundamental investigation of Laser Ablation Glow Discharge Time-Of-Flight Mass Spectrometry (LA-GD-TOFMS)

Glow Discharge (GD) spectroscopy is a well known and accepted technique for the bulk and surface composition analysis, while laser ablation (LA) provides analysis with high spatial-resolution analysis in LIBS (laser-induced breakdown spectroscopy) or when coupled to inductively coupled plasma spectrometry (ICP-OES or ICP-MS). This work concerns the construction of a Laser Ablation Glow Discharge Time-Of-Flight Mass Spectrometry (LA-GD-TOFMS) instrument to study the analytical capabilities resulting from the interaction of a laser-generated sample plume with a pulsed glow discharge. Two ablation configurations were studied in detail. In a first approach, the laser-generated plume was introduced directly into the GD, while the second approach generated the plume inside the GD. The ablated material was introduced at different times with respect to the discharge pulse in order to exploit the efficient ionization in the GD plasma. For both LA-GD configurations, direct ablation into the afterglow of the pulsed glow discharge leads to an ion signal enhancement of up to a factor of 7, as compared to the ablation process alone under the same experimental conditions. The LA-GD enhancement was found to occur exclusively in the GD afterglow, with a maximum ablation S/N occurring in a few hundred microseconds after the termination of the glow discharge. The duration of the enhanced signal is about two milliseconds. Both the laser pulse energy and the position of the ablation plume (with respect to the sampling orifice) were found to affect the amount of mass entering the afterglow region and consequently, the enhancement factor of ionization.     

A radiofrequency glow-discharge-time-of-flight mass spectrometer for direct analysis of glasses

A radiofrequency (rf) glow-discharge (GD) ion source coupled to a commercial on-axis time-of-flight mass spectrometer (TOFMS) has been developed for the direct analysis of non-conducting samples. Different instrumental configurations of the rf-GD source, including the optional use of a sampler cone and the possibility of allowing electrical floating of the discharge, were evaluated first with a conducting sample. Higher ion signals were obtained when the GD was electrically floating and no sampler cone was used. A homogeneous glass was then analyzed using two different rf-GD configurations—with a sampler cone and discarding the use of the sampler cone. The atomic mass spectra obtained with the TOFMS using both configurations were compared. Analyte signals were systematically higher for the latest mode which avoids the sampler cone. The analytical capability of the proposed rf-GD–TOFMS system for the analysis of thick glasses, up to 6 mm, has been investigated in terms of sensitivity, isotopic ratio accuracy, and mass-resolving power. Different homogeneous glasses (including glasses as thick as 6 mm) have been analyzed and major and minor elements were detected. Isotope ratio accuracies of about ±1% and mass resolving powers of about 700 were observed.     

Hot cell based time-of-flight mass spectrometry

A time-of-flight mass spectrometer has been designed and is presently being installed within a radiological controlled hot cell at Argonne National Laboratory-West. Direct solid sampling is performed by laser ablation or glow discharge sputtering followed by supersonic expansion into the TOFMS source chamber. Once the atom/ion beam enters the source chamber, enhanced ionization can be accomplished by laser ionization or electron impact. An assortment of samples may be analyzed ranging from irradiated nuclear fuel elements and cladding materials to eutectic salts, radioactive waste materials and environmental samples.     

Design and construction of glow discharge-ion trap mass spectrometer (GD-ITMS) and its application for the analysis of volatile organic samples

In this study, we present a newly designed see-through type hollow cathode glow discharge (St-HCGD) cell developed for the analysis of volatile organic materials in an ion trap mass spectrometer. The cell was interfaced with a homemade ion trap mass spectrometer by adopting skimmer and sampler in an optimized dimensions based on the computer simulation done by SIMION software. The St-HCGD cell has a relatively small size (4×4×7 cm) with the diameter of the inner tube of 0.25′′. The anode and cathode were made of stainless steel-304 and helium was used as a buffer gas for discharge to enhance the Penning ionization process rather than sputtering process. Mass spectra of volatile organic samples such as benzene, toluene, cyclohexane were obtained by using the St-HCGD-ITMS.