W. W. Harrison

Microsecond-pulsed Grimm glow discharge as a source for time-of-flight mass spectrometry

A Grimm-type glow discharge ion source has been constructed for a time-of-flight mass spectrometer (TOF-MS). This source was operated in the microsecond-pulsed mode. Special features of pulsed Grimm GD-TOF-MS are described, including resolving power, stability, sensitivity and the temporal separation of discharge gas species and analyte ions. The analytical characteristics of a pulsed Grimm GD-TOF-MS system were also evaluated. These preliminary studies demonstrate the potential of this technique for the elemental analysis for solids.

A pulsed Grimm glow discharge as an atomic emission source

A high voltage, microsecond regime pulse dc glow discharge was applied to a standard Grimm source on a modified LECO SA-2000 direct reader spectrometer and to a duplicate Grimm source coupled to a scanning monochromator. The effects of experimental conditions, including pulse voltage, pulse frequency, pulse width and Ar pressure, on Cu atomic and ionic emission intensities were examined and compared with continuous discharge results. The pulsed Grimm source has a higher sputtering rate, greater signal intensity and lower detection limits. The pulsed signal for atomic lines shows a different temporal response from ionic lines. Pulsed Grimm GD-AES also offers control features advantageous in the measurement of thin surface layers.

Microsecond pulsed glow discharge as an analytical spectroscopic source.

A pulsed glow discharge, operating in the microsecond regime, has been found to be advantageous for the examination of solid samples. We have studied the spectroscopic response by atomic emission, absorption, fluorescence, and mass spectrometries. Results to date show enhanced efficiency for analytical response of the sputtered sample atoms. This type of discharge also permits acquisition of useful diagnostic information concerning glow discharge processes.

Temporal considerations with a microsecond pulsed glow discharge

A high instantaneous power microsecond pulsed glow discharge reveals many interesting features as an ion source for atomic mass spectrometry and as a photon source for atomic emission spectroscopy. Its advantages over a conventional continuous source are high signal intensity, temporal profile resolution, additional sputtering control and high sample utilization efficiency. It is demonstrated how these features may be useful in solids elemental analysis by mass spectrometry, including thin-layer analysis. In addition, the complementary possibilities of a pulsed atomic emission source are shown for a hollow-cathode lamp and for a glow discharge cell.

Microsecond pulsed glow discharge optical emission spectrometry — investigation of temporal emission characteristics

A high voltage, microsecond regime, pulsed glow discharge has been applied to a standard Grimm type source on a LECO SA-2000 multichannel optical spectrometer. The spectrometer system has been equipped with fast time-resolved electronics in order to study the temporal response of the elemental emission lines. The aim of the research was to investigate possible gains in signal-to-background ratio (S/B) and detection limits compared to a conventional direct current (dc) mode of operation. It was found that the high instantaneous pulse power in itself gives approximately a factor 2 increase in S/B. By utilising time-resolved detection, gains in the range 5–10 are achieved. For nitrogen, it was found that the background from leakage of air could be reduced to approximately 1/3 compared with the dc mode. Another interesting observation was that certain nearly non-conducting materials, which cannot be run in dc mode, could be run in the microsecond pulsed mode.

Quantitative depth analysis using microsecond pulsed glow discharge atomic emission spectrometry

The development of microsecond pulsed glow discharge atomic emission spectrometry (GD-AES) for quantitative depth profile analysis is described. The quantification method, which is based on sputtering rates, has been previously applied to depth analysis employing a dc glow discharge source. Optimal conditions of the glow discharge source, including voltage, pressure, pulse width and pulse frequency, are utilized in order to give the best depth resolution. Pulsed operation of the glow discharge affords the advantages of two additional parameters, pulse width and pulse frequency, which allow better control over the amount of sample being removed via cathodic sputtering. This allows thin layer samples to be analyzed that are very difficult using the dc source. Under the optimized parameters, thin coatings of varying depths of Cu deposited on steel were quantified. The quantitative method predicted Cu deposition depths in accord with the nominal depths.

Glow discharge source interfacing to mass analyzers: theoretical and practical considerations.

The fundamental requirements for the optimum mechanical interface between a glow discharge ion source and a mass spectrometer are described in this paper. Specifically, the properties of a typical glow discharge ion source are compared and contrasted to those of a typical inductively coupled plasma ion source. The critical parameters and theoretical considerations of glow discharge and inductively coupled plasma ion source interfaces are reviewed, and the results of experiments using both quadrupole and time-of-flight mass spectrometers with a glow discharge source are presented. The experimental results clarify several important problems in the glow discharge ion sampling process. Our findings indicate that a shock wave structure does not occur in the supersonic expansion of the glow discharge ion source. Ions of different masses have similar initial kinetic energies in the glow discharge; thus, the angle of the skimmer cone is not a critical parameter for efficient ion beam extraction. Another consquence is that space charge effects in glow discharge ion sources repel heavy ions farther off axis than light ions. Thus, there are distinct and fundamental differences between glow discharge and inductively coupled plasma ion sources which are relevant to both ion sampling and ion extraction processes.

A double microsecond-pulsed glow discharge ion source

A double pulsed glow discharge was compared to the single pulsed mode as an ion source for atomic mass spectrometry. During the time-window between the first applied voltage pulse and signal collection, the second pulse is applied. The double pulse was used to enhance analyte ionization and study the timing sequences within the pulsed glow discharge, the signals from which were followed using a time-of-flight mass spectrometer. Results showed that the double pulse glow discharge increased ion signals, provided that appropriate pulsing delays and gas flow rates were applied. The effect of the double pulse on methane-enhanced polyatomic cluster formation has also been evaluated.

Primary studies of the microsecond pulsed glow discharge as an emission source using a conventional hollow cathode lamp

Primary investigations on the operation of a glow discharge in the microsecond regime as an emission source have been carried out by using conventional hollow cathode lamps. By passing large currents of short duration repetitively through a conventional hollow cathode lamp, and making use of a gated detection system, light intensity enhancement of up to 3–4 orders of magnitude can be achieved with respect to dc operation. Intensity comparisons have been conducted on 4 hollow cathode lamps, As, Ca, Cu and Ti, for both the analyte and the filler gas species. The effects of variation of pulse conditions were examined in great detail on a Ti lamp. Temporally resolved voltage, current and emission profiles, with the analytical characteristics, demonstrate that micro-second pulsed operation does offer unique analytical advantages.

Factors influencing signal profiles in microsecond pulsed glow discharge atomic emission spectrometry

The effects of discharge conditions on the temporal profiles of sputtered atoms and gas species in a microsecond (µs) pulsed glow discharge (GD) plasma were investigated. A sequential scanning monochromator accompanied with a gated detection system was used to measure the spectral intensity. Substantial improvement in signal intensity has been realized compared with that of dc operation. Among the parameters investigated, voltage and pressure are dominant factors affecting emission profiles. The results are useful for describing various spectrochemical properties of the pulsed glow discharge and for parameter optimization.