Glow Discharge Atomic Emission Source Research Articles

The design and characteristics of direct current glow discharge atomic emission source operated with plain and hollow cathodes

A compact direct current glow discharge atomic emission source has been designed and constructed for analytical applications. This atomic emission source works very efficiently at a low-input electrical power. The design has some features that make it distinct from that of the conventional Grimm glow discharge source. The peculiar cathode design offered greater flexibility on size and shape of the sample. As a result the source can be easily adopted to operate in Plain or Hollow Cathode configuration. I– V and spectroscopic characteristics of the source were compared while operating it with plain and hollow copper cathodes. It was observed that with hollow cathode, the source can be operated at a less input power and generates greater Cu I and Cu II line intensities. Also, the intensity of Cu II line rise faster than Cu I line with argon pressure for both cathodes. But the influence of pressure on Cu II lines was more significant when the source is operated with hollow cathode.

On-line determination of sputtered depth in a radio frequency glow discharge atomic emission source by laser confocal displacement

A new method of sputtered depth determination was evaluated for glow discharge (GD) analysis of solid samples. Although sputter analysis methods like GD may characterize samples in terms of both composition and spatial (depth) distribution of those components, quantitative analysis may suffer significantly from uncertainties in sputter rate (depth) determinations. The study described in this paper was conducted in an effort to achieve true depth profile capabilities for the radio frequency glow discharge atomic emission spectroscopy (r.f.-GD-AES) source, however the results are applicable to any sputter-type analysis method. Detailed here is a unique system by which sputter depth information may be collected simultaneously with operation of the GD plasma, with only minimal influence from sample matrix or plasma conditions. As such, the method enables the determination of real-time sputter depth. This achievement was made possible by the use of a laser confocal displacement sensor (LCDS) that optically measures the distance between a mounted sensor head and the sample surface. Very simply, the LCDS measures the displacement of the sputter-exposed surface relative to the original sample. The results outlined in this report indicate that the laser confocal technique has great potential as an advancement to depth-resolved analysis by r.f.-GD-AES. Statistical figures of merit include an experimental depth resolution of -1 μm and a sample-to-sample reproductibility of ∼5% RSD. Methods of improving further the capabilities of the system described here are also outlined.

Evaluation of helium-argon mixed gas plasmas for bulk and depth-resolved analyses by radiofrequency glow discharge atomic emission spectroscopy

Studies were performed to determine the practical benefits of mixed discharge gases (Ar and He) in the bulk and depth-resolved analysis of solids using a radiofrequency glow discharge atomic emission source. This study examined the characteristics of analyte emission intensity and yield, sample sputter rate and crater shape as a function of added He gas, for both conductive and non-conductive sample matrices. In comparison with pure Ar plasmas, the addition of He does not ultimately improve the limits of detection in the bulk analysis of conductive (metallic) solid samples. However, non-conductive sample matrices such as glasses, which are greatly affected by discharge gas pressure to maintain sputtering conditions (crater shape, sputter rate), may benefit from the addition of He to the plasma gas as a means of enhancing the excitation conditions. The shape of the sputtered craters was minimally affected by the addition of He, indicating that Ar partial pressure is the parameter that most critically affects sputtering characteristics. Overall, the addition of He to the discharge plasma has been found to enhance analyte emission intensity without significantly influencing sputtering characteristics. In terms of depth-resolved analyses, optimized discharge conditions (specifically pressure) derived for the pure Ar case, which are sometimes accompanied by a loss of analytical sensitivity, can be augmented by the addition of He to the discharge to yield improved analytical responses while retaining the desired sputtering characteristics.

Characterization of helium/argon working gas systems in a radiofrequency glow discharge atomic emission source. Part II: Langmuir probe and emission intensity studies for Al, Cu and Macor samples

The application of a tuned Langmuir probe is extended to the measurement of the charged particle characteristics (electron and ion number density, average electron energy and electron temperature) in an analytical radiofrequency glow discharge (RF-GD) in helium. The effects of discharge operating conditions, such as RF power and pressure, on the charged particle characteristics for conducting (aluminum) and nonconducting (Macor) samples are studied. The differences in plasma characteristics between argon and helium working gases are discussed. Langmuir probe measurements are also performed in an argon/helium mixture. Variations of the emission intensities of sputtered analytes (copper and aluminum) are investigated when helium is introduced into an argon RF glow discharge plasma. It is recognized that, although the number of sample atoms in the plasma gradually decreases due to reduced sputtering, the emission intensities of various Al(I) and Cu(I) lines increase with helium addition. Measured electron and ion number densities also decrease with helium addition, whereas the average electron energy and electron temperature increase, accounting for the enhancement of emission intensities.

Characterization of helium/argon working gas systems in a radiofrequency glow discharge atomic emission source. Part I: Optical emission, sputtering and electrical characteristics

Studies are performed to determine the influence of discharge gas composition (helium/argon working gas mixtures) on the analyte emission signal intensities, sputtering rates, and DC-bias characteristics of an analytical radiofrequency glow discharge atomic emission spectroscopy (RF-GD-AES) source. As the partial pressure of He is increased from 0 to 15 torr, increased emission intensity is observed for a range of bulk and trace elements in NIST 1250 SRM (low alloy steel), regardless of the base pressure of Ar in the source (5 and 9 torr). In contrast to increases in analyte emission intensity of up to 300%, counterindicative decreases in the sputtering rates on the order of about 30–50% are observed. The magnitude of these effects depends on both the partial pressure of helium introduced to the source and the total pressure of the He and Ar gases. Use of relative emission yield (REY) to normalize changes in emission intensity to sputtering rates indicates that excitation efficiencies increase under these conditions. Increases in average electron energy and temperature appear to control this response. Decreases in both analyte emission intensities and sputter rates occur with increasing He partial pressure when the total pressure in the cell remains fixed (11 torr in these studies). Emission yields for the fixed pressure, mixed gas plasmas decrease as the partial pressure of He (He/Ar ratio) in the RF-GD source increases. In this case, decreases in electron number densities appear to dictate the lower REYs. Measurement of DC-bias values at the sample surface provide understanding with respect to the observed changes in sputtering rates as well as suggest the origins of changes in plasma electron energetics. Use of a diamond stylus profilometer provides both the quantitative sputter rate information as well as qualitative insights into the use of mixed gas plasmas for enhanced depth profiling capabilities. The analyte emission characteristics of these mixed gas plasmas can be further understood by examining fundamental Langmuir probe data for pure He and mixed Ar/He plasmas, which are presented in the following companion paper.

Direct solution analysis by glow discharge: electrolyte-cathode discharge spectrometry

A new glow discharge atomic emission source was developed for the direct determination of metals in aqueous solutions by applying an atmospheric glow discharge in the air gap (2–6 mm) between an electrolyte solution cathode and a W-rod anode. Cathode sputtering of the solution surface and subsequent excitations occur when the pH of the solution is below 2.5. The spectrum emitted contains the basic atomic lines of the dissolved metals from K 769.9 to Zn 213.8 nm, ion lines of Mg and Ca and strong OH, NH and N2 bands. Boiling of the cathode pole can be avoided by use of a flow-through technique. The electron temperature was found to be around 5000 K. The calibration curves of line intensities versus metal concentrations were linear in the 1–50 ppm range and showed strong positive dependence on the discharge current and on the hydrogen ion concentration of the solution. Pulse modulated operation with time-resolved signal processing promises a substantial increase in the signal-to-background ratio. The flow-through electrolyte acts as a continuously renewed cathode pole, thereby enabling the continuous direct multi-metal assay of solutions. This report is the first discussion on the analytical characteristics of electrolyte-cathode discharge (ELCAD). spectrometry as a new technique for metal monitoring in aqueous solutions.

Electrical and optical characteristics of a radio frequency glow discharge atomic emission source with dielectric sample atomization

A parametric study has been conducted on a radio frequency powered glow discharge atomic emission spectrometry (rf-GD-AES) source to evaluate its performance in the direct analysis of non-conducting solid materials. These experiments include both the emission and electrical characterization of this system with respect to discharge power, pressure, limiting anode orifice diameter, and sample size. The rf-GD-AES source has been demonstrated to operate interchangeably between conducting and non-conducting sample materials; however, the energy dissipated within the plasma appears to be reduced with the dielectric samples, resulting in lower emission intensities and sputtering rates. The power losses have also been found to be a function of the size, or thickness, of the sample materials. Despite these limitations of the system, preliminary emission data demonstrate that the rf-GD-AES system can be successfully employed in the direct, trace analysis of non-conducting sample materials.

Emission characteristics of a pulsed, radiofrequency glow discharge atomic emission device

The instrumentation, initial observations, and operating characteristics ofa pulsed radio-frequency glow discharge atomic emission source are described. Anomalies In the temporal emission intensity wave forms for some analyze transitions are a b reported. These anomalies take the form of a well defined intensity maximum located either near the beginning of the discharge pulse or just after power termination,depending on the particular transition. Still other analyze transitions demonstrate no such temporal irregularities.Selective, gated, detection of these emission anomalies suggests possible analytical advantages in terms of instantaneous emission intensities, which may translate into improved analytical sensitivities.

Characterization of a radio frequency glow discharge emission source

A radio frequency glow discharge atomic emission source is described. This source incorporates two optical windows, one positioned such that both the glow region of the discharge and the sample surface may be viewed simultaneously, and the other positioned such that only the glow may be viewed. Preliminary signal-to-noise ratios obtained with the two optical geometries are compared and found to be similar. However, optical monitoring of both the glow and sample surface results in significantly higher emission intensities, owing to the more complete viewing of the energetic regions of the discharge. As a result, this geometry may be considered to be analytically more advantageous. Additionally, data are presented to illustrate the dependences of the dc self-bias potential, excitation temperature, sputter rate, and emission intensities of sputtered species on discharge pressure and generator output power. Scanning electron micrographs of the surface of a machinable glass ceramic (MACOR) sample indicate that appreciable sputtering of electrically nonconductive solid samples occurs in the rf glow discharge. Comparison of data acquired with conductive and nonconductive samples is made to show the similar behavior of the discharge in the presence of the two sample types. Exemplary spectra are presented to illustrate the potential usefulness of the source for the emission spectrometric analysis of both electrically conductive and nonconductive solids.