GDS950 Glow Discharge Spectrometer: Glow Discharge Atomic Emission Spectrometer

Features

The glow discharge source brings several advantages, including:

• Simple, linear calibrations when compared to other sources
• Controlled excitation that occurs away from the sample surface
• Reduced reference material consumption
• Automatic cleaning cycles between samples saves time and minimizes matrix effects for increased precision

The detection system ensures stability, flexibility, and performance.

• Full wavelength coverage from 120 nm to 460 nm
• 30 pm (0.030 nm) resolution to differentiate even the most complex spectra

Multiple lamp options allow for analysis of various sample types:

• DC only lamp for electrically conductive samples
• DC/RF combination lamp to support both electrically conductive and non-conductive samples
• Pulsed RF capability for thermally sensitive samples

Optional CDP Analysis Support is available.

• Compositional depth profiling of solid electrically conductive samples
• Ideal for plating, galvanizing, cladding, and other conductive surface treatments

Cornerstone Software provides simplified analysis and reporting.

Applications

The GDS950 is ideal for bulk elemental determination in metals or other solid materials like steel, cast iron, titanium, and other metals. When equipped with the CDP option, it expands the capability to compositional depth profile analysis of surface features like galvanizing, plating, heat treatments, and cladding. Application capabilities are further expanded with the DC/RF lamp option, which expands both bulk and CDP to include solid electrically non-conductive materials like paint, glass, plastics, and more.

Theory of Operation

Glow Discharge Spectrometry (GDS) is an analytical method for direct determination of the elemental composition of solid samples. A prepared flat sample is mounted on the glow discharge source, and then the source is evacuated and backfilled with Argon. A constant electric field is applied between the sample (cathode) and the electrically grounded body of the lamp (anode). These conditions result in the spontaneous formation of a stable, self–sustained discharge, which is called a glow discharge.

The applied current is regulated by the power supply and the lamp voltage is held constant through regulation of the Argon pressure. As soon as the plasma is initiated, inert gas ions formed in the plasma are accelerated by the electric field toward the cathode. Through a process called cathodic sputtering, kinetic energy is transferred from the inert gas ions to the atoms on the sample surface, which causes some of these surface atoms to be ejected into the plasma.

Instrument models equipped with the Radio Frequency (RF) option use radio frequency energy (instead of direct current) to generate the glow discharge. LECO's proprietary True Plasma Power algorithm is used to correct for radiated and reflected power losses. True Plasma Power improves the ability of RF-equipped models to perform both quantitative bulk analysis and quantitative depth profile analysis for electrically conductive and electrically non-conductive samples.

Once the atoms are ejected into the plasma, they are subject to inelastic collisions with energetic electrons or metastable Argon atoms. Energy transferred by such collisions causes the sputtered atoms to become electrically excited. The excited atoms quickly relax to a lower energy state by emitting photons. The wavelength of each photon is determined by the electronic configuration of the atom from which it was emitted. Since each element has a unique electronic configuration, every element can be identified by its unique spectrochemical signature or emission spectrum.

A spectrometer is used to measure the emission signals from the glow discharge. To ensure that the media within the spectrometer is transparent to ultra-violet and visible light (120-460 nm), the entire optical system is purged with Argon. Photosensitive Charge-Coupled Device (CCD) arrays are positioned at the focal plane in such a manner that the complete emission spectrum is recorded from 120 to 460 nm.

The CCD arrays convert the spectrum into an electrical signal, which is digitized and processed to remove dark current signal, normalize the pixel response, extend the dynamic range, and eliminate pixelation. Since the number of photons emitted by each element is proportional to its relative concentration in the sample, analyte concentrations can be deduced by calibration with reference samples of known composition.