Surface chemical analysis - General rules for glow discharge optical emission spectrometry (GD-OES)
1 Scope
This document provides guidelines that are applicable to bulk and depth profiling GD-OES analyses. The guidelines discussed herein are limited to the analysis of rigid solids, and do not cover the analysis of powders, gases or solutions. Combined with specific standard methods which are available now and, in the future, these guidelines are intended to enable the regulation of instruments and the control of measuring conditions.
Although several types of glow discharge optical emission sources have been developed over the years, the Grimm type with a hollow anode accounts for a very large majority of glow discharge optical emission devices currently in use both for dc and rf sources. However, the cathode contact is often located at the back of the sample, in e.g. the Marcus type source, rather than at the front as in the original Grimm design. The guidelines contained herein are equally applicable to both and other source designs and the Grimm type source is used only as an example.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 3497 Metallic coatings - Measurement of coating thickness - X-ray spectrometric methods
Note: GB/T 16921-2005, Metallic coatings - Measurement of coating thickness - X-ray spectrometric methods (ISO 3497:2000, IDT)
ISO 5725-1 Accuracy (trueness and precision) of measurement methods and results - Part 1: General principles and definitions
ISO 5725-2 Accuracy (trueness and precision) of measurement methods and results - Part 2: Basic method for the determination of repeatability and reproducibility of a standard measurement method
ISO 5725-3 Accuracy (trueness and precision) of measurement methods and results - Part 3: Intermediate measures of the precision of a standard measurement method(trueness and precision) of measurement
ISO 5725-4 Accuracy (trueness and precision) of measurement methods and results - Part 4: Basic methods for the determination of the trueness of a standard measurement method
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3497, ISO 5725-1, ISO 5725-2, ISO 5725-3 and ISO 5725-4 apply.
4 Principle
Analysis by GD-OES involves the following operations:
a) preparation of the sample to be analysed, generally in the form of a flat plate or disc of dimensions appropriate to the instrument or analytical requirement (round or rectangular samples with a width of more than 3 mm, generally 20 mm to 100 mm, are suitable);
b) atomization and excitation of the analytes to be determined by means of ion sputtering and inter-particle collisions occurring in the glow discharge plasma;
c) measurement of the emission intensities of characteristic spectral lines of the analytes (for depth profiling, emission intensities are recorded as a function of time);
d) determination of the analyte concentrations contained in the sample by calibration with reference materials of known composition (for depth profiling, the sputtered depth as a function of time is also determined by calibration with reference materials of known composition and sputtering rates).
A diagram of a typical GD-OES system is presented in Figure 1. GD-OES is based on the use of a glow discharge device as an optical emission source. The glow discharge device consists of a vacuum chamber filled with a supporting gas, usually argon. The glowing plasma, from which the discharge takes its name, is maintained by a controlled high voltage of 200 V to 2000 V applied between the anode and cathode in the plasma gas. The solid sample to be analysed serves as the cathode.
Atomization of sample material in the glow discharge is the result of cathode sputtering, the destruction of the negative electrode (cathode) in a gas discharge due to the impact of fast charged and neutral particles. Ions formed in the plasma are accelerated toward the cathode surface by the electric field in the plasma. When an ion or neutral atom collides with the surface, its kinetic energy may be transferred to atoms on the surface, causing some of these surface atoms to be ejected into the plasma. Once in the plasma, these sputtered sample atoms may be ionized and excited through inelastic collisions with electrons or other species. The majority of these excited analyte atoms and ions then emit characteristic optical emission upon relaxing into the lower electronic state. The optical emission is analysed by an optical spectrometer containing a dispersive element, normally a diffraction grating. The intensities of element-specific spectral lines are translated to electrical signals by means of appropriate detectors. A polychromator is commonly employed, so that many elements can be quantified simultaneously. Spectral lines that are not contained in the line set of the polychromator can be accessed by means of a scanning monochromator, if one is available. CCD instruments also exist, where a spectrum over a wide spectral range can be measured continuously. In practice almost all elements in the periodic table can be determined, including metals, metalloids and non-metals.
