Principle of Grimm source
There are many different kinds of glow discharge sources. They can be classified in various ways. We can first of all distinguish between inductively coupled discharge and capacitively coupled discharge. Inductively discharges are generated by a strong alternating magnetic field, accelerating the electrons and thus generating a plasma. This type of discharge is used in ICP a well established analytical technique, but not subject of this web site. In capacitively coupled discharges, an electrical field is generated between two electrodes. Electrons are accelerated in this electrical field and can create a plasma. Most Neon tubes and low consumption "light bulbs" use capacitively coupled discharge.
For the direct analysis of solid samples, capacitively coupled discharges are used because the sputtering or etching of the cathode is an essential part of the analytical process.
The modern era of GD-OES began in 1967 with the Grimm source. All commercial sources in use today owe much to the original Grimm source. In the Grimm source the cathode block is copper and the DC voltage is applied to the cathode block which is in direct contact with the metal sample. It was first brought on the market by RSV Präzisionsgeräte GmbH, Germany.
A copper tube, anode, is brought very close to the solid sample to be analysed, the cathode. The copper tube is filled with argon at low pressure(~600 Pa) and a voltage (potential difference) is applied between the anode and the sample. The applied voltage can be radio frequency (RF) or direct current (DC). The sample is negatively biassed relative to the anode.
Electrons leave the more negative surface of the sample towards the anode. On their way to the anode they collide with argon atoms creating positively charged argon ions and high energy metastable argon atoms. The positively charged argon ions are attracted to the negatively biassed sample surface. Along the way they have many collisions with other argon atoms, losing much of the energy gained in the electrical field. Many of the ions are neutralised by collisions but continue towards the sample.
When the argon ions (plus neutrals) strike the sample surface they impart sufficient energy (>100 eV) to disrupt atomic bonds and eject atoms and electrons. This process is called sputtering. The sputtered atoms fly away from the sample surface and coat themselves on the anode or are removed by vacuum pumps.
Away from the sample, some of the sputtered atoms have collisions with high energy electrons or metastable argon atoms and are excited to high energy states. When they then de-excites they emit a photons, which create a 'glow'. This glow is then analysed with one or more optical spectrometers.
It sounds complicated and it is. The plasma created in the source is very small and therefore difficult to measure directly and to model mathematically. The plasma has the same diameter as the inside diameter of the anode, typically 4 mm. No plasma is created between the front face of the anode and the sample as the distance there, typically 0.1-0.2 mm, is too small. It is therefore called a restricted plasma.
Immediately in front of the sample is a dark space called the cathode fall. It extends for about 0.5 mm. Here there is no glow but most of the drop in plasma potential occurs here. So most of the ions are created near the beginning of the cathode fall. After the cathode fall is the negative glow region, where most emission occurs. The negative glow region extends for about 2-3 mm away from the sample. Normally there is no positive glow region in GD-OES sources, used for analytical purposes.