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The main problem with an RF coil is corrosion, whether from heat or chemical oxidation. Corrosion reduces the efficiency of energy transfer to the plasma increasing the stress applied to other components in the RF generating system. In particular, the RF Power Tube or RF amplifying FET's have to generate more power to deliver the same amount of energy to the plasma.
Coils are the transmitters of this RF energy. The efficiency of different materials in conducting RF energy plays an important role in the overall plasma efficiency. Metal oxides do not conduct RF as well as pure metal. In particular copper sulphate is a very poor conductor, whereas silver oxide has a much smaller effect on efficiency. The most RF efficient metals are pure silver IAC (International Annealed Copper Standard) 105, annealed copper IAC 100, and gold IAC 74.
If heat is the biggest problem then the best way to deal with this is to use a coil that is plated in a highly reflective material such as silver. Silver plating however does produce a tarnish appearance after a period of use. To obtain the best life out of a coil in this situation, the coil should be cleaned frequently with a tarnish remover. This will eventually lead to the removal of the silver plating. However, if the tarnish is not removed, excessive heat can build up due to higher absorption and lead to the plating flaking away from the copper tubing. This exposes bare copper areas that will deteriorate at a faster rate. Copper will oxidise much quicker, thereby leading to possible arcing between coils, and lower energy transfer to the plasma.
Chemical corrosion is best dealt with using a gold-plated coil. The plating process is critical in the performance of the coil. Gold plating can produce porous coatings, which allow a chemical attack on the copper tubing, thereby rendering the plating useless. The plating process used in the production of the Glass Expansion gold plated coils is designed to remove the porous effect yet still maintain the high purity plating. In time, the gold plating will show signs of copper sulphate formation on the surface. This occurs due to copper migration through the plating. For best performance these deposits should be removed using an abrasive metal polish that is free of acid. Unfortunately once this starts to occur the migration will occur at a faster rate and the coil should be replaced.
Periodic cleaning of your coil will extend the lifetime and produce a more consistent plasma. This in turn will lead to better analytical results. It will also reduce stress on the RF generator.