NEWS Glass Expansion Newsletter | October 2017 | Issue 44 GE News Why change your RF Coil? Introduction Radio Frequency (RF) alternating current has been used for quite some time to induce a heat source in a localized region. Development started in the 1960’s with the first commercial ICP available in 1974.1 Originally the RF alternating current was generated by a valve or power amplifier tube, which was connected to the RF load coil via a matching circuit. This matching circuit was generally made up of a fixed capacitor and a tuning or variable capacitor. This tuning capacitor was adjusted to balance the system and lower the reflected power back to the RF generator. Today many if not all use a solid state RF generator and amplifier; some have matching circuits and others allow the frequency to change to match the plasma load (free-running generators). All but one of the ICP instruments use a coil to generate the plasma inside a torch. The coil is designed to match the rest of the RF system. RF Coil Plating So why are some RF load coils plated? This goes back a long way to when the purity of copper was not as high as it is now. Silver plating of load coils was found to give higher conductivity of the RF energy. Once the purity of copper increased it was shown that the conductivity of the copper load coil was better than the silver-plated coil.2 So why are most RF load coils still plated? The answer is a little more complex. The ICP’s of today use two RF frequencies 40 and 27 Mhz to generate the plasma. For these two frequencies the majority of the current will flow in the outer 10-13 micron layer for copper and silver and 12-15 micron for gold. The environment inside the torch box can be highly corrosive, and bare copper will corrode faster leading to a buildup on the outside of the coil which will lower the conductivity of the coil. It will also make the surface of the coil uneven, meaning the current has to travel a larger distance which increases the resistance of the coil. Plating the coil with a very high purity silver gives better corrosion resistance, as silver oxide does not affect the resistance. However, silver is attacked by sulphides,2 which leads to a buildup on the silver surface. Alternatively, gold can be used to provide better corrosion resistance. However, there are a couple of issues with gold in an RF system. Firstly the conductivity of pure gold is only around 75% that of pure copper, and secondly the thickness of the layer of gold plating required to give a non-porous layer would mean that the majority of the current will flow through the gold plated layer. This lowers the conductivity of the coil. For this reason, a thinner layer is typically used which is not thick enough to stop the porosity; hence the gold plated coils will eventually corrode. The addition of a PTFE coating over the silver coating will give very good corrosion resistance without the increase in resistance. However the constant GE Employment Opportunity Glass Expansion Inc. is looking for an experienced ICP analyst to join our marketing team in Massachusetts USA as a Product Specialist. US Citizenship required, relocation expenses will be reimbursed. Please contact Jerry Dulude (jdulude@geicp.com) for details. A Glass Expansion specialists will be on hand to answer your questions and a selection of Glass Expansion products will be on display at the following Conferences: Matsue, Japan, November 12 - 17, 2017. http://www.apwc2017.org/ Omni Amelia Island Plantation Resort Amelia Island, FL January 8 – 13th (Expo January 8 – 11th) Booth # 8 & 9 http://icpinformation.org George R Brown Convention Center Houston, TX January 17 – 18th Booth # 616 www.gulfcoastconference.com In this issue: Application Spotlight...........................1 – 3 GE News..................................................1 New products............................................4 Instrument News.......................................5 From Teledyne Leeman Labs Application Spotlight 2017 Asia-Pacific Winter Conference on Plasma Spectrochemistry 2018 Winter Conference on Plasma Spectrochemistry Gulf Coast Conference (2017 rescheduled)
GE Newsletter October 2017
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