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)
/
/icpinformation.org
/www.gulfcoastconference.com
link