A gold coil is a base copper coil, coated with gold in order to provide extra corrosion resistance. Corrosion originates from the potential for condensation of acidic vapors on the cooled coil surface. Gold is more corrosion resistant than Copper, but the coating is somewhat porous. Therefore corrosion of the copper can still result and the RF coil can turn green (copper sulphate and chlorides on the surface). Corrosion of the RF coil may result in a poor appearance, but the coil can still perform acceptably for some time.
There are no significant differences in electrical performance for an RF Coil between Copper, Gold or Silver for sustaining an ICP. Therefore there should not be any need for a change in tuning parameters.
What is crucial is the alignment of the RF coil and there is a significant risk of damage during plasma operation if the coil is misaligned. Agilent recommend that only a trained service engineer should replace the RF coil for this reason.
With alignment in mind - any cleaning of the RF coil should be performed very carefully and it is not recommended to clean the coil frequently.
I would suggest checking the alignment of the RF coil and potentially look at any other sources (things that you may have changed at the same time as the RF coil)
This topic has recently been discussed on the PLASMACHEM-L forum, if you have not seen this information you can search the archives for the topic "RF Coil Material of construction"
Hope this helps!
ICP-MS Supplies Product Manager
I have gone ahead and marked this question as answered, if you still have more questions, feel free to come back and we will be happy to work with you.