Skimmer cone green deposits

Hello! I have an issue with the skimmer cone at ICP-MS 8900. After 3 - 4 days of running the instrument, the skimmer cone showed a green color, at the same time we register a sensitivity decrease. Even when proceeding with the cleaning procedure (cotton wipe with HNO3 2%, washing with distilled water, sonicating 10 min) the green's skimmer cone didn't disappear. Consideration: -The samples analyzed were the same as always, the problem shows lately -The green deposits on the skimmer cone were recovered and analyzed. Nickel was the most abundant element, samples are nickel free. -The exhaust flow is ok. -The sampler cone isn't green, it is dirty as usual because of the sample matrix. -The background noise at m/z 60 is higher than before. All your comments will be helpful!