We get an interference next to mass 7 peak.

Almost every time we run the performance check we get this peak at mass 6, next to mass 7. We tried cleaning and/or changing skimmer plus cone, but we still get this interference. We use high quality nitric acid and milli-Q water. A while ago we had the omega lens cleaned as well, and it got a little better, but it only lasted two days! After two runs there it was again. Has this happened to anyone? We are grateful for any help or advice. Thank you! 

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