We get an interference next to mass 7 peak.

Hi Mercedes,

That peak is from residual internal standard solution.  Li6 is <1% abundance naturally.  For the internal standard solution they concentrate the Li6 so that Li can be an analyte at mass7.  When you start the plasma, make sure your internal standard capillary is in 1% nitric acid.  That way it has the full warmup time to wash out the ISTD solution from the line before starting the performance report.  As long as the right peak Li7 is greater than the left Li6 peak and your axis position is 6.9-7.1 amu you can run without a concern.

Thank you Tom, your advice has been very very helpful. Me and my team are very grateful. Your answer was detailed, well written and very clear. Thanks!!! 

Glad I could help.  This is a common issue.  The internal standard line is much smaller diameter than the sample line, so it takes a lot longer to flush it out.  The 15 minute warmup should be enough with a 1% nitric acid solution to clear it.  Even if there is a small peak there, as long as it is smaller than the Li7 peak it will find the correct peak.  Best Regards.