Internal standard fluctuates

When we run standards (or ICAL) the internal standards get slightly higher the higher the concentration of the standard.  The analyst has mentioned this with samples as well (dirty samples vs clean samples).  When we run blanks or clean samples the IS goes lower, esp the last IS.  It will typically fail the IS criteria when compared to the daily check standards.   I ran 15 blanks and the IS is stabile during the runs.  Then I put in 4 standards and the IS gets higher.  This tells me that something still isn’t right with the system.  We have changed all the parts that we are able to change.  Columns, traps, liner/septum, cleaned source….changed source body, repeller assy, drawout plate.  We also checked the pump oil in the diffusion pump.  The autosampler just went through a PM visit and they were not able to find anything wrong with the autosampler. 

We also see tune problems.  The tune will pass, we run an ICAL, then the next day the tune won’t pass again.  It is normal for it to take 1-3 tries to get the tune to pass….but this instrument takes WAY too many.  It seems more like a game of chance than anything else. 

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