Having carryover issues with selenium and molybdenum (7900)

I run a 7800 doing EPA 200.8 MET and 6020B methods. Recently, we've had an instrument serviced and have been seeing Mo and Se carryover into ICB and CCBs at levels > ½RL. 

Here are the things I've tried to address this:

  • Loaded an old batch and tune from before the PM
  • Replaced the nebulizer
  • Cleaned the rotary valve
  • Replaced the spray chamber
  • Cleaned the torch
  • Replaced cones
  • Dramatically increased rinse time by a minute 

So far, no luck. I'm suspecting it's something in the software, like some tune settings or something like that. 

Does anyone have any ideas on what it could be?


After doing some research I've come to the conclusion that the issue is the switching valve. When it switches over to "inject" the Mo comes through. It seems the inside isn't being cleaned well enough to. 

  • Replaced both the stator and rotor
  • Cleaned the heck out of the valve

No matter how long I have my rinses (within reason) it still carries over. 

  • Bump. Added some more info and context. 

  • You can split your system.  try the self-aspiration with the nebulizer.  Splitting your system.  Check if you see with the blank in self -aspiration the Mo and Se in tune monitor screen. If it's there focus on the torch box and everything else related to between the gate valve and nebulizer.   If it's gone. Focus on the sample intro.  You may want to try to rinse for at least 20min with 10% HNO3 and 1%HCl max (2%). Then go back to normal operation. Try to monitor Mo and Se during the rinse to see if you have an active site that will be cleaning with the rinse solution. Lastly, check the sample probe for any trace of dirt or restriction since you run 6020 these can stress part.  Make sure the sample probe you use is 1mm (double band blue).  Could be the blank that is contaminated.  (Same tube reused, same volumetric flask... etc) 

  • I'll give this a shot tomorrow and see what happens.

    Could be the blank that is contaminated.  (Same tube reused, same volumetric flask... etc) 

    One thing to note: The contamination washes out with a second injection from the same blank. It isn't the bottles or solution that's contaminated

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