Having carryover issues with selenium and molybdenum (7900)

Do you rinse with HCl?  Mo, Sb, Se, Hg, Ag, Al can get "hung up" in the capillary.  Rinsing using a mixture of nitric and HCl will help remove those.  With 200.8, you will want to run a nitric only rinse after the nitric/HCl mix will clear out the Chloride to minimize interferences.  Be careful how long and at what concentration you add the HCl.  The standard Ni sample cone does not stand up well against HCl >0.5%.  For that much HCl we suggest using the Ni plated sample cone.  Typically to clean stubborn elements, 2%nitric with 2%HCl will do it.  It just depends on how high they are in solution to be cleaned out. You could try a short rinse in 2%nitric with 0.5%HCl for 10-15 seconds before going to your Nitric rinse for the time you have it set now.  Play with the timing of the two rinses to remove the Mo and Se without getting interference on As, Se and V from Cl-.

Are you using Preemptive Rinse in your Batch?  This can help minimize the time the high concentration sample is in the line.

> Do you rinse with HCl? 

Yes, the rinses are as follows:

The rinse port and vials 1 and 2 are both 2% nitric 1% HCl. Rinse vial 3 is 4% nitric 2% HCl. I've been running it this way for ages and haven't had any interference issues until lately but I suppose I could try changing out Rinse 3 for only 2% HNO3 and adjusting the timings and post run so it's flushing the loop and spray chamber with that. 

>The standard Ni sample cone does not stand up well against HCl >0.5%.  For that much HCl we suggest using the Ni plated sample cone. 

Yep, that's what we use

>You could try a short rinse in 2%nitric with 0.5%HCl for 10-15 seconds before going to your Nitric rinse for the time you have it set now.  Play with the timing of the two rinses to remove the Mo and Se without getting interference on As, Se and V from Cl-.

So what I'm hearing here is that I should be rinsing with only HNO3, with maybe a short rinse of HCl early on that gets washed out by a later HNO3 rinse. 

HCl is to help move along the "sticky" elements like Mo, and some others. The HNO3 only at the end of things is to get rid of the Cl- as you can't use gas mode for 200.8 Drinking Water analysis.  As, Se, V have Cl- interferences in no-gas.

I see from the program you are using an ISIS, have you pulled that off and cleaned the valve head and rotor seal?  With the valve off, and all connections removed, I spray soap solution through the openings for all the ports to clean them out.  Then spray DI through them to rinse out the soap.  For the rotor seal, make sure there is no material in the small channels of the face.  Clean with a cotton tip swab and some DI.

>you can't use gas mode for 200.8 Drinking Water analysis. 

A point of clarification: We're doing 200.8 MET, so we have gas.

>have you pulled that off and cleaned the valve head and rotor seal? 

Yeah, I've cleaned that out. There was no change

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)