Trace Metal Analysis in Drinking water - Se_78 Question

Recently i have run into a issue with my Se_78 CCV.  In our lab, we analyze drinking water for 24 different parameters and our CCV has 50ppb of each parameter in it.  Our mass spec 7900 is set up with a ISIS and a SPS4 autosampler.  We use Sc, In and Tb as our internal standard.  Se is the only analyte we scan for using H2 mode, the rest are done in He mode.  
Lately, our Se CCV starts to drift as the run goes on.  It starts out at 50ppb and then continues to decrease, always down toward 40ppb, never up.  I have tried making the CCV from a fresh stock solution, different cal stds, different internal standard stocks.  Samples are scanned in triplicate and RSDs are always below 3%.   Any help or insight would be appreciated.
Thank you,

  • Seems that there is something going on with the H2 mode. If the RSD% is low for the CCV measurement then the ISIS3 isn't the suspect however the ready time of the complete H2 mode is quite short to detect drift I guess. Is the pressure regulator of the H2 set between the 20-60 kPa and stable?

    Is it possible that the H2 line is purge for a night with 1 ml/ml and then start the batch again. If then the stability is better then it's a problem with the H2 supply or H2 quality change in the gas line.  

    is the H2 the first or second mode. maybe adding some seconds of stabilization time between the different tune modes could help stabilizing. 

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