ICP-MS 7900 Question

I use a Agilent 7900 MS.  Recently we have run into some problems with our Fe parameter.  In our drinking water lab, we filter all received samples using a .45 micro filter.  There has been a shortage of our filters that we normally receive from SCP (back ordered) and in the meantime, we have ones from delta.  Lately, our Fe counts using the delta filters are weird.  RSDs can be all over the place for Iron only using these delta filters.  All other parameters (23 others) are all fine, no RSD issues.  My QCs, CRMs, checks are all fine for all other parameters, even Fe QCs are correct and where they should be (our QCs, Crms, are not filtered).  I have gone back and run several days previous to when we started using the delta filters (when we used SCP) and all those samples are fine, no wonky RSDs, etc.  My question is, has anyone else ever run into this problem?  Is there a way i can check to see if there is some type of iron interference occuring?  Any help, insight, or advice would be greatly appreciated.
Thank you,

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