I don't know about ideas, but I have two questions:
1. It's so basic I hesitate to ask, but so important I must. Do you
perform the Hg profile each day before analyzing samples? If you don't,
a small shift will eat your lunch.
2. Are you also doing IEC's? Zn (in particular 213) has some
coincidental emission spectra that cannot be corrected by MSF alone (Fe
an Ni I believe). There are several other IEC's that still must be done
in addition to the MSF, but substantially fewer than the traditional
There are a few other possibilities, but these would seem most obvious
and a good place to start.