[Protein-analysis] ionic strength vs. gel filtration

[atahualpa]

Hi!
I am using Superdex 200 column by Pharmacia. I have run my protein in 150mM  
NaCl on it and it was eluting at a given volume. Than, because of an 
experimental setup I had to switch to 50 mM NaCl and I saw my protein much 
later than previously. I have seen somewhere in this newsgroup that 
histagged proteins may make this kind of tricks in low ionic strength (and 
my protein has a his tag). May it be a case indeed? How does it work?
Thanks on advance.