To Anonymous on Tuesday, April 29, 2003 - 10:59 am: from topic: Effect of sample prep (composition) on retention time in GPC, which asked:
“ ” \ Hi, recently I was analysed metal co-ordination polymer on GPC using 0.025 M NH4PF6 in DMF as eluent on three column bank consists of styragel columns HT 2, HT 4 and HT 6. Calibration curve was generated by using polystyrene standards and molecular weight estimated as ~30000. Then I started study the effect of additive concentration (NH4PF6)on molecular mass of the polymer. I was obtained strange results. With 0.01M NH4PF6 in DMF, gave molecular mass ~ 19000 and with 0.035 M it was ~41000. In all the cases retention times of standards were almost same, but sample retention time was drastically changed. Sample preparation was done in eluent with low concentration.
Can anybody help me in conclusion about molecular weight of my polymer? “ “

I did copy You question and opened the new topic, as problem I see as more detailed and more general. I don’t know how works NH4PF6 in you case, but DMF is an eluent, where we can suspect some troubles in GPC. General idea of GPC is that there no adsorption effects of sample to stationary phase (in opposition to HPLC). Styragel columns are polystyrene-divinylbenzene (PSV), hydrophobic columns. This type of column works very well with tetrahydrofurane eluent. THF is well wetting polystyrene and is enough strong eluent to avoid adsorbing most of substances.
DMF is more polar and in some cases is not proper to perform analyse without additional adsorption. For example: if you add some small amounts of water to DMF with PSV column, esters will be eluted in time longer than total permeation time. This retention time depends to concentration of water and to kind of ester (dimethylphtalate, dibuthyl-, dioktyl- etc). After finding proper parameters, someone can analyse in one run: molecular weight distribution of polyvinylchloride, identification of kind of plasticizer (according to retention time) and concentration of plasticizer. (pay attention to proper molecular weight standards!). This is only simple example for existing of adsorption in GPC and to connection both mechanism. You can check or is it possible, that your problem is in additional adsorption.
As you use DMF, I understand you can not use THF– this one should be bettor for styragel type column. I work a lot with DMF and I prefer polyhydroksymetacrylate or polyvinylalcohol columns.
Jan

Jan, Thank you for your response. My sample having better solubility in DMF and ACN only. In fact I tried with DMF alone but sample peak shown big shoulder at higher molecular weight side (eluted at 17 min). Stnadards eluted between 22-31 min (MW range 11300-565000).And then added 0.05% LiBr to supress these solvent interations with analyte. This time no sample peak was eluted. I thougt it might have eluted with column elution volume. Do have any reason to explain this?
One literature method suggested to use 0.01M NH4PF6 as additive in DMF for the similar type of metal coordination polymers as my polymer is also PF6 salt.