# Why should I add lithium bromide to my GPC eluent?

I'm running gel permeation chromatography (GPC) of a polar, uncharged polymer (acrylamide-based) in a polar solvent (dimethylformamide). The literature recommends adding a salt to the eluent; for my particular experiment, I'm using 0.05 M LiBr (example). Other salts are reported (Sodium Sulfate, for example) in the literature, both for this eluent/polymer mixture, and for similar polymers and eluents. Why is this done? Most papers simply report the eluent and the additives they used, with no mention of their reasoning. One paper just identifies the eluent, with no mention of any added salts; given the prevalence of the use of salts, I'm not sure if this is an oversight on their part, or if they just didn't use a salt.

If I were working with a polyelectrolyte, I would know that adding a salt would screen the charges along the polymer, reduce the $A_2$ effect, and make my data much easier to interpret. One GPC column manufacturer claims (under "What additives are important...") that dipole-dipole interactions can have a similar effect, but I'm not not sure how much to trust that claim. In addition to confirming or denying that this is the reason, I would also appreciate pointers to any relevant literature.

• Could you provide a link to one of the papers describing the addition of salts to GPC eluents? I have never added salts to my GPC eluents, so I am curious about this technique. Maybe it is specific to polyacrylamides? Nov 27 '12 at 23:03
• @BenNorris, that's really interesting that you have never done this. I added some links to hopefully clarify. Nov 29 '12 at 22:52
• Also, I think I may have found a paper that answers this definitively. When I get the time, I will write that up as an answer (but certainly do appreciate any other answers in the meantime!). Nov 29 '12 at 22:53
• well most of my GPC has been done in THF and chloroform, in which most most salts are insoluble, so my experiences might not be typical. Nov 30 '12 at 0:29
• This is my first experience with GPC, so I wouldn't jump to any conclusions. Your experience may be more typical than mine. Jan 31 '13 at 20:09