use of LiBr and DMF in SEC/GPC??

[rick1112]

hi

i recentyl came across SEC/GPC using LiBr and DMF as mobile phase,could some one tell me the function/reason for use of LiBr and DMF in SEC/GPC??

thanks a lot

[Uwe Neue]

In SEC, solvents are chosen to completely dissolve the polymer into a random coil without any interaction with itself or with the packing. Particular choices depend on the investogator. However, DMF with 0.05M LiBr is a reasonable generic choice since it works for many different polymers. Look in my book on "HPLC Columns" on page 148.

[rick1112]

hi uwe...
thanks for your reply..
well i am working for a SEC/GPC method for PEG/PEO...presently i am working on "water" as my mobile phase ....as i was browsing through i stumpled upon a claim where DMF witn LiBr and Methanol with LiBr was used for increasing resolution for PEG/PEO..i could not understand the exact function of this particular choice..especially LiBr...could some one help me out...??
thanks a lot

[dillon0313]

I have also used LiBr and DMF in GPC in order to get better resolution. My understanding is that high molecular weight molecules can complex together due to the large amount of hydrogen bonds that form between them, which can give you a false reading where the results indicate that the molecular weights are larger than they actually are. Adding LiBr helps to dissolve these complexes by taking the place of the hydrogen bonds so that the high MW molecules cannot complex together, thus giving you more accurate results.

[dillon0313]

See [Hann, N. D. (1977). "Effects of lithium bromide on the gel-permeation chromatography of polyester-based polyurethanes in dimethylformamide."] more a more detailed answer.