Fellow Chromatographers-
You've been most helpful to me in weeks past. Now a new question. When using aqueous size exclusion chromatography (analyte ~250,000 g/mol) why would one opt to use NaNO3, NaN3, phosphate, or sulfate in the mobile phase? And is there any specific reasons for using a specific analyte?
Many thanks in advance,
-Jeff

The purpose of adding electrolyte in the case of aqueous size exclusion chromatography is to force the analyte into a pseudorandom coil configuration so that the calibration will be valid (assuming that the water-soluble calibration polymer is a well behaved nonionic polymer). This is generally necessary when the analyte polymer has some amount of fixed charge which prevents random coil configuration due to intramolecular electrostatic repulsion. Depending upon the nature and the amount of fixed charge, various amounts of electrolyte should be added in order to suppress this electrostatic repulsion. Too much electrolyte can cause HIC retention on the size exclusion phase. In some cases it is not possible to completely suppress the effect and in these cases it is necessary to calibrate with known molecular weight standards of the analyte itself (assuming the analyte is a well behaved homopolymer or copolymer). The choice of electrolyte depend somewhat upon your analyte polymer. Nitrate will work better in suppressing repulsion for cationic polymers although this electrolyte absorbs in the ultraviolet. Phosphate and sulfate are better at suppressing the effects of anionic charge and both are ultraviolet transparent. Azide is generally added only to prevent bacterial growth in the eluent bottle but it is not terribly stable (degrading, among other things to nitrate) and solutions should be made up frequently when using this additive.

Chris-
Thanks a bunch for the education. I'm fairly new to SEC method development and your help is greatly appreciated.
-Jeff