We are working with a mixture that contains benzoic acid, butyl paraben and amines. The pre-mixed MP consists of Phoshate buffer pH 2.1, MeOH, ACN and THF. It is my understanding that as you increase the chain length of ion-pairing reagent, the benzoic acid and butyl paraben should remain unaffected. However, we see a slight shift (less retention) for benzoic acid and a more pronounced shift with butyl paraben (~2 min) when going from C5 - C9 sulfonates. We are using a C8, 3um, 75 x 4.6mm column. Any insight on this would be appreciated

That's a common question in the method development courses that we teach. What you're seeing is quite typical. As you increase the chain length (or concentration) of the ion-pair reagent, the effect is to stick more of the IP reagent on the surface of your packing. The results are:
- bases are much more strongly retained (I'm assuming an anionic IP reagent like a sulfonate here) because they interact with the charged groups now on the stationary phase.
- neutrals (including benzoic acid at pH 2.1) are slightly less retained because part of the stationary phase to which they would normally stick is now covered by IP reagent and hence, unaccessible.

If you have access to a copy of the 2nd edition of "Practical HPLC Method Development" by Snyder, Kirkland, and Glajch [Wiley, 1997], take a look at page 323 and you'll see what I mean.

-- Tom Jupille / LC Resources Inc.

The slightly decrease of the retention time of your uncharged compounds indicates a slight lessening of the interaction with the stationary phase which became more polar by adsorption of IP.