Two situations for the separation of postively charged compounds using IP reagents (sulfonates)

(1) 25mM NaH2PO4 pH 2.5, 75mM NaCl
(2) 100mM NaH2PO4 pH 2.5

Assume 25mM phosphate is high enough to maintain the buffer capacity of the MP when organic is added. Would you and should you expect similar results using the two buffers above?

Thanks in advance

Since I have no comparable experimental data to use as reference, let's look at what theory predicts. First order effects predict that in this situation similar separation will result if the concentration of the ion-pair reagent stays the same, and if the cation composition of the buffer stays constant (which it does in this example: 100 mM of Na+ in each case). However, there are secondary effects involved which cannot be easily predicted. The high ionic strength of these two buffers combined with the very different nature of the anions (Cl- vs H2PO4-) is very likely to affect activity coefficients of the involved species, which will effectively result in a change in the equilibrium constants involved in determining values of k. I would expect a change in retention (k) but little change in selectivity (alpha).

I agree with the comment above. To a first approximation, the most important factor will be the nature and concentration of the buffer cation. In the case cited above, since the concentration and type of cation are constant, one wouldn't expect a major effect upon substitution of phosphate for chloride as the counter-anion. However, one must also keep in mind the relative solubility of your cationic analytes in chloride and phosphate forms. Since chloride is much less hydrated than phosphate, one would expect the chloride form of most organic cations to be significantly less water-soluble than the phosphate form of the same cations. As a consequence, one might predict somewhat higher K' values when using a chloride containing eluent of the same ion strength. An even greater effect would be expected by substituting 75 mM sodium perchlorate.

Just adding one more aspect of method ruggedness:
In ion pair RPC the retention is depending extremely on the ionic strength of the eluent. At low ionic strength you will find much higher retention than at high ionic strength. The dependence of the k-value on this changes is a robustness parameter for the method and should be measured and indicated in the method development report.

To get a robust method you have to adjust your eluent ionic strength extremely precise.
Therefore I would not expect to have the same chromatograms with the two different buffers mentioned above, as their ionic strength is not identical.
See also in Horvath et al, Anal.Chem.49, 2303 (1977), Fig.11.