When AA is replaced by an IPR in the eluent, the separation selectivity is also clearly affected as is visible in Figs. and. For each additive, the resolution (Rs) between the FLP and the respective impurity was calculated and plotted in Fig. , where a negative or positive Rs value indicates elution before or after the FLP, respectively. The N-1 and N+1 impurities are baseline separated from the FLP using AA (no IPR) in the eluent and the resolution decreases when AA is replaced by IPRs. This effect is similar to the decreased resolution of the poly(dT) ONs, where the separation between ONs comprising different number of nucleotides decreased.