Are two liquid chromatography columns in tandem better than one?: Answers from the hydrophobic subtraction model

An approach is described for determining if there is an intrinsic advantage, from a selectivity and resolution perspective, of using two different UHPLC/HPLC reversed-phase columns in tandem for a separation of a given sample compared to a single U/HPLC reversed-phase column that provides the same plate number. Retention data for 16 compounds extracted directly from the hydrophobic subtraction model (HSM) database at HPLCColumns.org are used to simulate and then compare the critical resolution of those compounds obtained using HSM conditions (isocratic elution at 35°C using 50% acetonitrile, 50% aqueous phosphate buffer at pH 2.8 or 7) for each of 662 U/HPLC single columns or 218,791 combinations of tandem columns and assuming a modest plate number of 8000. The critical resolution obtained for 16 additional "n-1" samples created by the systematic removal of one of the original 16 compounds was also compared using single- and tandem-column LC, as was the critical resolution obtained for thousands of synthetic samples generated by randomly varying HSM solute descriptors for each synthetic compound. When all possible single-column or tandem-column results were compared, a significant advantage was observed with tandem-column liquid chromatography (TC-LC), with an average increase in critical resolution of 0.63 (pH 2.8) or 0.75 (pH 7) units observed for the synthetic samples with the smallest number of components (m = 5). As the number of components in a sample increased, the average improvement in critical resolution (∆R_s,crit) using TC-LC gradually decreased from about 0.70 for m = 5 to 0.18 for m = 32 components. The average improvement in critical resolution achieved by switching from SC-LC to TC-LC was also lower when a smaller number of columns and column combinations were available to explore, as would be the case for a finite column inventory in a real laboratory. Nevertheless, on average there does appear to be an intrinsic advantage of tandem-column liquid chromatography, however small, which can be amplified by using high efficiency columns.