Question

A sample containing solutes A and B is injected to a 15cm reverse phase column. A 15% and later a 20% aqueous methanol mobile phase is used to analyze a sample containing components A and B. If the flow rate of the mobile phase is kept the same for both

1. will the selectivity factor increase or decrease or stay the same for the two runs, why?

2. will the number of theoretical plates for peak A increase or decrease or stay the same for the two runs? assume A has a small retention time than B.

Answer 1

A sample of A and B are injected into a reverse phase column with 15% and then 20% methanol as the mobile phase.

1. The selectivity factor would stay the same for the two runs.

changing the polarity of the mobile phase would alter the retention factor for the components to some extent, however, the selectivity that is dependent upon the amount of substance injected would remain unchanged.

2. The number of theoretical plates for peak A would be geater in case of 20% methanol as compared to 15% methanol mobile phase. As the polarity of solvent phase decreases, the retention time increases, which in turn means there is an increase in the number of theoretical plates. The component would spend more time on the column.