Effect of pH of Eluent on Elution Volume and Plate Number (ODP)

By using an alkaline eluent with a higher pH than the pKa sample value, basic drugs with tertiary amino groups will be retained by the column because the amino groups are not dissociated. As shown here, at pH11, the elution volumes of most drugs show the highest values and the separation of the drugs is the best at pH11. Also theoretical plate numbers of most drug components show the highest values at pH11. For the analysis of basic drugs using Asahipak ODP columns, sharp peaks can be obtained by using an alkaline eluent. In addition, the separation of both drug components and their impurities becomes easier because the components are retained more strongly by using an alkaline eluent.

No.	Sample	pKa
1	Scopolamine	7.6
2	Lidocaine	7.9
3	Tetracaine	8.5
4	Diphenhydramine	9.0
5	Procaine	9.0
6	Procainamide	9.2
7	Atropine	9.9

Column       : Shodex Asahipak ODP-50 4D (4.6mmID*150mm)
Eluent       : 25mM Phosphate buffer/CH3CN=60/40
Flow rate    : 0.6mL/min
Detector     : UV(254nm)
Column temp. : 30deg-C