Basic Drugs : Effects of Eluent pH

Under acidic conditions, it is possible to avoid non-specific adsorption of ODS columns, however retention of basic drugs become too short to realize good separation. This problem is caused by the increase of hydrophilicity due to the dissociation of amines when basic drugs having short amines are analyzed and very popularly observed in the analysis using reversed phase columns. Procainamide and procaine are analyzed under pH7 (neutral) and pH3 (acidic). Large peak tailing is observed for ODS-A and ODS-B under pH7 and the peak tailing can be solved by decreasing eluent pH to pH3. However, the separation of two peaks is insufficient for all columns including ODP.
For the analysis of basic drugs having short amines, sharp peaks can be obtained by using an alkaline eluent and it is recommended to use Asahipak ODP column for the purpose.

Sample :
1. Procainamide hydrochloride 0.1 mg/mL
2. Procaine hydrochloride 0.2 mg/mL

Columns      : Shodex Asahipak ODP-50 4D (4.6 mm I.D. x 150 mm),
               ODS-A, ODS-B, ODS-C (4.6 mm I.D. x 150 mm)
Eluent       : 25 mM Phosphate buffer/CH3CN=60/40
Flow rate    : 0.6 mL/min
Detector     : UV (254 nm)
Column temp. : 30 °C

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