Principles and Practice of Chromatography - Chromatography Applications > Liquid Chromatography Applications > Analysis of Î˛-blockers > Page 88
As the acetonitrile concentration increased, however, the concentration of adsorbed tetrabutyl ammonium salt would also be reduced and it would be eluted from the reverse phase with a resulting reduction in the ionic interactions of the solutes with the stationary phase. At higher concentrations of acetonitrile, the tetrabutyl ammonium salt would be completely removed and the interactions of the solutes with the stationary phase would become almost exclusively dispersive. This is an example where the phase system is complex and is required to be so, because limited column efficiency demanded clever phase selection to obtain the necessary differential retention.
Analysis of b -blockers
Another liquid chromatography application that involves some novel derivatization is the separation of some 'b blockers', the enantiomers of propranolol, metroprolol and atenalol as fluorescent enhanced derivatives on cellulose tris(3,5–dimethylphenylcarbamate) coated on silica gel. This technique was developed by Yang et al.  developed and involved the synthesis of their fluorescent derivatives, which were formed by reacting them with an electrophilic fluorogenic reagent, 4-(N-chloroformethyl-N- methylamino-7-N- N–dimethylaminosulfonyl-2, 1, 2–benzoxydiazole (DBD- COCl). These derivatives show intense fluorescence at long excitation wavelengths (450Ex and 560Em nm) and thus significantly reduce the level of detection. It was found that Propranolol, Metoprolol and Atenolol, three very commonly used b blockers, reacted directly with reagent forming 1:1 adducts by reaction with the secondary amino group of the b-blockers.
The DBD-COCl reagent reacted readily with the drugs under mild conditions, with no catalyst, and the reaction was relatively fast and was complete in about 5 min. The initial separation was carried out on a TSKgel ODS- 80T column, 15 cm long, 4.6 mm I.D., packed with 5 mm particles. The solvent gradient used to develop the separation commenced with 50% aqueous solution of acetonitrile which was increased to 80% acetonitrile over a period of 40 min. for Propanolol; from 40% to 48% over 15 min. and the increasing to 100% at 30 min. for Metroprolol and from 33% to 42% in 10 min. and then to 100% at 60 min. for Atenolol. The separations obtained are shown in figure 48. Peak 2 represents the DBD derivative of each drug, peak 1 DBD-COOH and peak x was ascribed to the anhydride produced by the reaction of DBD–COCl with DBD–COOH. The drug derivatives were collected and then subjected to separation on a chiral column.