Comparison of the performance of butanol and pentanol as modifiers in the micellar chromatographic determination of some phenethylamines

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A procedure was developed for the determination of several phenethylamines (amphetamine, arterenol, ephedrine, phenylephrine, phenylpropanolamine, mephentermine, methoxyphenamine, pseudoephedrine and tyramine), using micellar mobile phases of sodium dodecyl sulfate (SDS), a C 18 column and UV detection. The drugs were eluted at short retention times with conventional acetonitrile–water or methanol–water mobile phases. In contrast, in the micellar system, they were strongly retained due to association with the surfactant adsorbed on the stationary phase, and needed the addition of butanol or pentanol to be eluted from the column. These modifiers allowed a simple way of controlling the retention. The chromatographic efficiencies obtained with the hybrid mobile phases of SDS–butanol and SDS–pentanol were also very high, mostly in the N=3000–7000 range, significantly greater than those achieved with a conventional acetonitrile–methanol–water mobile phase. Butanol and pentanol yielded similar selectivities, but the latter modifier permitted significantly shorter retention times than butanol, and was preferred to expedite the analysis of the pharmaceuticals. Most binary combinations of the nine phenethylamines can be resolved with these mobile phases. A mobile phase of 0.15 M SDS–5% pentanol was used to assay five of the phenethylamines (amphetamine, ephedrine, phenylephrine, phenylpropanolamine and pseudoephedrine) in 22 pharmaceutical preparations, which contained diverse accompanying compounds. The results agreed with the declared compositions and with those obtained with a mobile phase of methanol–acetonitrile–0.05 M phosphate buffer (pH 3) 10:5:85, with no interferences and relative errors usually below 2%. However, with the aqueous–organic mobile phase, the retention time for phenylephrine was too low and could not be usually evaluated.