Starting from our previously developed method for the analysis of the atypical antipsychotic risperidone, similar chomatographic conditions were initially tried for the analysis of arp in human plasma. In particular,a mixture of a 34 mM phosphate buffer,pH 3.0 and acrtonitrile(73/27,v/v) was use as the mobile phase (flow rate 1.0ml min) on a C8 (150mm * 4.6mm i.d.,5 um) reversed-phase column. Under these conditions, the analyte peak showed long retention time (>20min) because of its high affinity for the stationary phase,with concomitant peak broadening. Thus, in order to have a lower retention of the analyte,