pH-mediated stacking in capillary electrophoresis for analysis of opiates in saliva

Abstract

Both acid-mediated and base-mediated stacking in capillary electrophoresis were developed and validated in the present paper for opiates analysis in saliva. Optimum conditions for the determination of morphine, codeine and 6-monoacetylmorphine in both acid-mediated and base-mediated stacking CE were investigated. 100 mmol L⁻¹ phosphate buffer (pH 3) containing 20% methanol and 5% isopropanol (v/v) was used as the background electrolyte for CE separation. The samples were introduced electrokinetically for 99 s at 10 kV in both stacking modes. An acid plug (100 mmol L⁻¹ HCl) was introduced for 2 s at 2 kV after sampling in acid-mediated stacking. 12.5% aqueous ammonia and methanol (50 : 50, v/v) was introduced for 3 s under a pressure of 0.2 psi before sampling in base-mediated stacking. The methods showed a significant decrease in LODs (≤7 ng mL⁻¹), while providing good separation efficiency, and reproducibility (RSD ≤ 19%, n = 5). The methods provided 1000-fold sensitivity enhancement compared with the normal hydrodynamic injection method. Beyond this, sample pretreatment was simple, which only involved diluting with methanol. Even in acid back liquid extraction in base-mediated stacking, the procedure for sample preparation was effort- and time-saving. These methods can be used to analyze trace amount of opiates in human saliva.