Head-column field-amplified sample stacking in binary-system capillary electrophoresis: The need for the water plug

Summary

Strategies to improve the sensitivity of drug monitoring in microliter amounts of biological fluids by capillary electrokinetic methods are currently being explored in our laboratory. Head-column field-amplified sample stacking is the most effective method of sensitivity enhancement—this approach is robust and highly reproducible when applied correctly. For analysis of opioids as standards or in plasma extracts by binary capillary electrophoresis with ethylene glycol, the data presented in this work unambiguously demonstrate that the water plug initially inserted at the column inlet is essential for establishing a steady current during separation, and thus the highest reproducibility. Internal calibration shows thatRSD imprecision values otherwise up to approximately 30% are reduced to values significantly below 10%. The water plug also results in higher detector responses at elevated solute levels (≥100 ng mL⁻¹) and leads to increased sensitivity when the sample (standards and extracts of body fluids) is dissolved in water. The water plug does not, however, furnish higher sensitivity in the analysis of opioids as standards or in plasma extracts that are prepared in the optimized sample solvent or buffer. The optimum length of the water plug cannot, furthermore, be obtained merely by dipping the capillary inlet into water (insertion of water by capillary action). The water zone obtained in this way must be elongated by approximately 0.6 mm by deliberate hydrodynamic introduction of additional water from a different vial. These head-column field-amplified sample-stacking conditions are shown in operation on two different commercial instruments. As an illustration, data are presented depicting the analysis of dihydrocodeine and nordihydrocodeine in plasma and urine specimens of individuals to which dihydrocodeine was administered.