Application of capillary electrophoresis for drug analysis

Capillary zone electrophoresis (CZE) and micellar electrokinetic capillary chromatography (MECC) have application in determining concentration of drugs and metabolites in body fluid. In CZE, nanoliter amount of specimen is applied at one end of a fused silica capillary column (usually 15-100 cm long with an internal diameter between 25 and 75 ^m) filled with buffer. When a high-voltage DC field is applied, charged solutes migrated through the capillary column by the combined action of electrophoresis and electro-osmotic bulk flow and are separated and detected at the other end of the column. Common detection techniques applied are on-column direct and indirect absorbance, direct and indirect fluorescence detection, occasionally radiometry or amperometry, as well as off-column detection, such as mass spectrometry. Only charged particles can be separated by CZE and neutral compounds cannot be analyzed by this method. In MECC, the buffer contains charged micelles (such as dodecyl sulfate), and both charged and uncharged solutes are separated based on differential partitioning between the micelles and the surrounding buffer. The separation is of chromatographic nature and elution order is based on partitioning. For charged solutes, separation is based on both chromatography and charged effect including electrophoresis. This technique can separate acidic, neutral, and basic drugs in human serum. Several drug levels determined by this technique correlated well with values obtained using immunoassay and HPLC (68).

Teshima et al. used CZE for simultaneous determination of sulfamethoxazole and trimethoprim in human plasma. The authors used ethyl acetate to extract these drugs from human plasma and used UV detection at 220 nm. The analysis was performed at 20 kV and 25°C using 15 mM phosphate buffer (pH 6.2) (69). Another report described CZE determination of methotrexate, leucovorin, and folic acid in human urine using a capillary column (60 cm x 75 ^m) and 15 mM phosphate buffer solution. The analysis was carried it using 25 kV at 20°C (70). A method of coupling CZE with electrospray ionization mass spectrometry has also been reported for TDM of lamotrigine in human plasma. Tyramine was used as the internal standard. The linearity of the assay was from 0.1 to 5.0 ^g/mL of serum lamotrigine concentration, and the limit of detection of was 0.05 ^g/mL. The run time was less than 6min (71). A solid phase extraction followed by CZE analysis of tobramycin in serum has also been reported (72). Determination of several analgesic and anti-inflammatory drugs (acetaminophen, ibuprofen, indomethacin, and salicylic acid) using CZE and micellar electrokinetic chromatography has also been reported (73). Huang et al. described a micellar electrokinetic chromatography with direct UV detection for determination of concentration of cisplatin in human serum. The main hydrolytic metabolite was determined using a CZE method (74).

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