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Pharmacokinetic Investigations with Direct Injection of Plasma Samples: Possible Savings Using Capillary Electrophoresis (CE)

Capillary electrophoresis (CE) is often regarded as a separation technique of choice because of its high selectivity and its cost advantages compared to LC. RSD% of 0.5% have become standard for quality control assays. Using CE, sample pretreatment can often be significantly reduced, leading to notable savings of labor and reagent costs. Moreover, errors from sample pretreatment steps are avoided. A number of pharmaceuticals (e.g. acetaminophen, salicylic acid, sulfamethoxazole, theophylline, tolbutamide, and trimethoprim) have been determined in human plasma on underivatized fused silica capillaries by MEKC without sample pretreatment, the total analysis time being only 10 min. An sodium dodecyl sulfate‐containing borate buffer (60 mM with 200 mM SDS) at pH 10 has been used. Between runs, proteins adsorbed to the capillary wall are removed by a rinsing regimen consisting of SDS buffer and either acetonitrile (e.g. 50% v/v) or isopropanol (e.g. 10% v/v). Other rinsing approaches are discussed (salts, enzyme containing solutions, organic solvents, sodium hydroxide, hydrofluoric acid). The separation system is tested in a concentra‐tion range between 10 ng/mL and 100 μg/mL, the detection limit being about 5 ng/mL. The sensitivity has been substantially improved compared to preceding work using field‐amplified injection mechanisms and efficient computer algorithms that take advantage of multiwavelength detection. Correlations between the limit of quantitation (LOQ), the limit of detection (LOD) and the signal/noise ratio are discussed. A day‐to‐day precision for relative peak areas of 1 to 2% relsdv (n > 40) has been reached in the upper concentration range. Thus, not only drug monitoring but also pharmacokinetic investigations from blood plasma have become possible without further sample pretreatment.