Velocity-difference induced focusing of xanthine and purine metabolites by capillary electrophoresis using a dynamic pH junction

Velocity-difference induced focusing (V-DIF) of analytes by a dynamic pH junction represents a simple yet effective on-line preconcentration method to improve concentration sensitivity in capillary electrophoresis (CE). Differences in buffer type, pH and conductivity between sample and background electrolyte (BGE) segments of the capillary are properties used to optimize purine focusing within a multi-section electrolyte system. This method permits the injection of large volumes of sample (up to 450 nL or about 18% of capillary length), resulting in over a 50-fold improvement in sensitivity with baseline resolution. The limit of detection (S/N=3) for xanthine is determined to less than 4.0×10−8 M under optimum conditions when using UV detection. Analysis of micromolar amounts of xanthine in pooled urine is also demonstrated without sample pretreatment. A dual mechanism involving dynamic pH and isotachophoretic modes is proposed to enhance analyte focusing performance when employing buffer pH junctions based on different types of electrolyte co-ions.