Preconcentration of methylxanthines on hyper-cross-linked polystyrene followed by their determination by high-performance liquid chromatography

We studied the possibility of hyper-cross-linked polystyrene application to the dynamic sorption preconcentration (solid-phase extraction) of methylxanthines (caffeine, theophylline, theobromine, diprophylline, and pentoxifylline) from aqueous solutions. The conditions of preconcentration were optimized as follows: solution volume of 25 mL (pH ～ 6), solution flow rate 0.7 mL/min, microcolumn size 25 × 2.7 mm, adsorbent weight 0.055 g. The compounds were desorbed into a 0.5-mL portion of methanol and determined in the eluate by reversed-phase HPLC with spectrophotometric detection at 280 nm. Sorption preconcentration provided more than 30-fold reduction of methylxanthine detection limits. The detection limits for methylxanthines are 1 (theophylline, theobromine), 2 (caffeine, diprophylline) and 4 (pentoxifylline) ng/mL. The procedure was applied to the analysis of urine-based model mixtures.