Pyrolyzed cotton balls for protein removal: Analysis of pharmaceuticals in serum by capillary electrophoresis.

Because of the inherent affinity of proteins for bare, fused silica capillaries, the analysis of protein-containing samples has proven a challenging task for capillary electrophoresis. The adsorption of proteins to the capillary walls effectively changes the zeta potential and thus affects the electro-osmotic flow leading to significant shifts in migration time, peak broadening, and poor reproducibility. While there are several well-known methods to remove proteins from samples prior to the analysis (including precipitation) or to prevent their adsorption to the capillary (semi-permanent coatings), those approaches are often expensive, time consuming, or simply unreliable. Aiming to address these needs, this manuscript reports on the use of pyrolyzed cotton balls, as a simple and widely accessible hydrophobic material to remove proteins from serum samples. The material retains enough flexibility so it can be placed directly into the sample vials and has enough capacity to capture more than 75% of the proteins in the sample (1% dilution of 1 mL of serum). The advantages of the material are demonstrated by performing the analysis of five representative drugs (in serum) by capillary electrophoresis obtaining a change in migration time of only 5 ± 1%, after 10 consecutive runs.