No abstract

The cationic surfactants didodecyldimethylammonium bromide (DDAB) and dioctadecyldimethyl-ammonium bromide (DODAB) have previously been shown to form semi-permanent coatings that effectively prevent adsorption of cationic proteins in fused silica capillaries with inner diameters of 25-75 µm. This paper investigates the impact that narrower capillary diameters (≤25 µm) have on the stability of surfactant bilayer coatings and the efficiency of separations of model cationic proteins and neurotransmitters. Using a DODAB-coated 5 µm i.d. capillary 210 consecutive protein separations (1050 min) were performed without recoating the capillary between runs. Separation efficiencies of 1,400,000-2,000,000 plates per m (340,000-430,000 plates) were obtained. Migration time reproducibilites of 6.8% RSD were observed for 300 injections performed over a 30 day period without any regeneration of the coating. Neurotransmitters were separated with efficiencies ranging from 470,000-610,000 plates per m (110,000-140,000 plates) in a 5 µm capillary.

No abstract

The two-tailed cationic surfactant dioctadecyldimethyl ammonium bromide (DODAB) produces semi-permanent coatings that yield strongly reversed electroosmotic flow (EOF), for example -0.31 ± 0.01 cm kV s at pH 3.5. Moreover, these coatings are easy to prepare, regenerable, cost effective, and yield high efficiency (520 000-900 000 plates per m) separations of cationic proteins over many runs under acidic (pH 3.5) conditions. Given the quaternary amine functionality of DODAB, we were surprised to observe that DODAB coatings become unstable at pH > 7. At pH 7.2, the EOF of a DODAB coated capillary drifted from reversed to cathodic over only 5 runs, and protein separations became severely compromised. By pH 12, no EOF reversal was observed. Electrophoretic and mass spectrometric studies demonstrate that the coating decomposition involves a surface conversion of the quaternary amine in DODAB to a variety of products, although the exact mechanism remains elusive. Regardless, the results herein demonstrate that semi-permanent coatings based on cationic two-tailed surfactants such as DODAB are limited to separations using acidic buffers.

No abstract