Efficient separation of organic anions in beverages using aminosilane-functionalized capillary electrophoresis with contactless conductivity detection.

BACKGROUND: Capillary electrophoresis (CE) has the advantage of rapid anion analysis, when employing a reverse electroosmotic flow (EOF). The conventional CE method utilizes dynamic coatings with surfactants like cetyltrimethylammonium bromide (CTAB) in the run buffer to reverse the EOF. However, this method suffers from very slow equilibration leading to drifting effective migration times of the analyte anions, which adversely affects the identification and quantification of peaks. Permanent coating of the capillary surface may obviate this problem but has been relatively little explored. Thus, permanent capillary surface modification by the covalent binding of 3-aminopropyltriethoxysilane (APTES) was studied as an alternative. RESULTS: This study investigates the effect of APTES concentration for surface functionalization on EOF mobility, separation efficiency, and reproducibility of anion separation. The performance data was complemented by X-ray photoelectron spectroscopy (XPS) and contact angle (CA) measurements. The XPS measurements showed that the coverage with APTES was dependent on its concentration in the coating solution. The XPS measurements correlated well with the EOF values determined for the capillaries tested. A standard mixture of 21 anions could be baseline separated within 10 min in the capillaries with lower EOF, but not in the capillary with the highest EOF as the residence time of the analytes was too short in this case. Compared to conventional dynamic coating with CTAB, APTES-functionalized capillaries provide faster equilibration and long-term EOF stability. The application of APTES-functionalized capillaries in analyzing different beverages demonstrates the precision, reliability, and specificity in determining organic anions, providing valuable insights of their compositions. SIGNIFICANCE: APTES coating on capillaries provides a facile approach to achieve a permanent reversal of the stable EOF to determine anions. The control of the coverage via the concentration of the reagent solution allows the tailoring of the EOF to different needs, a faster EOF for less complex samples where resolution is not challenging, while a lower EOF for higher complex samples where the focus is on separation efficiency. This enhancement in efficiency and sensitivity has been applied to analyzing organic acids in several beverages.

Silica nanolayer coated capillary by hydrothermal sol-gel process for amines separation and detection of tyramine in food products.

A hydrothermal sol-gel method for reproducible formation of silica nanolayer on the wall of silica capillaries was developed for electrochromatography. The formulation was optimized by observation of uniform gel formation on an optical microscope. The variables of the formulation include types of solvent, water-TEOS ratio, CTAB and urea contents, and mixing method. The procedure produced a coating of silica ca. 100 nm thick layer on the wall of the capillary. Surface morphology of the coating was characterized by SEM, contact angle and chemical composition by FT-IR spectroscopy and X-ray powder diffraction. The coating reduced the electroosmotic mobility producing enhanced separation performance. Eight standard amines (including tyramine and benzhydrylamine, as an internal standard) were separated with peak resolution Rs ≥ 2 for all adjacent peaks and plate number N ≥ 3.0 × 104 m-1. Calibration was linear from 5 to 200 µg L-1, with r2 > 0.9985 and instrumental LOD of 4.9 µg L-1. Five samples of food products were diluted and analyzed for the amines using the coated capillary and only tyramine was detected. Intra-day and inter-day precisions were less than 1.2%RSD. Percent recoveries of spiked tyramine in samples were 95 ± 3 to 106 ± 7% (n = 3).