Abstract
This work presents an innovative silica-layer coated capillary with comparison study of the silica-layer coated capillary and the fused-silica capillary for the separation of seven phenolic acids viz. p-hydroxyphenylacetic acid (PHPA), p-coumaric acid (PCA), p-hydroxybenzoic acid (PHBA), caffeic acid (CFA), (3,4-dihydroxyphenyl)acetic acid (DHPA), gallic acid (GLA), and 2,3,4-trihydroxybenzoic acid (THBA), together with caffeine (CF), by capillary electro-chromatography (CEC) and micellar electrokinetic chromatography (MEKC), respectively. The running buffer was 25.0 mM borate at pH 9.0, with addition of 50.0 mM sodium dodecyl sulfate for the MEKC mode. The non-coated capillary could not separate all seven phenolic acids by CEC or MEKC. This was achieved using the coated capillary for both CEC and MEKC. The innovative coated capillary with CEC had plate number N ≥ 2.0 × 104 m−1 and resolution Rs ≥ 1.6 for all adjacent pairs of peaks. The capillary was also able to separate GLA and THBA which are structural isomers. Although MEKC mode provided comparable efficiency and selectivity, the reduced EOF of the coated capillary led to longer separation time. The linear calibration range of the seven phenolic acids and caffeine were different but the coefficients of determinations (r2) were all > 0.9965. The precisions of the relative migration times and peak area ratios of analyte to internal standard were 0.1–1.8% and 1.8–6.8%, respectively. There were no statistical differences in the efficiency of separation of the phenolic acids and caffeine for three coated capillaries. It was applied to the analysis of caffeine and phenolic acids in brewed tea using tyramine as the internal standard. The tea samples were diluted prior to analysis by CEC. The separation was less than 15 min. Caffeine, gallic acid and p-coumaric acid were detected and quantified. Caffeine and gallic acid contents were 10.8–15.0 and 2.6–4.8 mg g−1 dry tea leaves, respectively. p-Coumaric acid was detected in only one of the samples with a content of 0.4 mg g−1. Percent recoveries of spiked diluted samples were 90 ± 9 to 106 ± 13%, respectively.Article highlightsSilica-layer coated capillary is first reported for simultaneous separation of seven phenolic acids by non-MEKC analysis.Performance between coated, and non-coated capillaries with analysis by CEC and MEKC were compared.Plate number, resolution, capillary reproducibility, and electroosmotic flow mobility are investigated.Graphical abstract
Highlights
Tea is a health-promoting drink as shown by nutrition research [1]
The compounds were employed in the study of their separation using capillary electro-chromatography (CEC) and micellar electrokinetic chromatography (MEKC) employing conventional fused-silica capillary and the innovative silica-layer coated capillary, respectively
The optimized concentration of 50.0 mM sodium dodecyl sulfate (SDS) was employed for MEKC mode
Summary
Tea is a health-promoting drink as shown by nutrition research [1]. The chemical composition of tea depends on its production process. The main gallic ester component of tea, (−)-epigallocatechin gallate (EGCG) and its dimer, can decompose to gallic acid at high temperature during the manufacturing process and polymerize into pigments [2, 6]. Capillary electrophoresis with UV detection method (CE-UV) for analysis of phenolic acids and polyphenols are given in Table 1 [8, 16,17,18,19,20, 29]. The method, with tyramine as internal standard, is applied to the analysis of caffeine and the seven phenolic acids in brewed tea samples using direct injection into the capillary electrophoresis system. The developed method is validated in terms of linear calibration range, limit of detection, precision and recovery of spiked samples
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