Pattamaporn Hemwech¹ · Apinya Obma¹ · Sasinun Detsangiamsak¹ · Supa Wirasate² · Pimchai Chaiyen³ · Prapin Wilairat⁴ · Rattikan Chantiwas^1*

¹Department of Chemistry and Center of Excellence for Innovation in Chemistry and Flow Innovation-Research for Science and Technology Laboratories (FIRST Labs), Faculty of Science, Mahidol University, Rama VI Rd., Bangkok 10400, Thailand.

²Department of Chemistry and Center for Surface Science and Engineering, Faculty of Science, Mahidol University, Salaya, Nakhorn Pathom 73170, Thailand.

³Department of Biomolecular Science and Engineering, School of Biomolecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), Wangchan Valley, Rayong 21210, Thailand.

⁴Analytical Sciences and National Doping Test Institute, Mahidol University, Rama VI Rd., Bangkok 10400, Thailand.

*Rattikan Chantiwas, rattikan.cha@mahidol.ac.th; rattikan.cha@mahidol.edu

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 R_s ≥ 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 compara ble 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.

Reference:

P. Hemwech, A. Obma, S. Detsangiamsak, S. Wirasate, P. Chaiyen, P. Wilairat, R. Chantiwas, Capillary electrophoresis-UV analysis using silica-layer coated capillary for separation of seven phenolic acids and caffeine and its application to tea analysis, SN Applied Sciences 3 (2021) 1-14. doi: 10.1007/s42452-021-04849-1