title: Observation and confirmation of oxidation reactions occurring on ultra-high-performance liquid chromatography columns.
abstract: BACKGROUND
  Ultra-high-performance liquid chromatography coupled with electrospray ionization mass spectrometry (UPLC/ESI-MS) has been frequently used for chemical analysis. A redox reaction in the ESI source has been observed during the ionization process. However, it is still unclear whether this redox reaction can take place on UPLC columns.
  
  
  METHODS
  In this study, the oxidation reactions potentially occurring on UPLC columns were investigated using polyphenols including baicalin, baicalein, propyl gallate (PG), quercetin-3-rhamnoside (QR), rutin, naringin and 2,3,5,4'-tetrahydroxystilbene-2-Ο-β-D-glucoside (THS-G) as model compounds. The on-column oxidation reaction was ascertained by post-column infusion of antioxidants such as ammonium sulfide ((NH4)2S). The oxidized products were reduced to their parent forms in the ESI source. This on-column oxidation reaction was further confirmed by means of post-column infusion of baicalin solution.
  
  
  RESULTS
  On-column oxidation reactions were observed and confirmed for baicalin, baicalein, PG, rutin, and QR. The exact reaction site was located at the outlet frits of the UPLC columns. (NH4)2S was proved to be the most suitable reducing agent among the tested antioxidants for eliminating negative effects caused by on-column oxidation reaction. It was subsequently proposed to be an efficient additive to suppress oxidation reactions in the ESI source.
  
  
  CONCLUSIONS
  Oxidation reactions can take place at the outlet frits of UPLC columns. Ascertaining on-column oxidation reactions and consequently eliminating relevant negative effects are of great interest for determination of oxidation-sensitive compounds such as polyphenols.
authors.affiliation: State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China.|University of Chinese Academy of Sciences, Beijing, 100049, China.
authors.forename: Caiming
authors.initials: C
authors.lastname: Tang
authors.affiliation: College of Natural Resources and Environment, South China Agricultural University, Guangzhou, 510642, China.|Guangzhou Quality Supervision and Testing Institute, Guangzhou, 510110, China.
authors.forename: Jianhua
authors.initials: J
authors.lastname: Tan
authors.affiliation: State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China.|University of Chinese Academy of Sciences, Beijing, 100049, China.
authors.forename: Jiabin
authors.initials: J
authors.lastname: Jin
authors.affiliation: Guangzhou Quality Supervision and Testing Institute, Guangzhou, 510110, China.
authors.forename: Shaofeng
authors.initials: S
authors.lastname: Xi
authors.affiliation: Guangzhou Quality Supervision and Testing Institute, Guangzhou, 510110, China.
authors.forename: Huiyong
authors.initials: H
authors.lastname: Li
authors.affiliation: College of Natural Resources and Environment, South China Agricultural University, Guangzhou, 510642, China.
authors.forename: Qilai
authors.initials: Q
authors.lastname: Xie
authors.affiliation: State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China.
authors.forename: Xianzhi
authors.initials: X
authors.lastname: Peng