You'll need to narrow down your compounds of interest first. Then select your column. Then your injection, chromatography and detection parameters. The reason you are not seeing clear results is that your search is too broad.

Major manufacturers, like Agilent, thermo fisher and Restek have white papers that study compounds and they usually have clear instructions. That's a good place to start.

Dear Mahsa Lotfi Marchoubeh , thank you for your answer and suggestion. I will look into your suggestions. However, I want to reveal all the chemical components found in the tea infusion of the plant, not a specific ingredient.

Thank you so much dear Prof. Dr. Noel W Davies ,

Your answer is really very descriptive and helpful for me. Our institution currently only has GC-MS and HPLC, I can use GC-MS from those available to look at the full contents of the tea infusions. So I have to look for GC-MS methods to reveal the water-soluble chemical content of the teas. Before I answer you, I found a few articles about it. I think these articles provide nice methods for analyzing water-soluble and non-volatile components of plants by GC-MS. I leave the article links here as I think it may be of interest to you and other researchers.

Links: https://www.scielo.br/j/rbfar/a/PXkSNDr6xPkJskH3zMdfbLh/?format=pdf&lang=en

https://recipp.ipp.pt/bitstream/10400.22/15358/1/ARTGRAQ_DelerueMatos_2018_18.pdf

https://www.mdpi.com/1420-3049/20/2/3431

Noel W Davies

You are right professor, what I am trying to do is very complex, time-consuming and tiring. I don't know what else to do as we don't have LC-MS. With HPLC, components of teas such as catechins are already analyzed, so unlike other studies, I want to look at all the phytochemicals dissolved in the tea infusion. Because I want to move my work to a different point on the compounds dissolved in tea infusion, so I will not limit my work to only analyzing chemical content.