Sorry for asking such a basic question...
When analyzing flavonoids or cannabinoids, i saw that the use of ACN and MeOH changed the elution order of the components. In RP-condition, isn't it that the highly polar component elutes first? Why is this happening?
Hello.
Rather than typing a very long answer, I am attaching a link below that will give you a clear picture regarding your question. Points 4 and 5 from the attached article will likely serve you best.
https://www.shimadzu.com/an/service-support/technical-support/lib/methanol-acetonitrile.html
Edit: I forgot to mention that you are correct about the elution. Analytes with higher polarity elute before analytes with low polarity in reversed phase HPLC. ACN is usually the stronger solvent compared to MeOH when it comes to eluting out polar analytes (it goes like Water -> MeOH -> ACN), but the elution order change that you saw is not unusual and is described in the article above.
Hi!
You are correct that for reverse phase, less polar solvents are stronger. As an extreme example, one can do non-aqueous reverse phase to elute very non-polar compounds (1).
The simple answer to why to why the selectivity sometimes changes when the solvent changes is because some compounds will interact with the solvent differently, changing the partition constant between the compound and the solvent. Acetonitrile has the triple bond and no hydroxyl group, and so it may interact more with double bonds via pi-pi interaction.
You may wish to familiarize yourself with Snyder's selectivity triangle (2, Figure 4). Various solvents are grouped by selectivity. You'll note the alcohols are all in Group 2. This means changing ethanol for methanol will do very little to change the selectivity, although it is a stronger (less polar) solvent.
(1) http://www.teledyneisco.com/en-us/liquidChromatography/Chromatography%20Documents/Posters/Facile%20Isolation%20of%20Carotenoid%20Antioxidants%20from%20Solanum%20Iycopersicum%20Poster.pdf
(2) https://www.drake.edu/media/departmentsoffices/dusci/pdfdocuments/Chromatography-selectivity-triangles.pdf
Kindly visit..
https://chem.libretexts.org/Courses/BethuneCookman_University/B-CU%3A_CH-345_Quantitative_Analysis/Book%3A_Analytical_Chemistry_2.1_(Harvey)/12%3A_Chromatographic_and_Electrophoretic_Methods/12.04%3A_Gas_Chromatography
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