[Rosalind Franklin]

Why are we adding Trifluoroacetic acid to our solvent system while doing HPLC? It says it's used as an ion-pairing agent but I did not understand the exact mechanism of action. Also, what kind of compounds are generally purified using HPLC? Someone told me that only charged molecules which cannot be purified using regular column chromatography can be purified using HPLC. Is this true? Also, how are the mobile phase for HPLC decided? Is it always water/acetonitrile?

[marquis]

Ok.  You are talking about one specific kind of chromatography. A different kind of chromatography, often used with polymers, is size exclusion chromatography (SEC).  In that case, the mobile phase is often tetrahydrofuran.   Then there is reverse phase,  normal phase , and ion chromatography.  Then all the different column chemistries added on top of that.  I'm hearing a lot of things that don't make sense. But rather than argue them, the better path would probably be to look them up on line (say with Waters or Agilent web sites), by calling a tech service line, or by reading. Good luck!