Terminology: By selecting a 7.8 x 300 mm HPLC column, you are running what we refer to as analytical scale chromatography, not "Preparative" scale chromatography. Semi-Preparative scale would utilize HPLC columns which are ~ 10 mm to 24 mm in ID, and Preparative scale includes HPLC columns which are ~ 25 mm ID and larger.

Flow Rate: INITIAL flow rate can be approximated based on column dimensions (est for a 7.8 x 300 mm column would be 1 to 2 ml/min), but in fact, the actual flow rate used will be determined once you actually develop the method of analysis, not before. So no one can tell you what the best flow rate of your own method should be except you.

Wavelength: IF your samples have moderate to strong chromophores (many phenolic compounds are good examples), than use a scanning UV/VIS detector for detection (DAD or PDA should be used for method development). This must be determined experimentally. If you do not know, then set your DAD to scan all wavelengths within an appropriate range while analyzing the mixture and record the spectra for each peak seen. Here are two links to related articles on wavelength/bandwidth selection:

• [ https://hplctips.blogspot.com/2013/08/wavelength-selection-for-method.html ]
• [ https://hplctips.blogspot.com/2011/09/uv-vis-hplc-detector-signal-bandwidth.html ]

Thank you sir for your valuable suggestions.

I'd call that "semi prep" at best, you will only collect milligram quantities each run. I used 214 and 278 nm for flavanoids.

Although this is flash chromatography, this may be helpful: http://www.teledyneisco.com/en-uk/liquidChromatography/Chromatography%20Documents/Posters/Purification%20of%20Phenolic%20Flavonoids%20with%20FC%20Poster.pdf#search=phenolic