It seems my compound of interest tightly bound to the Alumina column. Most of my impurities in the fraction had been eluted out but still no elution of my compound of interest even after 30% methanol in chloroform. Did anyone have an experience with Alumina column?
Hi.
If u use 30% methanol and most of impurities is remove from column, u must make optimalization on TLC Alumina N and Silica 60 (compare).
1. Prepare 3 bikers with 10%, 30% and 50% of methanol in chloroform for AluminaN and for Silica60.
2. After chromatography check Rf for all of them. The best if analyte will be have Rf=0,25 - 0.35 and will be separated from others compounds in minimum 0.1 - 0.15 Rf (trough spots)
3. Know u can try build column.
1g of silica or alumina needs aprox. 2 ml of solvents, If u use 50 grams, adsorbent needs aprox. 100 ml of solvent for self calibration. This mean volume of column (CV) is 100 ml. If your analyte have Rf=0.3 from TLC , this mean:
CV/Rf = Ret. volume will be: 100 ml / 0.3 = 333 ml. U need for 333 ml of mobile phase for remove your analyte from column.
Is general way to optimalize open column separation. I use this for separation many componds and its work. Even in flash chromatography (Buchi).
Looks like your compound is a salt. If it is, then it is well known that the anion is lost to some unknown anions like Cl- (evidence from crystallography). Thus it gets stuck to the column as it is a whole new compound and has different property. So the TLC before the column and after will look different. The TLC will not show you the sticking property as the time spend on TLC is negligible compared to the column chromatography.
I have run with 50:50 (MeOH/CH2Cl2 v/v) in a similar situation. I had to put back the desired anion back again to get my desired product. In my case I used NaPF6 or NH4PF6 to turn it into a PF6 salt........may this help
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