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Our lab found a Premium Rf silica from not too popular Sorbent Technologies that is noticeably better than silica from several major suppliers, especially for difficult separations.
This statement is from personal observation and as such is subjective. However, it is worthwhile to try a different silica brand if the separation persistently fails.
When calculating the amount of silica to be used, it is much better to measure silica by weight, not by volume, because the density of silica is variable. So if there is a mixture composed from 2g of A and 3g of B and the desired amount of silica is Possible solution Change elutant - say replace hexane by toluene or dichloromethane.
Try crystallization instead of a column Compound is too acidic Add Acetic Acid to elutant. Consider to run an alumina column instead of silica one Compound is moisture Try dry neutral Alumina instead of silica. Dry silica under vacuum at 60 - 80C overnight. Alumina or Florisyl usually won't work in this case Yet another thing to consider before doing a column is the TLC spot shape.
Round or slightly elongated spot shapes cases 1, 2 and 3 usually mean that the column will be successful.
A long tail like in case 4 is a sure indication of trouble. Note that there is no gain in using more silica than needed. Besides wasting time, expensive silica and organic solvents, you will be faced with yet another problem: The common graphic representation of van Deemter equation is shown.
A minimum on the graph represents the shortest height of a theoretical plate.
This means that a given column reaches the maximum counts of plates which is the best possible separating power.
In practice all that translates into a simple fact that for every chromatography separation there is an optimum flow rate value. The picture of the columns represents the practical outcome when only one compound was loaded onto the column.
The same amount of elutant was pushed through all three columns, but flow rates were different. If flow rate was too slow then the substance band diffuses excessively up and down. For overly fast flow rate, there was not enough time for the compound to equilibrate with the column packing.
The compound was forcefully dragged down the column leaving behind a long tail. There are two more implications that result from the van Deemter equation: It is not the flow rate, but the linear velocity of the elutant that is important.
Hence, small diameter columns must be run much slower that larger diameter ones to achieve the same separation efficiency. Finer sorbent particles allow for a longer actual path for the elutant.Locke and Latham's goal setting theory states that several conditions are particularly important in successful goal achievement.
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