Question

Briefly explain why is a solid liquid extraction used prior to running a TLC analysis on an unknown compound and why ethanol is a chosen solvent. Also explain what the mobile phase for TLC is and why it is chosen. If something did not go as planned during the experiment, explain what can go wrong.

Answer 1

Solid-liquid extraction is used prior to TLC examination, here one think is that our solid sample may contains various or lot of compounds therefore we can’t analyse or check how many number of components present in given sample.

But what types of extraction we want, it depends which type of solvent we have to be chosen.

Just if we want fractions of non-polar and polar separately, than directly alcohol is not chosen, because in alcohol all polar as wellas semi-polar compounds will be extracted.

If we run firstly hexane/dichloromethane (either separately or may be in proportions), all non-polar fatty acids, alkanes and steroids will be present in our extractant then decant supernatant liquid and distilled under vaccum we will get some waxy like compounds and we will check by TLC and mobile phase will be taken like hexane/ dichloromethane and 1-2 drops acetic acid.

Here aim is extraction based on increasing order of polarity and then use alcohol after that polar compounds glycosides of steroids, terpenides and sugars, amino acids may be extracted.

But in this process, our polar components may be lost their nature, activity and loss of weight.

Therefore if we choose alcohol is primarily (main) solvent, then all polar and some semi polar components extracted firstly, the supernatant liquid will be dist. Under vaccum and then check with TLC.

And in this concentrated sample we can use docholoromethane as a solvent for extract semi-polar from polar in this mixture.

Since some polar compounds are present in extract, therefore choose the mobile phase like: dichloromethane(9ml): ethanol(1ml):acetic acid(0.5 ml).

If more polar than we can choose: Butanol:water: aceticacid(5:4:1).

We will try best separation for TLC, using different proportion of solvent and choose best one.