You would have made any paper wet with water or liquid anytime. But have you ever noticed closely what happens to the ink after becoming wet?
Most of the time it smudges and the ink blurs, but not all the time the same happens. Sometimes it spreads up in the paper as some colored streaks. That is typically chromatography in process.
By definition, What is a chromatogram?
Chromatography is a way of separating a mixture of chemicals that are in either gas or liquid form. This separation is carried over by letting one substance slowly creep over another substance which would be commonly solid or liquid.
In the paper and ink scenario, we have ink which is a liquid substance creeping over the surface of the paper which is a solid substance.
Two Phases in Chromatography
In the chromatography process, there are typically two phases involved, they are
- Mobile phase
- Stationary phase
You have a mixture of chemicals or elements in one state of matter(liquid or gas), which is slowly moving and spreading over the other state of matter(solid or liquid) which is stable at a position and doesn't have any movement.
The moving substance is termed as mobile phase, and the substance that stays still without movement is termed as Stationary phase.
How does it Work?
The important thing to note is that the Chromatogram is a surface test.
Now, imagine and compare chromatography with a running race involving multiple athletes. At the starting line, we have a substance that is a mixture of chemicals in the form of unidentified liquid or gas as like multiple athletes waiting for the blowing whistle to run.
When the whistle blows and the race starts, athletes with different stamina and strength perform differently. In the same way, different chemicals in the mixture, travel at a different speed over a stationary substance.
Some chemicals travel long and some short distance and smudges away. This is because as the chemicals start to move over the solid substance, some of its molecules will be sucked and merged with the surface of the solid.
It sticks on that surface temporarily until it is being pulled back into the liquid substance that they got separated from. This process involving the exchange of molecules between the liquid and solid surface is a gluing effect and is generally termed as adsorption.
Adsorption is different from absorption since in absorption the molecules of the substance are trapped permanently in the body of the other, whereas in adsorption it is temporary.
Since the mobile phase we have is a mixture of quite a few different liquids, each one undergoes an adsorption process differently and spends more or less time in the solid or liquid phase.
Some of the liquids in the mixture spend much more time on the solid substance than in the liquid and hence move slowly over the solid substance underneath. Whereas some liquid spends less time on the solid substance than in liquid and would be traveling a bit faster.
Imagine, the mixture is of multiple glue-like liquids and some sticks more to solid and some doesn't.
For an effective chromatography process, the liquid mobile phase needs to separate as much as possible while moving over the solid stationary phase. The stationary phase would generally be a highly adsorbent material for this reason.
Different types of chromatography
There are many different types of chromatography. The commonly used ones are:
Paper chromatography
Most of us would have experienced this type during a school experiment. Similar to the one that we discussed at the starting of this article about what chromatogram is.
You put a spot of ink near the edge of a filter paper and hang the paper vertically while dipping the edge that contains the ink into a solvent usually water or alcohol. The dissolved ink which is the mobile phase slowly travels up the paper which is the stationary phase and gets separated into different components.
Column chromatography
In this method, the stationary phase used is a glass jar instead of paper. This vertical glass jar will be filled with highly adsorbent solid such as silica crystals.
The glass jar with the adsorbent is considered the column. The mobile phase now drips into the vertical jar and starts to split into different molecules. These will be removed and analyzed later.
If the column is actually a film of glass, metal, or plastic coated with a thin layer of highly adsorbent substance, then it is called thin-film chromatography.
Gas chromatography
One of the common and widely used types is gas chromatography. In this process, the column chromatography is being utilized but with a modification to the mobile phase.
The mobile phase for this process would be a mixture of gases. This type is largely automated and widely used with help of laboratory equipment called a gas chromatography machine.
