& Techniques

The word chromatography comes from the Greeks and means to write with colours which is literally
translated from its Greek roots chroma and graphein. Chromatography is an analytical
technique used for separating a mixture of chemical substances. Through this we
can see the different components that make up a substance. There are 2 types of
chromatography that we studied which are simple chromatography (paper
chromatography) & Thin Layer Chromatography. Although these types of
chromatography are named differently they do the same thing, which is as stated
before the separate a mixture of chemical substances. This is done by applying
the substance to the chromatography paper/slide and then placing it in some
sort of tube with a solvent in. The solvent will then travel up the
chromatography paper/slide and the different substances will all separate out
and you can work out the rf values through this and discover which each colour
or separation line represents. In chromatography you have 2 different phases,
the mobile and stationary phase, the mobile phase is usually the solvent which
makes the substance travel, and then the stationary phase is the thing that stays
still, usually the chromatography slide or paper.

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Column/Absorbance ChromatographyPartition ChromatographyPaper ChromatographyThin Layer Chromatography (TLC)Gas-Liquid Chromatography (GLC)Gas-Solid ChromatographyIon Exchange Chromatography

Behind Chromatography

There’s one thing that links all chromatography techniques
together is that there’s a stationary phase and a mobile phase.

The stationary phase (solid or liquid supported on a solid)
and the mobile (a liquid or gas) are used to analyse the compounds which make
up a mixture or a substance. The stationary phase is where the compounds within
the mixture stay still and are ‘stationary’. The mobile phase is the stage
where the substance separates into its different parts/compounds.

The process of moving the compounds with the solvent is known
as elution and the solvents used are eluting solvents.


Capillary action is where the molecules of a liquid have to
ability to move themselves along a surface which is solid against the forces of
gravity, this is because the bond between the liquid and the solid surface’s
intermolecular forces are stronger then between each substance. An example of
this is when a plant will move the water molecules through the roots for
respiration because of capillary action.


Polarity affects how attracted molecules are to a different
substance. Molecules have both a positive and negative charged side. The
negative charged side would be attracted to a positive charged side of another
molecule. How polar something is will depend on the charge difference within
the molecules. If there’s an increase of polarity in the solvent during an
experiment the components within the mixture will begin to move quicker. The
way a substance will separate is determined by how polar something is, and the
difference in polarity in the different molecules.




Simple chromatography, otherwise known as paper
chromatography is where you use a piece of chromatography paper, which is the
stationary phase and place a small dot of your substance onto it. Then you have
a beaker, test tube or something which you can place your solvent into to let
the solvent run up the chromatography paper, the solvent in this chromatography
is the mobile phase. This is the simplest of chromatography techniques as it’s
the most basic. There are various pros and cons to this chromatography
technique and one of the pros being that it’s very easy to use and is also very
cheap. A con to this chromatography technique is that it’s not the most
accurate chromatography technique compared to others. It’s used as a
qualitative analytical technique for separating coloured substances such as
plants and seeing the different components which make this useful to identify
organic substances which are unknown. Simple chromatography is used a lot within
the crime world, it’s good for the analysis of blood samples, identifying the
different substances which were responsible for a fire or also testing blood
after death to indicate levels of alcohol, poisonous substances or drugs within
the body.

Thin Layer

Thin layer chromatography (TLC) is a more complicated
chromatography technique to simple chromatography but only being the
chromatography slide is a sort of glass slide instead of a piece of paper which
means you need to be more careful whilst applying the spot of your substance
and whilst handling the slide. The slide will be the stationary phase in this
technique and similar to the paper chromatography the solvent will be the
mobile phase. Now the same with paper chromatography, TLC has some pros and
cons to it. One of the main pro’s is that it’s very easy to use and requires
minimal skill you just need to be careful whilst applying the substance and
whilst handling the slide. And similar to paper chromatography, TLC isn’t the
most accurate compared to others such as gas chromatography. Thin layer
chromatography is good for separating non-volatile mixtures, such as for
forensic testing. TLC is a cheap chromatography method which is used a lot
within the food industry, and has many different applications such as lipid separation
and food dye analysis.


Now with these two types of chromatography they’re suitable
for different things. TLC is normally better used for chromatograms as the
separation between the different elements of a substance is usually more
visible on a TLC plate then a piece of chromatography paper.


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