By: Arafath and Nick

What is it Chromatography is a technique that is

used to separate the substances present in a mixture.

It is widely used to determine the identity of a substance (Qualitative)

It also is used to determine the amount of the identified/unidentified substance (Quantitative)

How Does It Work? Small amounts of samples (e.g. solids, liquids,

gases) are inject into an injection port which heats up and vaporizes the sample into a gas(Stationary Phase).

This gas then gets constantly pushed by the Carrier Gas(Mobile Phase) through a column made of glass or stainless steel tubing which is normally 20m in length and coiled into a round shape.

This gas(Stationary Phase) while getting pushed is constantly adsorbing and desorbing.

How Does It Work? After it has passed through the Column a

detector detects how long it took for a particular gas to reach the detector. Different gases will take different times by adsorbing and desorbing. This is how they separate into there different gases. The time taken for a gas to reach the detector is called the Retention Time which is used for Qualitative Analysis. The output of the this technique is a chromatogram.

The Two Phases

Mobile Phase – The mobile phase is the inert carrier gas this gas will move over the stationary Phase

Stationary Phase –Is the liquid or solid in the column, The sample adsorbs and desorbs to this giving us the retention time.

What is it used for Chromatography is mainly used to

identify: Drugs present in the human body Sugars in fruit juice Hydrocarbons in oil Pollutant gases in exhaust fumes Pesticides in water and soil Calculating accurate concentration of a

particular unknown substance.

How to read the chromatogram There are 2 thing we can read of a

chromatogram: Retention Time: The time take for each

peak(i.e. time taken for a substance to pass thorough the chromatograph)

Peak Area: Represents the amount of sample which is present (i.e. Concentration). This can be calculated by using a simple ruler(Calculate the area of a triangle) or these days computers automatically calculate them when the date is given from the detector.

A Typical Chromatogram

Components of GC Carrier Gas – Is a non reactive gas (inert)

usually Helium (Mobile Phase) Injection Port – Where the sample is

injected into (if the sample is a liquid it must be vaporised)

Collum – A coil of wire where a liquid is held on the walls of it (stationary Phase)

Oven – Heats the column Detector – Where the retention time is

detected and a chromatogram is produced

What is the retention time When a mixture passes through the

column depending on its viscosity it will adsorb (stick) to the liquid on the walls of the column and then desorbs (un-stick), the substances that stick the least will come out first so will have a lower retention time (usually less soluble elements) and the substance that sticks the most has a higher retention time (more soluble)

In summary the retention time is how long a substance sticks to the walls then desorbs

Have a Look- Chromatography

Advantages of GC High Resolution- Many compounds can be

resolved easily. High Speed- Analysis is done in a matter of

minutes High Sensitivity- Can detect even the

smallest of samples. High Accuracy- Accurate results Easy, Well Known- It is an easy technique

compared to others used and one of the most well known techniques

Limitations of GC Samples must always be VOLATILE(i.e.

when the sample is heated the carrier gas should be able to easily carry it.

“Dirty” Samples require some clean up.(eg. Urine)

Must use another instrument (e.g. Mass Spectrometer) for confirmation of identity.

Some training and experience is needed.

Samples of GC

Gasses, Liquids or Solids Molecular weight 2 to ~800. Organic or Inorganic (usually Organic

e.g. Hydrocarbons). Sample must be Volatile