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