Gas Chromatography (GC)

1. By: Arafath and Nick Gas Chromatography (GC)

2. 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)

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. A Typical Chromatogram

9. 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

10. 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

11. Have a Look- Chromatography

12. 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

13. 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.

14. Samples of GC • Gasses, Liquids or Solids • Molecular weight 2 to ~800. • Organic or Inorganic (usually Organic e.g. Hydrocarbons). • Sample must be Volatile