**Gas Chromatography**

Here is where your sample for analysis is injected into the GC.

It's not advisable to use large compounds in GC as it can lead to blockages forming in the column

Different types of carrier gas can be used but Neon is a good choice as it is unreactive, helium or nitrogen are also used.

The carrier gas is the

mobile phase

in GC.

In the oven is a column, which contains an extremely thin layer of either a liquid, polymer or inert solid. This is the

stationary phase!

As samples are carried through the column by the carrier gas they interact with the sides of the column. The more they interact the longer it takes for them to get through the column (elute).

The oven is there to keep samples gaseous. It's temperature can be altered in order to ensure the sample always remains this way

Housed inside the oven is the column

The detector is there to recognise when samples elute. This information is then plotted onto a graph

**How can Gas Chromatography be used in Quantitative Analysis**

**Area Percent Method**

This technique is the simplest but also the most prone to errors. It assumes that the detector responds equally to all samples however this is not strictly true.

To calculate area percent firstly take the area of the peak and then divide it by the sum of all the areas of analytes in the chromatogram

For cholesterol

peak area

total peak area

____________

320

850

____

=

=

0.38

therefore 38% of the sample is cholesterol

**Single Point External Standard**

To do this you have to use the following two equations

response

factor

=

peak area

__________

sample amount

amount of

analyte

=

peak area

________

response factor

Firstly analyse a sample containing a known amount of analyte or analytes and record the peak area. \use this to calculate the response factor

Inject a sample with the unknown analyte concentration and record the peak area. Use the equation to calculate the amount of analyte

An injection containing benzene at a concentration of 1,000 micrograms/ml is made and results in a peak area of 50,000. Calculate the response factor for benzene.

An injection of the sample with the unknown concentration of benzene has a peak area of 37,000. Calculate the amount of benzene present

**Multiple Point External Standard**

For this method we assume that analyse reponse is linear over a range of concentrations

Best used when concentration range is large or single method is not linear

Place in the GC solutions of known concentration and plot peak areas on a graph.

Then run your unknown concentration and where it breaks the line of best fit will tell you the solution concentration

**Single Point Internal Standard**

An internal standard can be used which has a predictable retention time and area, this way if abnormalitites occur they can be detected

The SPIS method requires at least two analyses. The first of which contains a known amount of internal standard and a sample your analysing. You would then calculate the response factor using the following the equation

Internal Response

Factor

=

area of internal standard x amount of compound of interest

_________________________________________

amount of internal standard x area of compound of interest

Then add a known amount of the internal standard tot he sample containing analytes of unknown concentrations. Then calculate the amount of unknown analyte using the following equation

amount of compound

of interest

=

amount of internal standard x area of compound of interest x Internal Response Factor

__________________________________________________________________

area of internal standard