GCSE Chemistry

a scientific method of finding what separate substances are in a mixture by making it flow, as a liquid or gas, through a material such as paper which different substances pass through at different speeds

the time taken for a substance to reach the detector at the end of a gas chromatography column 

an analytical technique that involves breaking molecules into chraged fragments and measuring their mass/charge ratio . Also know as mass spectrometry 

analyse coloured substances, such as the coloured pigments in plants and artificial colours used as food additives 

because some of the coloured substances are better at dissolving in the liquid than they are at bonding with the paper, so they travel further up the paper.

• Analyze – examine a mixture, its components,  and their relations to one another
• Identify – determine the identity of a mixture or  components based on known components
• Purify – separate components in order to isolate one of interest for further study
• Quantify – determine the amount of the a mixture  and/or the components present in the sample

• Pharmaceutical Company – determine amount of   each chemical found in new product
• Hospital – detect blood or alcohol levels in a patient’s blood stream
• Law Enforcement – to compare a sample found at  a crime scene to samples from suspects
• Environmental Agency – determine the level of   pollutants in the water supply
• Manufacturing Plant – to purify a chemical   needed to make a product

distinctive attraction to the mobile phase and the stationary phase.

• Compound is placed on stationary phase
• Mobile phase passes through the stationary phase
• Mobile phase solubilizes the components
• Mobile phase carries the individual components a certain distance through the stationary phase, depending on their attraction to both of the phases

solvent (such as water)

paper strip

a sample of the colour from the food is placed on a piece of chromatography paper along with samples of known dyes , then a solvent is added. the solvent soaks up the paper taking the dyes with it . The more soluble the colouring is , the further up the paper it travels . Different colourings move different distances up the paper

the further up the paper it travels

because the water is the solvent and travels up the paper taking the substance with it

so the substances don't dissolve in the water

so the substances dont dissolve in the water

to allow the dyes to seperate completely

because the dyes have different solubilities in the solvent (mobile phase) and they bond/stick to the paper (stationary phase) at varying degrees .

gas chromatography 

seperate out a mixture of compounds and help you identify the substances present 

• Distance moved by spot
• --------------------------------

Distance moved by solvent

• a = cylinder of carrier gas with valve to control the flow 
• b = sample injector
• c = collector in which components are condensed 
• d = detector 
• e = chromoatogram
• f = column in temperature controlled oven 

it contians inert gas that is the mobile phase 

it is the column paccked with solid material , it is the stationary phase 

it is called a chromatogram . The number of peaks tells us the number of compounds present in the mixture . The retention time varies with the different components and is used to identify the substance . The relative are under each peak tells us the relative quantity of each component 

the sample is injected into the machine and is vapourised . The sample passes through a long column packed with a solid that is wound into a coil (stationary phase) . An inert gas (unreactive) (mobile phase) such as nitrogen , is passed through the column to move the sample through . Different substances in the mixture travel through the column at different speeds as they have different affinities for the mobile and stationary phase . This means that they reach the end of the column at different times and so are sepearted . 

quicker the retention time 

slower the retention time 

identify tiny amounts of a substance quickly and accurately it does this by measuring the mass of the particles in the substance 

lose an electron to form a molecular ion . the Mr of the substance = the mass of the molecular ion . often the molecular ion breaks apart and other lighter fragments are detected aswell .

to a gas chromatography machine . after a smaple has been seperated by chromatography , the mass spectrum of each compound can be recorded allowing it to be identified 

the peak furthest to the right is the molecular ion peak and gives the Mr of the substance - this means the Mr of this substance is 204 , this substance is mercury