Instrumental Analysis
What do you guys do all day? What do they do in CSI? What do you guys do all day? Well, you know, a bit of gas chromatography followed by some mass spectrometry.
You’ve all done paper chromatography in prep school Paper chromatography is used to separate dyes.
Why does paper chromatography work? Some dyes dissolve better than others in water. The soluble dyes tend to travel further up the paper. Filter paper is made of cellulose fibres. Some dyes are attracted to the cellulose fibres more than others. The ones that stick to the fibres less tend to travel further. The result is that each different dye travels a different distance up the filter paper.
Retention Factor (Rf) Values The further a dye travels, the higher its retention factor.
Gas Chromatography
In gas chromatography, a carrier gas pushes the sample through a long tube (column) containing silica. Different compounds take different amounts of time to travel all the way along; they have different retention times.
How many substances have been separated here?
Mass spectrometers can measure the mass of a molecule and therefore identify it
The peak furthest to the right hand side is called the molecular ion peak. This tells you the relative molecular mass (Mr) of the molecule.
Shannon Matthews Kidnap 2008 It turned out that she was kidnapped by her own Mum.
The police removed ten 20cm strands of Shannon’s hair on April 30th 2008. A GC-MS machine was used to look for the drug temazepam in tiny 1mm pieces of her hair. This evidence helped convict Shannon’s Mum.
GC-MS is a gas chromatograph connected to a mass spectrometer
Instrumental methods are: Advantages Disadvantages Instrumental methods are: quick (scans might take 20 seconds) Expensive (£30k for a GC-MS machine) sensitive (small quantities can be detected) accurate (you can find out the exact mass of the molecule)
Key words paper chromatography mass spectrometry molecular ion peak retention factor, Rf relative molecular mass, Mr gas chromatography retention time GC-MS column sensitive