Transcript Chromatography - Noadswood School

Chromatography & Rf Values
Noadswood Science, 2012
Friday, July 17, 2015
Chromatography & Rf Values

To understand chromatography, how it works, and how it can be used
for analytical scientists
Chromatography

Chromatography is a way to separate dissolved substances which have
different colours, such as ink and plant dyes
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It works because some substances dissolve in the liquid better than
the others

The better a substance dissolves,
the higher up the filter paper it
travels
Chromatography

Chromatography can be used to detect forgeries as most inks are
made of a mixture of dyes (a forged document will probably use
different ink)

Thin-layer chromatography (TLC) is very similar to paper
chromatography, but it uses thin layers of gel on a glass plate (it is
faster than paper and gives a better separation)
Chromatography

Look at the chromatography results: -
chromatography
paper
start line
in pencil
blue
purple
red
blue
purple
red
completed chromatogram
chromatography apparatus at start
solvent



Why is the start line drawn in pencil and why is the ink drawn well above the
solvent level?
In which direction did the molecule move?
Where would the red dot appear on our chromatogram?
Chromatography

Look at the chromatography results: -
chromatography
paper
start line
in pencil
blue
purple
red
blue
purple
red
completed chromatogram
chromatography apparatus at start
solvent

The start line is drawn in pencil so more colour spots are not added and the
ink is drawn well above the solvent level to stop it dissolving in the solvent

The molecules moved upwards
Dr. Green

Dr. Green has been killed. The murderer tried to cover their tracks, by
forging a suicide note. The only evidence we have is the ink that the killer
used to write the note!

You are a forensic scientist - your job is to identify which of the pens, taken
from the four main suspects was used to write this note

Complete the worksheet, and find out who the killer was
Chromatography

Chromatography can be used to separate mixtures of
coloured compounds – mixtures that are suitable for
separation by chromatography include inks, dyes and
colouring agents in food…

Simple chromatography is carried out on paper – a
spot of the mixture is placed near the bottom of a
piece of chromatography paper and the paper is then
placed upright in a suitable solvent

As the solvent soaks up the paper, it carries the mixtures with it (different
components of the mixture will move at different rates, separating the
mixture out)
Rf Values (Higher)

Different chromatograms and the separated components of the mixtures
can be identified by calculating the Rf value
Rf = distance moved by the compound ÷ distance moved by the solvent

The Rf value of a particular
compound is always the same – if
the chromatography has been
carried out in the same way

This allows industry to use
chromatography to identify
compounds in mixtures (precisely)