Chromatography & Rf Values

Noadswood Science, 2012

Chromatography & Rf Values

Wednesday, April 19, 2023

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

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: -

blue purple red blue purple red

solvent

start line in pencil

chromatography paper

completed chromatogramchromatography apparatus at start

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: -

blue purple red blue purple red

solvent

start line in pencil

chromatography paper

completed chromatogramchromatography apparatus at start

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)