What determines how far a substance travels in chromatography?

The balance between its solubility in the moving phase and its retention by the stationary phase.

Describe the stationary and moving phases in TLC.

A solid coating on a plate; a solvent that moves up the plate.

Describe column chromatography.

A column packed with a solid, with a solvent that moves down the column.

Describe gas chromatography.

A column packed with a solid, or a solid coated with a liquid, with a gas passed through under pressure at high temperature.

Why is the baseline in TLC drawn in pencil?

Pencil does not dissolve in the solvent, so it does not run up the plate.

Why must the solvent level be below the baseline?

Otherwise the spots would dissolve into the solvent in the beaker.

How are colourless spots seen on a TLC plate?

With ultraviolet light or a developing (locating) agent.

State the equation for Rf.

Rf = distance moved by the spot ÷ distance moved by the solvent front.

How are substances identified using Rf values or retention times?

By comparison with standards run under the same conditions.

What is retention time?

The time a component takes to pass through the column to the detector in GC.

Why does a more strongly retained component have a longer retention time?

It spends more time held by the stationary phase, so it passes through more slowly.

What is the use of mass spectrometry after GC?

It analyses each separated component, giving its M(r) and fragmentation pattern.

Chromatography techniquesAQA A-Level Chemistry: Flashcards

Card 1 of 130 of 13 known

Question

What does chromatography do?

Tap or press Space to reveal

Tap card or press Space to flip

See all 13 cards

What does chromatography do?: It separates the components of a mixture and helps identify them.
What determines how far a substance travels in chromatography?: The balance between its solubility in the moving phase and its retention by the stationary phase.
Describe the stationary and moving phases in TLC.: A solid coating on a plate; a solvent that moves up the plate.
Describe column chromatography.: A column packed with a solid, with a solvent that moves down the column.
Describe gas chromatography.: A column packed with a solid, or a solid coated with a liquid, with a gas passed through under pressure at high temperature.
Why is the baseline in TLC drawn in pencil?: Pencil does not dissolve in the solvent, so it does not run up the plate.
Why must the solvent level be below the baseline?: Otherwise the spots would dissolve into the solvent in the beaker.
How are colourless spots seen on a TLC plate?: With ultraviolet light or a developing (locating) agent.
State the equation for Rf.: Rf = distance moved by the spot ÷ distance moved by the solvent front.
How are substances identified using Rf values or retention times?: By comparison with standards run under the same conditions.
What is retention time?: The time a component takes to pass through the column to the detector in GC.
Why does a more strongly retained component have a longer retention time?: It spends more time held by the stationary phase, so it passes through more slowly.
What is the use of mass spectrometry after GC?: It analyses each separated component, giving its M(r) and fragmentation pattern.

Exam questions on Chromatography techniques

A student separates the dyes in a black ink by thin-layer chromatography (TLC). A line is drawn near the bottom of a plate coated with silica, spots of the ink and of three known dyes are placed on the line, and the plate is stood in a shallow depth of solvent in a covered beaker.
Explain why the different dyes in the ink travel different distances up the plate.2 marks
In a TLC experiment the solvent front moved 7.5 cm from the baseline. A pure dye P moved 4.5 cm from the baseline. A sample of an unknown ink, run on the same plate, gave two spots, at 4.5 cm and 1.8 cm from the baseline.
Calculate the Rf value of the second spot in the ink and state, with a reason, which of the two substances in the ink is more strongly retained by the stationary phase.2 marks
A forensic chemist analyses a mixture of volatile organic compounds using gas chromatography linked to mass spectrometry (GC–MS). The column is packed with a solid coated with a liquid, and an inert gas is passed through it under pressure at high temperature. The mixture gives three peaks, with retention times of 2.1, 3.8 and 5.2 minutes. A standard of compound S, run under identical conditions, has a retention time of 3.8 minutes.
Explain how gas chromatography separates the components of the mixture and how mass spectrometry is used after the separation.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).