Chromatography (AQA A Level Chemistry): Flashcards
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What are the mobile phase and stationary phase in chromatography?
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What are the mobile phase and stationary phase in chromatography?
A saturated compound is an organic molecule that contains only single carbon–carbon bonds and no double or triple bonds between carbon atoms.
In TLC the mobile phase is a .......... and the stationary phase is ........... In gas chromatography (GC) the mobile phase is an .......... and the stationary phase is a microscopic liquid film on a solid support.
In TLC the mobile phase is a liquid solvent and the stationary phase is solid silica or alumina on a plate. In gas chromatography (GC) the mobile phase is an inert carrier gas and the stationary phase is a microscopic liquid film on a solid support.
True or False?
In chromatography, a component with a high affinity for the stationary phase travels further and faster than one with a low affinity.
False.
A component with a high affinity for the stationary phase is retained longer and travels slower and less far. A component more soluble in the mobile phase travels further.
Define the Rf value and state the formula used to calculate it.
The Rf value (retention factor) is a dimensionless value that identifies a component in TLC.
Rf = distance travelled by component / distance travelled by solvent
It always lies between 0 and 1.
In gas-liquid chromatography the .......... of a component is the time elapsed from injection to when it exits the column. Components are identified by comparing this value with ...........
In gas-liquid chromatography the retention time of a component is the time elapsed from injection to when it exits the column. Components are identified by comparing this value with data book values.
True or False?
Column chromatography, TLC and gas chromatography all use the same stationary phase.
False.
Each technique uses a different stationary phase: TLC and column chromatography use solid silica or alumina, while GC uses a microscopic liquid film coated on a solid support inside a column.
A spot in TLC moves 3.6 cm and the solvent front moves 6.0 cm. Calculate the Rf value.
Rf = 3.6 / 6.0 = 0.60
The Rf value has no units because it is a ratio of two distances in the same units.
What is thin layer chromatography (TLC) used for?
Thin layer chromatography (TLC) is a technique used to separate and analyse the components of a mixture by their differing affinities for a solid stationary phase and a liquid mobile phase.
In TLC the stationary phase is usually .......... or .......... coated on a plate. The mobile phase is a .......... that travels up the plate by capillary action, carrying the sample components with it.
In TLC the stationary phase is usually silica (SiO2) or alumina (Al2O3) coated on a plate. The mobile phase is a liquid solvent that travels up the plate by capillary action, carrying the sample components with it.
True or False?
The baseline on a TLC plate must be drawn in pencil because pen ink would react with the solvent and the sample.
True.
Pencil is inert — it does not react with the solvent or sample components. Using any other medium would interfere with the chromatography.
Name three methods that can be used to locate colourless spots on a TLC plate.
UV light
Ninhydrin spray (a harmful skin/eye irritant)
Iodine vapour
In a TLC experiment, the solvent level in the beaker must remain .......... the baseline. A lid is placed on the beaker to ensure the atmosphere inside is .......... with solvent vapour.
In a TLC experiment, the solvent level in the beaker must remain below the baseline. A lid is placed on the beaker to ensure the atmosphere inside is saturated with solvent vapour.
True or False?
A component that is more soluble in the mobile phase will travel a shorter distance up a TLC plate.
False.
A component more soluble in the mobile phase is carried further up the plate, giving it a higher Rf value. A component with stronger affinity for the stationary phase travels a shorter distance.
Why is two-dimensional TLC more reliable than single-solvent TLC for identifying amino acids?
Some amino acids have very similar Rf values in the same solvent, making identification uncertain.
Two-dimensional TLC runs the plate through a second solvent at 90° to the first, giving two distinct Rf values per spot and allowing more confident identification.
Rf value
A dimensionless ratio used in chromatography equal to the distance travelled by a component divided by the distance travelled by the solvent front. Values always lie between 0 and 1.
What is the stationary phase in column chromatography?
A general formula is the simplest algebraic formula that represents any member of a homologous series. For example, alkanes have the general formula CnH2n+2.
In column chromatography, the solvent added on top of the sample is called the ........... The component with the greatest affinity for the stationary phase takes .......... time to elute through the column.
In column chromatography, the solvent added on top of the sample is called the eluent. The component with the greatest affinity for the stationary phase takes the longest time to elute through the column.
True or False?
The sample in column chromatography is added using a pipette to avoid disturbing the surface of the stationary phase, so that the sample flows from one level through the column.
True.
Disturbing the stationary phase surface can cause channelling (cracks), which prevents clean separation. The sample must be introduced carefully so it runs evenly from a single level.
How can the flow of solvent through a column chromatography column be increased in the laboratory?
By attaching a gas syringe to the top of the column to push the mobile phase through under pressure.
In industrial or research settings, a compressed air line is attached instead.
Coloured components can be identified in column chromatography by collecting fractions and analysing them separately. Colourless components cannot be located by UV light in column chromatography — that technique applies to TLC instead.
Coloured components can be identified in column chromatography by collecting fractions and analysing them separately (e.g. by spotting onto a TLC plate or using a spectrophotometer). Colourless components cannot be located using UV fluorescence in column chromatography — UV fluorescence is a detection method used in TLC, not column chromatography.
True or False?
Cracks in the stationary phase of a column are beneficial because they speed up the separation by providing extra flow channels.
False.
Cracks in the stationary phase are detrimental — they cause uneven flow and lead to poor separation of the sample components.
Why must fresh solvent be continuously added to the top of the column during column chromatography?
To prevent the column from drying out.
If the solvent level drops below the stationary phase, air bubbles can enter, disrupting the flow and causing poor separation.
What is retention time in gas chromatography?
A molecular formula is the formula that shows the actual number and type of each atom present in one molecule of a compound.
In gas-liquid chromatography the mobile phase is an .......... carrier gas (e.g. helium or nitrogen). The stationary phase is a .......... film on a solid support inside a long .......... column.
In gas-liquid chromatography the mobile phase is an inert carrier gas (e.g. helium or nitrogen). The stationary phase is a non-volatile liquid film on a solid support inside a long coiled column.
True or False?
In a GC chromatogram, the area of each peak is related to the amount of each component present in the mixture.
True.
The relative areas (sizes) of the peaks in a gas chromatogram are proportional to the relative quantities of each component in the sample.
Why must GC be used for gases and volatile liquids rather than for large non-volatile molecules?
The sample must vaporise when injected into the column so it can be carried through as a gas by the inert carrier gas.
Non-volatile compounds cannot enter the gas phase and would remain trapped at the injection point.
A component with a .......... retention time has a stronger interaction with the stationary phase. An unknown component can be identified by comparing its retention time with ...........
A component with a longer retention time has a stronger interaction with the stationary phase. An unknown component can be identified by comparing its retention time with data book values.
True or False?
In a GC chromatogram, the compound with the shortest retention time has the strongest interaction with the stationary phase.
False.
The compound with the shortest retention time has the weakest interaction with the stationary phase (or the highest volatility), so it passes through the column fastest.
State two factors that determine the retention time of a component in gas chromatography.
The volatility of the component (more volatile = shorter retention time)
The strength of attraction between the component and the stationary phase (stronger attraction = longer retention time)
What is the Rf value in thin-layer chromatography?
An electrophile is a species that accepts a pair of electrons from a nucleophile, forming a new covalent bond. Electrophiles are attracted to electron-rich regions, such as π bonds.
In TLC, the less polar components travel .......... up the plate and have .......... Rf values, because they are more soluble in the .......... phase.
In TLC, the less polar components travel further up the plate and have higher Rf values, because they are more soluble in the mobile phase.
True or False?
More polar components travel further up a TLC plate because they are more strongly attracted to the polar stationary phase.
False.
More polar components travel a shorter distance up the plate. Their stronger attraction to the polar stationary phase retains them near the baseline, giving lower Rf values.
A spot travels 2.82 cm on a TLC plate where the solvent front has moved 4.63 cm. What is the Rf value?
Rf = 2.82 ÷ 4.63 = 0.61
In TLC, knowing the .......... values of compounds helps compare the .......... of various molecules. Each spot is identified by matching its value against .......... standards run alongside.
In TLC, knowing the Rf values of compounds helps compare the polarity of various molecules. Each spot is identified by matching its value against reference standards run alongside.
A mixture of amino acids is analysed by TLC. What determines how far each amino acid travels up the plate?
The balance between solubility in the mobile phase and retention by the stationary phase. More soluble (less polar) amino acids travel further; more polar ones are retained closer to the baseline.
True or False?
A higher Rf value indicates a more polar component in TLC.
False.
A higher Rf value indicates a less polar component — it travels further because it is more soluble in the (less polar) mobile phase and less attracted to the polar stationary phase.
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