Chromatography

Class 12 Chemistry, unit 19, book pages 295–307

Practise 41 MCQs Take the unit quiz Review 25 cards

Chromatography separates a mixture into its components by letting a moving phase (the mobile phase) carry the mixture over a fixed phase (the stationary phase). Components that are held more strongly by the stationary phase move more slowly, so they separate. The $R_{f}$ value of a spot (paper and thin layer chromatography) and the retention time of a peak (column and gas chromatography) identify each component. This unit explains the phases and terms, thin layer chromatography and its forensic uses, how to read $R_{f}$ values and retention times, how to choose the phases, and how mass spectrometry joined to chromatography identifies tiny amounts of unknown substances.

Learning outcomes (5)

Describe the terms stationary phase, mobile phase, $R_{f}$ value, baseline and solvent front.
Explain the principles and applications of thin layer chromatography in forensic chemistry and analysis of unknown materials.
Interpret $R_{f}$ values and retention times in chromatograms to determine the composition of a mixture.
Explain the importance of selecting the appropriate stationary and mobile phase in chromatography and their impact on the separation of compounds.
Describe the use of mass spectrometry in combination with chromatography for identifying and quantifying small amounts of unknown materials in forensic analysis.

19What chromatography is

Put a drop of black felt-tip ink near the bottom of a strip of filter paper and dip the end in water. As the water climbs, the ink spreads into bands of blue, yellow and pink. The black ink was a mixture of dyes, and the paper has separated them. This is chromatography.

Chromatography: A technique for separating the components of a mixture by passing a mobile phase (liquid or gas) over a stationary phase (solid, or liquid held on a solid). Each component moves at its own speed because it is attracted to the two phases to a different extent.

The name comes from two Greek words: chroma, meaning colour, and graphein, meaning to write. The first separations were of coloured plant pigments, which appeared as coloured bands. Today colourless substances are separated too; they are made visible with UV light or a spray reagent, or detected by an instrument.

Fig 19.1: the process of chromatography. A mixture is separated into its components, which can then be analysed, identified, purified and quantified.

Why chromatography is useful

It separates complex mixtures into individual components, even when only a tiny amount of sample is available.
It checks the purity of a compound: a pure substance gives one spot or one peak.
It identifies components by comparing them with known standards run under the same conditions.
It quantifies components: the size of a spot or the area of a peak depends on the amount present.
It is used in pharmaceutical, food and chemical industries, environmental testing (purity of water), beverage testing, forensic science and drug testing, biochemical research and the petroleum industry (mixtures of hydrocarbons).

19.1Stationary phase and mobile phaseOften examined

Every chromatography has two phases. One stays still and the other moves over it, carrying the mixture with it.

Stationary phase: The phase that does not move. It is a solid (such as silica gel, alumina or paper) or a liquid coated on the surface of a solid. It holds back the components of the mixture by adsorption or by dissolving them.
Mobile phase: The phase that moves over or through the stationary phase and carries the components of the mixture with it. It is a liquid (the solvent or eluent) or a gas (the carrier gas).

Fig 19.2: stationary and mobile phases in paper chromatography. The paper is the stationary phase and the solvent that rises up it is the mobile phase.

Stationary and mobile phases in common kinds of chromatography.
Kind of chromatography	Stationary phase	Mobile phase
Paper	water held in the cellulose fibres of the paper	a liquid solvent
Thin layer (TLC)	thin layer of silica gel or alumina on a glass, plastic or aluminium plate	a liquid solvent (eluent)
Column	silica gel or alumina packed in a glass tube	a liquid solvent (eluent)
Gas (GC)	a non-volatile liquid coated on a solid support inside a long thin tube	an inert carrier gas (He or $N X_{2}$ )
High performance liquid (HPLC)	very fine particles packed in a steel column	a liquid pumped at high pressure

How the separation happens

The components of the mixture are shared between the two phases. There is a dynamic equilibrium: molecules keep moving from the mobile phase onto the stationary phase and back again. A molecule moves forward only while it is in the mobile phase.

A component that is attracted strongly to the stationary phase spends most of its time there, so it moves slowly (short distance on paper, long time in a column).
A component that is more soluble in the mobile phase spends most of its time moving, so it moves fast.

Two mechanisms hold a component on the stationary phase. In adsorption chromatography (TLC, column) molecules stick to the surface of a solid such as silica gel. In partition chromatography (paper, gas) molecules dissolve in a liquid held on a solid, and are distributed (partitioned) between this liquid and the mobile phase according to their partition coefficient.

19.1Rf value, solvent front and baselineOften examined

After a paper or thin layer chromatogram has been run, each component is a spot at some height. To describe how far a spot moved we compare its distance with the distance moved by the solvent. This ratio is the $R_{f}$ value.

Baseline (origin): The starting line, drawn in pencil near the bottom of the paper or plate, on which the spots of the mixture are placed. All distances are measured from it. It must be above the solvent level in the tank.
Solvent front: The highest level reached by the solvent as it moves up the paper or plate. It is marked with a pencil line as soon as the chromatogram is taken out, before the solvent evaporates.
$R_{f}$ value (retardation factor, also called retention factor): The ratio of the distance travelled by the solute (component) from the baseline to the distance travelled by the solvent front from the baseline. It has no unit and lies between 0 and 1.

R_{f} = \frac{distance travelled by the solute from the baseline}{distance travelled by the solvent from the baseline}

(19.1)

The solute is carried by the solvent, so it can never move farther than the solvent front. Therefore $0 \leq R_{f} \leq 1$ . A spot that stays on the baseline has $R_{f} = 0$ ; a spot that moves with the solvent front has $R_{f} = 1$ .

Fig 19.3:

R_{f}

calculations. The solvent front is

10 cm

above the baseline; the blue, purple and red spots are

8

6

and

2 cm

above it.

Book example (Fig 19.3):

R_{f}

of three dyes

On a paper chromatogram the solvent front is

10 cm

above the baseline. A blue spot is

8 cm

, a purple spot

6 cm

and a red spot

2 cm

above the baseline. Calculate the

R_{f}

value of each dye.

Write the formula. Both distances are measured from the baseline.
$R_{f} = \frac{distance moved by dye}{distance moved by solvent}$
Blue dye.
$R_{f} = \frac{8 cm}{10 cm} = 0.80$
Purple dye.
$R_{f} = \frac{6 cm}{10 cm} = 0.60$
Red dye.
$R_{f} = \frac{2 cm}{10 cm} = 0.20$

The red dye has the smallest $R_{f}$ , so it is held most strongly by the stationary phase. The blue dye is held least strongly and is most soluble in the solvent.

R_{f} (blue) = 0.80, R_{f} (purple) = 0.60, R_{f} (red) = 0.20

Distance moved from a known

R_{f}

A dye has

R_{f} = 0.45

in a certain solvent. How far above the baseline will its spot be when the solvent front has moved

12.0 cm

Rearrange the formula for the distance moved by the solute.
$distance of spot = R_{f} \times distance of solvent front$
Substitute.
$= 0.45 \times 12.0 cm = 5.4 cm$

5.4 cm above the baseline

Measuring from the right line

The baseline of a TLC plate is

1.5 cm

above the bottom edge. After development the solvent front is

9.5 cm

and the centre of a spot is

6.3 cm

above the bottom edge. Calculate the

R_{f}

of the spot.

Distances must be measured from the baseline, not from the edge of the plate. Subtract $1.5 cm$ from each.
$d_{spot} = 6.3 - 1.5 = 4.8 cm, d_{solvent} = 9.5 - 1.5 = 8.0 cm$
Divide.
$R_{f} = \frac{4.8}{8.0} = 0.60$

R_{f} = 0.60

19.1, Fig 19.4Paper chromatography

The book shows paper chromatography in Figs 19.2 to 19.4 and in its key points. It is the simplest kind of chromatography and the one used in the school laboratory.

Principle

Chromatography paper is made of cellulose fibres, which hold a thin film of water. This water is the stationary phase; the paper only supports it. The solvent rising through the paper by capillary action is the mobile phase. Each component is partitioned between the water in the paper and the moving solvent, so paper chromatography is partition chromatography.

Procedure

Draw a pencil baseline about $2 cm$ from the bottom of the paper.
Put a small, concentrated spot of the mixture on the baseline with a capillary tube. Put spots of known substances (standards) on the same line, a little apart.
Stand the paper in a tank containing the solvent, with the solvent level below the baseline. Close the tank with a lid so that the air inside is saturated with solvent vapour and the solvent does not evaporate from the paper.
Let the solvent rise until it is near the top. Take the paper out and mark the solvent front in pencil at once.
Dry the paper. Colourless spots are made visible with a locating agent, for example ninhydrin spray for amino acids (purple spots after warming) or UV light.
Measure the distance of each spot and of the solvent front from the baseline and calculate the $R_{f}$ values.

A paper chromatogram (like book Fig 19.4): standards A, B and C and an unknown mixture M run on the same paper. M gives two spots level with A and C.

Finding the composition of a mixture

In the chromatogram above the solvent front is

10.0 cm

from the baseline. Standards A, B and C moved

3.0

5.5

and

7.5 cm

. The mixture M gave two spots at

3.0

and

7.5 cm

. What does M contain?

Calculate the $R_{f}$ of each standard.
$R_{f} (A) = \frac{3.0}{10.0} = 0.30, R_{f} (B) = \frac{5.5}{10.0} = 0.55, R_{f} (C) = \frac{7.5}{10.0} = 0.75$
Calculate the $R_{f}$ of the two spots of M.
$R_{f} = \frac{3.0}{10.0} = 0.30 and R_{f} = \frac{7.5}{10.0} = 0.75$
Match equal $R_{f}$ values obtained on the same paper with the same solvent.
$0.30 = R_{f} (A), 0.75 = R_{f} (C)$

M contains A and C. It does not contain B, because no spot of M appears at $R_{f} = 0.55$ .

M = A + C

19.2Thin layer chromatography (TLC)Often examined

Thin layer chromatography works like paper chromatography, but the paper is replaced by a glass (or plastic or aluminium) plate coated with a very thin, even layer (about $0.2 mm$ ) of an adsorbent such as silica gel or alumina (aluminium oxide). The adsorbent is the stationary phase.

Principle

TLC separates substances by adsorption: by how strongly they stick to the surface of the adsorbent. Silica gel is very polar (its surface is covered with $- OH$ groups). Polar components, for example those with $- OH$ or $- NH X_{2}$ groups, are adsorbed strongly and move slowly (low $R_{f}$ ). Non-polar components are adsorbed weakly and are carried farther by the solvent (high $R_{f}$ ). So in TLC substances are separated mainly because of differences in polarity.

Fig 19.5: thin layer chromatography. Left: the plate stands in a closed jar containing the solvent (eluent). Right: the developed plate;

x

is the distance moved by the spot and

y

the distance moved by the solvent, so

R_{f} = x / y

The plate, the chamber and the mobile phase

TLC plates are usually bought ready-made. They are chemically inert and stable, and the layer of stationary phase is fine and evenly thin.
The chamber (a jar or tank with a lid) keeps a stable atmosphere saturated with solvent vapour, so the spots develop properly. It stops the solvent evaporating from the plate and keeps out dust.
The mobile phase (eluent) is a single solvent or a mixture of solvents. It must be pure and free from particles: the purer the mobile phase, the better the spots develop.

Working of TLC (19.2.1)

Coat the plate with silica gel or aluminium oxide and dry it (or use a ready-made plate).
With a pencil, mark small points on a baseline about $1$ to $2 cm$ from the bottom of the plate. Apply the sample solutions to these points with a capillary tube and let the spots dry.
Pour the mobile phase into the TLC chamber to a depth below the baseline. Add a moistened filter paper against the wall to keep the air saturated with solvent vapour (constant humidity).
Put the plate in the chamber and close it with the lid.
Let the plate develop. The sample spots must stay above the solvent level and must not dip into it.
When the solvent is near the top, remove the plate, mark the solvent front, and let it dry. Look at the spots under UV light (or spray with a locating reagent such as ninhydrin, or put the plate in iodine vapour). Calculate the $R_{f}$ values.

Fig 19.6: a developed chromatogram of ink spots. The questioned ink Q matches the standard S2 spot for spot, but not S1.

Applications of TLC (19.2.2)

Testing medicines such as sedatives and local anaesthetics, and checking the purity of drugs.
Biochemical analysis, for example separating substances obtained from food, and detecting amino acids (ninhydrin spray).
Identifying natural products such as essential oils and alkaloids.
Purifying samples and comparing them with authentic samples.
In the food industry, separating and identifying colours, sweeteners and preservatives.
In the cosmetic industry, checking dyes and other ingredients.
In organic synthesis, monitoring the progress of a reaction: spots of the reaction mixture taken at intervals show the starting material spot fading and the product spot appearing.

Advantages of TLC

TLC is one of the fastest, cheapest, simplest and easiest chromatography techniques. It needs only a tiny sample, a run takes a few minutes, several samples can be compared side by side on one plate, and corrosive spray reagents or UV light can be used to see colourless spots because the layer is inorganic.

Limitations of TLC (19.2.3)

The separation length is short compared with other chromatography methods (such as column or gas chromatography), so substances with close $R_{f}$ values may not separate.
It is an open system, so humidity and temperature can change the results.
It works only with very small amounts of substance, so it is used for analysis and not for purifying large amounts.

Differences between paper chromatography and TLC.
Point	Paper chromatography	Thin layer chromatography
Stationary phase	water held in cellulose fibres of paper	thin layer of silica gel or alumina on a plate
Mechanism	partition	mainly adsorption
Speed	slow (often one hour or more)	fast (a few minutes)
Spots	larger, more spreading	small and sharp, better separation
Locating agents	mild reagents only (paper chars with strong acids)	corrosive sprays and heating can be used
Reproducibility of $R_{f}$	less	better, because the layer is uniform
Cost	very cheap	cheap

19.2.4Thin layer chromatography in forensic chemistryOften examined

Forensic chemistry applies chemistry to the investigation of crimes. The samples are usually tiny and are complex mixtures (a drop of blood, a fibre, a trace of ink). TLC is ideal because it is quick, cheap, needs very little sample and compares the sample with known standards on the same plate.

Drug analysis. TLC separates and identifies illicit (forbidden by law) drugs such as cocaine and heroin. Comparison with known standards gives the composition and purity of the seized drug.
Poison report. In suspected poisoning, biological samples (blood, urine, tissues) are tested for toxic compounds, which identifies the poison involved.
Trace evidence. Fibres, paints and dyes are separated and compared with reference samples to link them to a particular source (a car, a piece of clothing).
Gunshot residue (GSR). Residue collected from a suspect's hands or from the crime scene is analysed for the organic components of gunpowder.
Explosives inspection. Components such as nitroaromatics and nitrates are separated and identified to show whether explosives were present at a crime scene or a suspected bomb-making site.
Document analysis. The dyes in the ink of a questioned document are compared with inks of known pens (Fig 19.6). Different spot patterns show different inks, which helps to detect forgeries.
Scientific screening. TLC is a quick preliminary screening test in toxicology. It shows which samples contain drugs, so only those are sent for more sensitive and expensive analysis by GC-MS or LC-MS.

Forensic laboratories also use gas chromatography (GC), high performance liquid chromatography (HPLC) and mass spectrometry, each for a different purpose.

19.3, 19.3.1Interpretation of Rf values and factors affecting themOften examined

An $R_{f}$ value tells us how much a solute 'likes' the stationary phase compared with the mobile phase. This gives clues about its properties: its polarity, its relative size and its solubility.

Fig 19.7:

R_{f}

values of three spots (0.40, 0.65, 0.80). The solvent front itself corresponds to

R_{f} = 1.00

$R_{f}$ value	What it means (silica gel or paper, polar stationary phase)
small (near 0)	held strongly by the stationary phase; usually a polar compound (for example glucose, amino acids)
large (near 1)	more soluble in the mobile phase, held weakly; usually a less polar compound
same $R_{f}$ as a standard (same conditions)	possibly the same compound
different $R_{f}$	different compounds
one spot only	the sample is probably pure (in that solvent)

Consistent $R_{f}$ values allow unknown substances to be identified by comparing them with known substances run on the same plate under the same conditions.
Different $R_{f}$ values show different compounds, while equal values suggest (but do not prove) that the compounds are the same.
Slight variations occur because of interactions with other substances and differences in concentration (an overloaded spot streaks and its centre moves a little).

Factors affecting Rf values (19.3.1)

Consider a mixture of A and B spotted on a plate. A and B separate because they have different affinities for the two phases. Their $R_{f}$ values depend on the following factors, so an $R_{f}$ value is meaningful only when these are kept the same.

Solvent (mobile phase). The solvent carries the solute, so it is the most important factor. On silica gel, a more polar solvent competes better for the polar surface and pushes all components farther, so $R_{f}$ values are higher in a polar solvent than in a non-polar one. A mixture of solvents is often used to adjust the $R_{f}$ values to a useful range.
Solute (sample). Compounds with polar groups such as hydroxyl ( $- OH$ ) or amine ( $- NH X_{2}$ ) bind to the silica (by hydrogen bonding) and travel slower than the solvent, so they have lower $R_{f}$ values. Non-polar compounds bind less and travel with the solvent, so they have higher $R_{f}$ values.
Temperature. Solubility of compounds rises with temperature, so temperature changes the $R_{f}$ values. Runs being compared must be at the same temperature.
Thickness of the layer. Silica binds the solute, so a thicker layer binds more strongly and gives lower $R_{f}$ values.
Other factors: the kind and quality of the paper or adsorbent, how saturated the chamber is with vapour, and the amount of sample applied.

19.3.2Retention timeOften examined

In column chromatography and gas chromatography the components are not left as spots on a plate. They come out of the end of the column one after another and pass a detector. So instead of a distance we measure a time.

Retention time ( $t_{R}$ ): The time taken by a component to travel through the column, from the moment the sample is injected to the moment the component is detected (the top of its peak). It is usually given in minutes.

t

signal

A

B

C

t_{R} (A) = 2.0

t_{R} (B) = 3.5

t_{R} (C) = 6.0

A gas chromatogram of a three-component mixture. Each peak is one component; its retention time

t_{R}

is measured from injection (

t = 0

) to the top of the peak (times in minutes).

The record of detector signal against time is a chromatogram. Each peak is one component. Under fixed conditions (same column, temperature and flow rate of the carrier gas) every compound has its own retention time, so:

the number of peaks gives the number of components;
the retention time of a peak identifies the component, by comparison with pure standards run under the same conditions;
the area under a peak is proportional to the amount of that component, so the composition can be calculated.

Reading a gas chromatogram

A mixture gives the chromatogram above, with peaks at

2.0

3.5

and

6.0 min

. Pure ethanol, propanone and methylbenzene, run on the same column under the same conditions, have retention times

3.5

2.0

and

6.0 min

. Identify the components.

Count the peaks.
$3 peaks \Rightarrow 3 components$
Match each retention time with a standard.
$t_{R} = 2.0 (A) 3.5 (B) 6.0 (C) \to propanone \to ethanol \to methylbenzene$

Peak A is the tallest and largest, so propanone is probably the major component; peak areas would give the exact percentages.

A: propanone, B: ethanol, C: methylbenzene

Long and short retention times (book, for a non-polar stationary liquid)

Longer retention time	Shorter retention time
non-polar components (with a non-polar stationary liquid)	polar components (with a non-polar stationary liquid)
strongly attracted to (very soluble in) the stationary liquid	weakly attracted to the stationary liquid
higher boiling point, vaporise less readily	lower boiling point, vaporise more readily, stay longer in the gas
travel slower through the column	travel faster through the column

19.1, 19.3.2Column chromatography

The book mentions column chromatography when it defines the phases and retention time. It is TLC turned into a tube, and it is used to purify and collect larger amounts of each component.

Column chromatography. The weakly held band A comes out first; the strongly held band B follows.

Plug the bottom of a vertical glass column (fitted with a tap) with cotton wool and pack it with a slurry of silica gel or alumina in the solvent. This is the stationary phase.
Add the mixture, dissolved in a little solvent, to the top of the column.
Keep adding the solvent (the eluent, mobile phase) at the top. It flows down under gravity. This washing is called elution.
The components move down as separate bands: the one held weakly moves fast, the one held strongly moves slowly.
Collect the liquid coming out (the eluate) in a series of test tubes (fractions). Each band is collected in its own fractions. Evaporate the solvent to get the pure component.

A column can be imagined as a stack of very thin layers called theoretical plates. In each plate the solute reaches one equilibrium (one partition) between the two phases. The plate height $H$ (height equivalent to a theoretical plate) is the distance the solute moves while undergoing one partition. For a column of length $L$ with $N$ plates:

H = \frac{L}{N}

A smaller plate height means more partitions in the same length, so sharper bands and better separation.

19.1, 19.3.2Gas chromatography (GC)

In gas chromatography the mobile phase is a gas. It is the best method for volatile compounds (compounds that vaporise easily without decomposing), such as solvents, fuels, perfumes and alcohol in blood.

A gas chromatograph. The components leave the column at different retention times and appear as peaks on the chromatogram.

Carrier gas (mobile phase): an inert gas such as helium or nitrogen flows through the column at a steady rate.
Injection port: a tiny volume of liquid sample is injected with a syringe and vaporised at once.
Column (stationary phase): a long, thin, coiled tube (metres long) containing a non-volatile liquid coated on a solid support (or on the inside wall). This is gas-liquid chromatography: the stationary phase is a liquid supported on a solid.
Oven: keeps the column at a controlled temperature, high enough to keep the components as gases.
Detector and recorder: respond to each component as it leaves the column and draw the chromatogram.

Components that are more soluble in the stationary liquid, or that have higher boiling points, stay longer in the column and have longer retention times. Components are identified by their retention times and measured by their peak areas.

19.4Selection of stationary and mobile phaseOften examined

A separation works only if the components of the mixture have different affinities for the two phases. If all components stick to the stationary phase they stay on the baseline; if none sticks they all run with the solvent front. The choice of phases therefore decides whether, and how well, the compounds separate.

Selection of the stationary phase (19.4.1)

The stationary phase is usually an adsorbent. A good adsorbent has:

high and selective adsorption capacity;
finely divided particles, to give a large surface area for adsorption;
high mechanical stability, so that it does not form dust;
chemical inertness towards the sample and the eluting solvents;
high purity;
easy availability.

It is chosen so that the components have different solubilities (or adsorptions) in the two phases. Then they move at different rates and separate. For example, polar silica gel separates compounds that differ in polarity.

The next step is to decide the separation needed. To separate closely related compounds with high resolution, use a stationary phase with small particles and a mobile phase of low viscosity and high flow rate. The components then diffuse less and interact more with the stationary phase, which gives sharper peaks and better separation.

Selection of the mobile phase (19.4.2)

The properties of the solvent matter more than the factory it comes from, but solvents from reputable manufacturers with strict quality control give consistent, pure solvent.

Key properties of the mobile phase.
Property	Why it matters
Purity	impurities give extra spots or peaks and spoil the analysis; use HPLC-grade or MS-grade solvents
Polarity	match it to the polarity of the analytes: it controls how far the components move and how well they separate
Viscosity	a suitable (low) viscosity gives the right flow rate and sharp peaks
Chemical reactivity	the solvent must be stable and must not react with the analytes or the stationary phase
UV transparency	with UV detection, the solvent must not absorb UV light at the detection wavelength
pH	for charged (ionisable) solutes, the pH controls their charge and so their movement

19.5Mass spectrometry with chromatography in forensic analysisOften examined

Chromatography separates a mixture but, on its own, identifies a component only by comparison with a standard. Mass spectrometry (Unit 17) identifies a pure substance from its molecular mass and fragmentation pattern, but a mixture gives a confusing spectrum. Joined together, the two methods separate and identify even tiny amounts of unknown substances.

Mass spectrometry is used in chemistry, biochemistry, pharmacy and medicine, from single cells to samples from outer space. It identifies the structure of unknown substances, analyses environmental and forensic samples, detects atmospheric pollutants, and controls the quality of drugs, foods and polymers. In forensic work it is used for drug testing, toxicology and the detection of explosive residues in tiny samples of human tissue, drugs or chemicals from a crime scene.

How mass spectrometry works in forensic science (19.5.1)

The sample is injected directly, or first passed through a chromatograph (GC or LC) that separates it into its components.
In the ionisation chamber each component is turned into positive ions by removing at least one electron. Bombarding with high-energy electrons is called electron ionisation (EI), the most common method.
\ce{M + e- -> M^{+\bullet} + 2e-}
The ions are accelerated into the mass analyser. A magnetic field deflects them by different amounts according to their mass-to-charge ratio (m/z).
The separated ion beams enter a detector, which gives an electric signal proportional to the number of ions hitting it.
The result is a mass spectrum of each component: relative abundance against m/z. The computer matches it with a library of known spectra to identify exactly which compounds were present.

GC-MS: the gas chromatograph separates the mixture; each component, as it leaves the column, is ionised and analysed by the mass spectrometer and identified from a spectral library.

For greater resolution, forensic analysts may use a time-of-flight mass spectrometer, which separates ions by the time they take to cross a fixed distance in a vacuum instead of by magnetic deflection.

Applications of mass spectrometry (19.5.2)

Finding which toxin was used in a poisoning, or whether drugs were involved and which drugs they were; this is evidence in court.
Finding the chemical composition of a sample or molecule (elucidating its structure and chemical properties).
Classifying biological products, in particular proteins, in many species.
Identifying unknown substances by measuring their molecular mass, and measuring the amounts of known compounds.
Detecting explosive residues, atmospheric pollutants and drugs in sport.

Advantages (19.5.4)

It detects substances at very low concentrations (parts per million or even parts per billion).
It accurately determines the atomic composition and molecular mass of a sample.
It analyses a wide range of compounds, including complex mixtures (especially when joined to chromatography).

Disadvantages (19.5.5)

It is less effective for identifying hydrocarbons that produce similar ions.
It cannot distinguish optical and geometric isomers, which have the same molecular formula and give almost the same fragments.
The equipment is expensive and needs trained staff to operate it and interpret the results.

Formula sheet

R_f value

R_f=\dfrac{\text{distance travelled by solute from baseline}}{\text{distance travelled by solvent front from baseline}}

both distances measured from the baseline, to the centre of the spot and to the solvent front

no unit; 0\le R_f\le1

Distance moved by a spot

d_{\text{solute}}=R_f\times d_{\text{solvent}}

d_{\text{solvent}}: distance of the solvent front from the baseline

Unit: cm

TLC plate (Fig 19.5)

R_f=\dfrac{x}{y}

x: distance moved by the spot; y: distance moved by the solvent

no unit

Retention time

t_R=t_{\text{detection}}-t_{\text{injection}}

time from injection of the sample to the top of the component's peak

Units: min or s

Plate height

H=\dfrac{L}{N}

L: length of column; N: number of theoretical plates

Units: cm or mm

Key terms

Chromatography: A technique that separates the components of a mixture by passing a mobile phase over a stationary phase; components move at different rates because of their different affinities for the two phases.
Stationary phase: The phase that does not move: a solid (paper, silica gel, alumina) or a liquid held on a solid.
Mobile phase: The liquid or gas that moves over or through the stationary phase and carries the components of the mixture.
Eluent (eluant): The solvent used as the mobile phase in TLC or column chromatography; the liquid leaving a column is the eluate.
Developer: The solvent (mobile phase) that develops a chromatogram by rising up the paper or plate; also used for a spray reagent that makes spots visible.
Adsorbent: A finely divided solid, such as silica gel or alumina, on whose surface molecules are held; the stationary phase in TLC and column chromatography.
Adsorption chromatography: Separation based on how strongly components stick to the surface of a solid stationary phase (TLC, column).
Partition chromatography: Separation based on how components distribute between a liquid stationary phase and the mobile phase (paper, gas-liquid).
Baseline (origin): The pencil line near the bottom of the paper or plate on which the samples are spotted and from which all distances are measured.
Solvent front: The highest level reached by the solvent as it moves up the paper or plate.
R_f value: Retardation (retention) factor: distance travelled by the solute from the baseline divided by distance travelled by the solvent front from the baseline.
Chromatogram: The record of a chromatographic separation: the developed paper or plate with its spots, or the graph of detector signal against time.
Retention time: The time a component takes to pass through a column, from injection to detection (top of its peak).
Thin layer chromatography (TLC): Chromatography on a plate coated with a thin (about 0.2 mm) layer of adsorbent such as silica gel or alumina.
Locating agent: A reagent sprayed on a chromatogram to make colourless spots visible, such as ninhydrin for amino acids.
Carrier gas: The inert gas (helium or nitrogen) that is the mobile phase in gas chromatography.
Plate height: The distance a solute moves while undergoing one partition between the phases; smaller plate height means better separation.
GC-MS: Gas chromatography joined to mass spectrometry: GC separates the mixture and MS identifies each component.
Electron ionisation (EI): Formation of positive ions by bombarding molecules with high-energy electrons, which knock out an electron.
Forensic chemistry: The use of chemical analysis to investigate crimes and provide evidence for courts.

Multiple choice questionsp. 303–304

In a scenario where a person may have died from a drug overdose at a crime scene, how can mass spectrometry contribute to determining the cause of death?

AMass spectrometry can detect the presence of a toxin but not specify which toxin.
BCarpet fiber analysis can be utilized to match fibers found on the victim.
CTissue samples can be examined to identify both the presence and quantity of toxins.Answer
DMass spectrometry can ascertain the presence of a toxin but not quantify it.

Why

Mass spectrometry is both qualitative and quantitative: the fragmentation pattern identifies which drug or toxin is present, and the peak heights show how much is present, down to parts per billion. Options A and D each deny one of these abilities, and B describes fibre comparison, which does not show the cause of death.

Detecting amino acids by spraying the plate with ninhydrin solution exemplifies which type of chromatography?

AColumn chromatography
BThin layer chromatographyAnswer
CPaper chromatography
DLiquid chromatography

Why

The word plate points to thin layer chromatography: after development the TLC plate is sprayed with ninhydrin, which turns amino acid spots purple. A column is never sprayed.

Note. Ninhydrin is also sprayed on paper chromatograms, but paper is called a sheet or strip, not a plate.

iii

What does the retardation factor represent?

AThe ratio of the distance traveled by the substance from the baseline to the distance traveled by the solvent from the baseline.Answer
BThe ratio of the distance traveled by the solvent from the baseline to the distance traveled by the substance from the baseline.
CThe ratio of the distance traveled by the substance from the top line to the distance traveled by the solvent from the top line.
DThe ratio of the distance traveled by the solvent from the top line to the distance traveled by the substance from the top line.

Why

R_f=\dfrac{\text{distance moved by substance from baseline}}{\text{distance moved by solvent from baseline}}. Option B is the inverse (it would be greater than 1), and distances are never measured from the top.

In chromatography, the stationary phase may be ______ supported on a solid.

ASolid or liquidAnswer
BLiquid or gas
CSolid only
DLiquid only

Why

The stationary phase is either a solid adsorbent (silica gel, alumina) or a liquid held on a solid support (water in paper fibres, the liquid in a GC column). A gas cannot be stationary.

Note. Strictly, only the liquid is 'supported on a solid'; a solid stationary phase is itself the support. Option A is the intended answer because the stationary phase can be solid or liquid.

Which type of chromatography involves separating substances in a mixture over a 0.2\,\mathrm{mm} thick layer of an adsorbent?

AGas liquid
BColumn
CThin layerAnswer
DPaper

Why

In TLC the plate is coated with a very thin (about 0.2\,\mathrm{mm}) layer of adsorbent such as silica gel or alumina.

What is the distance that the solute moves while undergoing one partition?

ARetention distance
BDistribution constant
CPlate heightAnswer
DColumn packing length

Why

A column is treated as a series of theoretical plates, each giving one partition (equilibrium) of the solute between the phases. The length of column for one partition is the plate height, H=L/N. The distribution constant is a ratio of concentrations, not a distance.

vii

In TLC substances are separated because of differences in:

ApolarityAnswer
Bmolecular size
Cconcentration
Delectronegativity

Why

Silica gel is polar. Polar compounds are adsorbed more strongly and move less; non-polar compounds move farther. So separation depends mainly on differences in polarity.

viii

Which chromatographic technique is best suited for separating volatile compounds based on their interaction with a stationary phase?

APaper chromatography
BThin-layer chromatography (TLC)
CGas chromatography (GC)Answer
DHigh-performance liquid chromatography (HPLC)

Why

Volatile compounds vaporise easily, so they can be carried as gases by the carrier gas through a GC column, where they separate by their interaction with the stationary liquid.

Which chromatographic technique is often used for monitoring the progress of reactions and identifying compounds based on their retention factors?

APaper chromatography
BThin-layer chromatography (TLC)Answer
CColumn chromatography
DGas chromatography (GC)

Why

TLC is fast and needs only a drop of the reaction mixture, and it gives R_f values (retention factors). Samples taken at intervals show the starting material spot fading and the product spot appearing.

Short answer questionsp. 304

Answer each part with a definition or statement first, then the reason, then an example.

i3 marks

What causes colors to separate during chromatography?

Solution

Coloured inks and pigments are mixtures of dyes. The dyes separate because each is attracted to the stationary phase (paper or silica gel) and dissolves in the mobile phase (solvent) to a different extent.

As the solvent rises, a dye that is very soluble in the solvent and weakly held by the stationary phase is carried a long way (high R_f). A dye that is strongly adsorbed, usually because it is more polar, spends more time on the stationary phase and moves only a short distance (low R_f). So the dyes end up as separate coloured spots at different heights, as when black ink separates into blue, yellow and red spots.

ii3 marks

Is it possible for the R_f value to exceed 1?

Solution

No. The R_f value can never be greater than 1.

R_f=\dfrac{\text{distance moved by solute}}{\text{distance moved by solvent front}}

The solute moves only because the solvent carries it, so it cannot travel farther than the solvent front. The numerator is therefore always less than or equal to the denominator, and 0\le R_f\le1. A solute that does not interact with the stationary phase at all moves with the front and has R_f=1 (the maximum). If a calculation gives more than 1, the solvent front was not marked in time or the distances were measured wrongly.

iii3 marks

Explain the primary use of thin-layer chromatography (TLC)?

Solution

The primary use of TLC is the quick qualitative analysis of mixtures: to find how many components a mixture has, to identify them by comparing their R_f values with standards on the same plate, and to check the purity of a compound (a pure compound gives one spot).

It is also used to monitor the progress of a reaction (the reactant spot fades and the product spot appears) and to choose a solvent for column chromatography. In forensic science it is used to screen drugs, poisons and inks. It is preferred because it is fast, cheap, simple and needs only a tiny sample.

iv3 marks

Differentiate between thin-layer chromatography (TLC) and paper chromatography?

Solution

Thin layer chromatography	Paper chromatography
Stationary phase is a thin layer of silica gel or alumina on a glass, plastic or aluminium plate.	Stationary phase is water held in the cellulose fibres of the paper.
Separation is mainly by adsorption.	Separation is by partition.
Fast: a run takes a few minutes.	Slow: a run can take an hour or more.
Spots are small and sharp; separation is better.	Spots spread more; separation is poorer.
Corrosive locating sprays and heating can be used.	Corrosive sprays cannot be used because paper chars.

In both, the R_f value is calculated in the same way and both are used for qualitative analysis of small samples.

v3 marks

What do 'stationary phase' and 'mobile phase' refer to in chromatography?

Solution

Stationary phase: the phase that does not move. It is a solid, such as silica gel, alumina or paper, or a liquid held on the surface of a solid. It holds back the components of the mixture by adsorption or by dissolving them. Example: the silica gel layer on a TLC plate.

Mobile phase: the phase that moves over or through the stationary phase and carries the components of the mixture with it. It is a liquid solvent (eluent) in paper, TLC and column chromatography, or an inert carrier gas in gas chromatography.

Components separate because each one divides itself between the two phases to a different extent: the more strongly it is held by the stationary phase, the slower it moves.

vi3 marks

What type of chromatography is used in forensic science?

Solution

Thin layer chromatography (TLC) is the main type used in forensic science, because it is fast, cheap and needs only a tiny sample. It is used to analyse illicit drugs, poisons in blood or urine, inks of questioned documents, dyes in fibres, gunshot residue and explosives, and as a quick screening test.

For confirmation and for measuring amounts, forensic laboratories use gas chromatography (GC) and high performance liquid chromatography (HPLC), usually joined to a mass spectrometer (GC-MS or LC-MS). The chromatograph separates the sample and the mass spectrometer identifies each component, which gives evidence acceptable in court.

vii3 marks

Describe how you would analyze the chromatograms to determine if two ink samples are from the same source.

Solution

Place a small spot of each ink (for example, ink scraped from the questioned document and ink from the suspect's pen) side by side on the same pencil baseline of one TLC plate or paper, and develop it with the same solvent in a closed tank. Mark the solvent front and dry the chromatogram.

Then compare the two lanes. The inks are from the same source only if they give the same number of spots, with the same colours and the **same R_f values** (spots level with each other). Any extra, missing or shifted spot shows a different ink. For confirmation, repeat with a second solvent.

The questioned ink Q matches S2 spot for spot but not S1, so Q and S2 could come from the same pen.

viii3 marks

Explain the importance of the solvent choice in chromatography and how it affects the separation process.

Solution

The solvent is the mobile phase that carries the components, so it is the most important factor deciding the R_f values and whether the components separate at all.

Polarity. On polar silica gel, a more polar solvent pushes all components farther (higher R_f); a less polar solvent leaves them near the baseline. If the solvent is too polar all spots run with the front; if it is not polar enough all stay on the baseline. A good solvent (often a mixture) gives R_f values between about 0.2 and 0.8.
The solvent must dissolve the components, be pure, and not react with the sample or the stationary phase.
Its viscosity controls the flow rate and sharpness of spots or peaks; for UV detection it must not absorb UV light.

ix3 marks

Assess the limitations of paper chromatography when used to separate complex mixtures?

Solution

Paper chromatography is simple and cheap, but it is poor for complex mixtures:

Poor resolution. Spots spread (diffuse) along the paper fibres, so components with close R_f values overlap.
Short separation length and slow: the run takes a long time but the distance is only a few centimetres.
Two different compounds can have the **same R_f**, so identification is not certain.
It handles only tiny amounts and is mainly qualitative; amounts are hard to measure.
Colourless components need locating agents, and corrosive reagents cannot be used because paper chars.
R_f values change with temperature, humidity, paper quality and solvent, so results are hard to reproduce.

For complex mixtures TLC, HPLC or GC (with MS) give far better separation.

x3 marks

What are the essential characteristics of the substance used as a developer?

Solution

The developer is the solvent (mobile phase) that rises up the paper or plate and develops the chromatogram. It should:

be pure and free from particles, so that it gives no extra spots;
have a suitable polarity, so that the components move different distances (R_f roughly between 0.2 and 0.8);
dissolve the components of the mixture but not react with them or with the stationary phase;
be volatile, so that the chromatogram dries quickly, but not so volatile that it evaporates from the plate during the run;
have a low viscosity, so that it moves up at a reasonable speed and gives sharp spots;
be cheap, easily available, non-toxic and safe to handle.

Long answer questionsp. 305

i7 marks

Is mass spectrometry quantitative or qualitative? Give a reason to support your answer.

Solution

Answer

Mass spectrometry is both qualitative and quantitative. It tells us what a substance is (qualitative analysis) and how much of it, or of each isotope, is present (quantitative analysis).

Reasons: qualitative analysis

The m/z value of the molecular ion peak gives the relative molecular mass of a compound, because the ion carries a +1 charge. For example \ce{M+} at 46 fits ethanol.
The fragmentation pattern (fragment peaks and the base peak) is like a fingerprint. Matching it with a library of known spectra identifies the compound and its structure.
The M+2 peak shows chlorine (3:1) or bromine (1:1), and the number of peaks of an element shows its isotopes.
In forensic science it identifies which drug, poison or explosive is present in a tiny sample.

Reasons: quantitative analysis

The height of each peak is proportional to the number of ions of that m/z reaching the detector. So peak heights give the relative abundances of isotopes, from which the relative atomic mass is calculated.
The M+1 peak height gives the number of carbon atoms: n=\dfrac{100\times I_{M+1}}{1.1\times I_M}.
By comparing the signal with standards of known concentration, the amount of a drug or toxin in blood or tissue is measured, down to parts per million or parts per billion.

Example of a quantitative result

Boron shows peaks at m/z=10 (19.90\%) and m/z=11 (80.10\%). The relative atomic mass follows from the peak heights:

A_r=\dfrac{(10\times19.90)+(11\times80.10)}{100}=\dfrac{199.0+881.1}{100}=10.80

Conclusion

The positions of the peaks (m/z) identify substances, and the heights of the peaks measure amounts. Therefore mass spectrometry is used for both qualitative and quantitative analysis, for example to identify a toxin and to find its quantity in a victim's tissue.

ii7 marks

Evaluate the effectiveness of chromatography as a method for environmental monitoring of pollutants. What are the strengths and weaknesses in this application?

Solution

Introduction

Environmental monitoring means measuring pollutants in air, water and soil, such as pesticides, polycyclic aromatic hydrocarbons (PAHs), volatile organic compounds (VOCs) from fuels and solvents, and ions such as nitrate. The samples are complex mixtures and the pollutants are present in tiny amounts, so a method must separate them and measure very low levels. Chromatography does this well.

Techniques used

Technique	Pollutants monitored
Gas chromatography (GC), GC-MS	VOCs in air, pesticide residues, hydrocarbons from oil spills
HPLC, LC-MS	non-volatile pesticides, PAHs, drugs and dyes in water
Ion chromatography	anions and cations in water, such as \ce{NO3-}, \ce{SO4^2-}, \ce{Cl-}
TLC	quick field screening of samples before laboratory analysis

Strengths

Separation of complex mixtures: hundreds of compounds in one water or air sample can be separated in one run.
High sensitivity: with suitable detectors or MS, pollutants are detected at ppm to ppb levels, below legal limits.
Identification and quantification together: retention times (and mass spectra) identify each pollutant; peak areas give its concentration.
Small samples are enough, and many samples can be run automatically, which suits regular monitoring.
Versatility: gases, volatile liquids, non-volatile organics and ions can all be analysed by choosing the right technique.

Weaknesses

Cost and skill: GC-MS and HPLC instruments are expensive and need trained staff and regular maintenance.
Sample preparation: pollutants often have to be extracted and concentrated first, which is slow and may lose some analyte.
Limited range for each method: GC needs volatile, heat-stable compounds; non-volatile ones need HPLC.
Co-elution: two compounds with the same retention time can overlap, so a mass spectrometer is often needed to confirm.
Standards needed: identification by retention time requires pure reference compounds.
Not real-time: samples are usually taken to a laboratory, so results are delayed; solvents used create waste.

Evaluation

Chromatography, especially GC-MS and LC-MS, is the most effective and widely accepted method for monitoring organic pollutants, because it separates, identifies and measures them at very low levels in one analysis. Its weaknesses are mainly cost, time and sample preparation, not accuracy. For quick field checks, simple TLC or portable sensors are used first, and doubtful samples are then sent for full chromatographic analysis.

iii8 marks

Design an experiment to investigate the effectiveness of different chromatographic techniques in separating and identifying components of a mixture. Evaluate the experimental design, including the choice of chromatographic techniques, sample preparation methods, detection systems, and data analysis strategies. Discuss the potential challenges and limitations of the experiment.

Solution

Aim

To compare paper chromatography, thin layer chromatography and column chromatography for separating and identifying the dyes in a mixture of food colours (or the pigments of a leaf extract).

Materials

A mixture of three food dyes (for example red, yellow and blue) and the three pure dyes as standards; chromatography paper; silica gel TLC plates; a glass column with silica gel; solvents (for example a water and ethanol mixture, and a less polar mixture); capillary tubes; developing tanks with lids; pencil and ruler; test tubes; UV lamp.

Sample preparation

Dissolve a small amount of the mixture and of each standard in the same volatile solvent to give concentrated solutions.
Filter the solutions so that no solid particles block the paper, plate or column.
Use the same concentration and the same spot size for every run, so that results can be compared fairly.

Procedure

Paper and TLC: draw a pencil baseline 1.5\,\mathrm{cm} from the bottom. Spot the mixture and the three standards side by side. Develop in a closed, saturated tank with the solvent below the baseline. Mark the solvent front and dry.
Repeat paper and TLC runs with a second solvent of different polarity to test the effect of the mobile phase.
Column: pack a column with silica gel, add 1\,\mathrm{cm^3} of the mixture at the top and elute with the solvent. Collect equal fractions, note the time and volume at which each coloured band comes out, and evaporate the solvent.
Run each technique three times to check reproducibility.

Detection

Coloured dyes are seen directly. Colourless components are located under UV light (TLC plates with a fluorescent indicator) or by a spray reagent. Fractions from the column are checked by running them on TLC. If available, a GC or HPLC with a detector (and GC-MS) can be used to confirm the identities.

Data analysis

Calculate R_f=\dfrac{\text{distance moved by spot}}{\text{distance moved by solvent front}} for every spot, and compare with the standards to identify each component.
Compare the techniques by: the number of components separated, the sharpness and spacing of spots (resolution), time taken, reproducibility of R_f over three runs, and the amount of pure component recovered (column).
Record results in a table and give the mean and range of R_f values for each dye.

Evaluation of the design

The design is a fair test: the same mixture, concentrations and solvents are used, and only the technique is changed. Standards on the same plate allow direct identification. Expected result: TLC gives the best separation for analysis in the shortest time, paper is cheapest but gives broader spots, and column chromatography is slowest but is the only one that collects usable amounts of each pure dye.

Challenges and limitations

Two dyes may have very similar R_f values and overlap; a second solvent or two-way chromatography may be needed.
Temperature, humidity and tank saturation change R_f values, so they must be kept constant.
Overloaded spots streak and uneven column packing gives poor bands.
Identification by R_f alone is not certain; confirmation needs GC-MS or LC-MS, which are expensive.
Solvents can be flammable or toxic, so work in a fume cupboard and dispose of waste safely.

FBISE 2026 questions

Questions from the FBISE HSSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

2026 Annual I, Set B, MCQ 171 mark

A compound that strongly interacts with the stationary phase will:

AMove slowerAnswer
BMove faster
CHave R_f=1
DBe colourless

Why

A compound that is held strongly by the stationary phase spends most of its time there and moves forward only while it is in the mobile phase, so it moves slower and has a low R_f. R_f=1 is for a compound that does not interact at all and moves with the solvent front; colour has nothing to do with the interaction.

2026 Annual I, Set B, Q.2 (xiii) OR1.5+1.5 = 3 marks

Describe the following terms related to chromatography. a. Stationary phase b. Solvent Front

Solution

a. Stationary phase (1.5 marks)

The stationary phase is the phase that does not move during chromatography. It is a solid such as silica gel, alumina or paper, or a liquid coated on the surface of a solid. The components of the mixture are held back by it (by adsorption or by dissolving in it) to different extents: a component held strongly moves slowly. Example: the silica gel layer on a TLC plate, or the water in the fibres of chromatography paper.

b. Solvent front (1.5 marks)

The solvent front is the highest level reached by the solvent (mobile phase) as it moves up the paper or TLC plate. It is marked with a pencil line as soon as the chromatogram is removed from the tank, before the solvent evaporates. The distance from the baseline to the solvent front is the denominator of the R_f value:

R_f=\dfrac{\text{distance moved by solute from baseline}}{\text{distance moved by solvent front from baseline}}

2026 Annual I, Set D, MCQ 11 mark

The maximum distance traveled by the solvent is referred as:

AStarting point
BSolvent frontAnswer
CR_f value
DMobile phase

Why

The highest level the solvent reaches on the paper or plate is the solvent front. The starting point is the baseline, R_f is a ratio of distances, and the mobile phase is the solvent itself, not a distance.

2026 Annual I, Set D, Q.2 (xiii) OR1.5+1.5 = 3 marks

Describe the following terms related to chromatography. a. Mobile phase b. R_f value

Solution

a. Mobile phase (1.5 marks)

The mobile phase is the phase that moves over or through the stationary phase and carries the components of the mixture with it. It is a liquid (solvent or eluent) in paper, thin layer and column chromatography, and an inert gas such as helium or nitrogen (the carrier gas) in gas chromatography. A component that is more soluble in the mobile phase moves faster.

b. R_f value (1.5 marks)

The R_f value (retardation factor) is the ratio of the distance travelled by a solute from the baseline to the distance travelled by the solvent front from the baseline. It has no unit and lies between 0 and 1.

R_f=\dfrac{\text{distance moved by solute}}{\text{distance moved by solvent front}}\qquad\text{e.g. }\ \dfrac{6\,\mathrm{cm}}{10\,\mathrm{cm}}=0.60

Under the same conditions each compound has a fixed R_f, so comparing R_f values with standards identifies the components of a mixture. A low R_f shows a polar compound held strongly by the stationary phase.

Exam focus

Describe stationary phase, mobile phase, solvent front, baseline and R_f value. (short)
Asked in both 2026 new-syllabus sets (Q.2 (xiii) OR) and in two 2026 MCQs; the first SLO of the unit.
Go to answer
Calculate R_f values from a chromatogram and use them to identify the components of a mixture. (numerical)
The core numerical skill of the unit: measure from the baseline, divide, compare with standards.
Go to answer
Can an R_f value exceed 1? Give a reason. (short)
Book short question ii; a favourite 3-mark reasoning question.
Go to answer
Principle, working and applications of thin layer chromatography. (long)
A whole book section (19.2) and an SLO; three book exercise MCQs test it.
Go to answer
Applications of TLC in forensic chemistry; comparing two ink samples. (short)
SLO 2 and book short questions vi and vii.
Go to answer
Factors affecting R_f values; effect of solvent polarity. (short)
Section 19.3.1 and book short question viii on solvent choice.
Go to answer
Retention time: definition and how it identifies components. (short)
SLO 3 pairs retention time with R_f values; long versus short retention times are easy marks.
Go to answer
Selection of stationary and mobile phase. (short)
SLO 4 and section 19.4; the list of adsorbent and solvent properties is directly examinable.
Go to answer
Use of mass spectrometry with chromatography (GC-MS) in forensic analysis; advantages and disadvantages. (long)
SLO 5 and book long question i (qualitative or quantitative).
Go to answer
Differentiate between TLC and paper chromatography. (short)
Book short question iv; a table answer scores full marks quickly.
Go to answer