Interpreting Chromatograms

Section 1 of 6Reading a chromatogram

Number of spots

• Number of spots A pure substance gives one spot only.
• A mixture separates into two or more spots, one for each component.

Matching spots

• Matching spots If two substances are the same, they produce spots at the same height.
• A known reference sample is often run alongside an unknown one to make comparison easy.

Aligned spots

• Aligned spots Spots in a horizontal line across the paper are probably the same substance, even if the colours cannot be seen in black and white diagrams.

Section 2 of 6Example conclusions

Observation	Conclusion
Brown ink gives three spots	The ink is a mixture of three dyes
Red, yellow and blue references each give one spot	Each reference is a pure substance
Brown ink spots line up with red, yellow and blue spots	The brown ink contains all three of those dyes
A spot that does not line up with any reference	It is a different substance not among the references
A spot that stays on the start line	That substance is insoluble in the solvent used

Section 3 of 6Calculating Rf

1

Use a ruler to measure from the start line to the centre of the spot: this is the distance moved by the substance.

2

Measure from the same start line to the solvent front: the distance moved by the solvent.

3

Use R_f = distance moved by substance ÷ distance moved by solvent.

4

Give the answer as a number between 0 and 1 with no units.

5

Example: a spot 3 cm up the paper when the solvent has moved 6 cm gives R_f = 3 ÷ 6 = 0.5.

Section 4 of 6Using Rf to identify substances

1Measure R_f

• Measure R_f Calculate the R_f for each unknown spot.

2Compare with reference

• Compare with reference Look up the R_f values of known substances measured with the same solvent.

3Match

• Match A match in R_f means the substances are probably the same.

4Same conditions only

• Same conditions only R_f changes if the solvent changes, so values are only comparable under the same conditions.

Section 5 of 6Key terms

Term	Meaning
Chromatogram	The paper showing the separated substances after chromatography.
Rf value	The retention factor: distance moved by substance divided by distance moved by solvent.
Solvent front	The highest point reached by the solvent on the paper.
Reference substance	A known sample used for comparison.

Section 6 of 6Exam technique

1

Measuring the substance distance to the top of the spot instead of its centre.

2

Measuring distances from the bottom edge of the paper rather than from the start line.

3

Giving R_f a unit such as cm, or a value greater than 1.

4

Comparing R_f values from two chromatograms run in different solvents.