Question 1

How do I calculate Rf value in TLC?

Accepted Answer

To calculate the Rf value, measure the distance from the baseline (where the sample was spotted) to the centre of the spot after development. Then measure the distance from the baseline to the solvent front. Divide the first distance by the second. For example, if a spot moved 3.2 cm and the solvent front moved 8.0 cm, Rf = 3.2/8.0 = 0.40. Always use the same units and record the solvent system used.

Question 2

What is the difference between GC and HPLC?

Accepted Answer

Gas chromatography (GC) uses a gaseous mobile phase (e.g., helium) and is suitable for volatile compounds that can be vaporised without decomposition. High-performance liquid chromatography (HPLC) uses a liquid mobile phase under high pressure and is ideal for non-volatile or thermally unstable compounds. GC columns are typically longer and narrower, while HPLC columns are shorter and packed with smaller particles. Both techniques provide high resolution and are used for quantitative analysis.

Question 3

Why do some compounds have the same Rf value in TLC?

Accepted Answer

If two compounds have identical Rf values under the same conditions, they are likely the same compound. However, different compounds can occasionally have similar Rf values, especially if they have similar polarities. To confirm identity, you can use a different solvent system or a more sensitive technique like GC-MS. Always run a known standard alongside your sample for comparison.

Question 4

How does temperature affect gas chromatography?

Accepted Answer

Temperature affects the vapor pressure and partitioning of compounds in GC. Increasing temperature generally reduces retention times because compounds spend more time in the mobile phase. However, if the temperature is too high, separation may be poor (low resolution). Temperature programming (starting low and increasing gradually) is often used to separate mixtures with a wide range of boiling points, giving good resolution for early eluting compounds and reasonable times for later ones.

Question 5

What is the stationary phase in TLC made of?

Accepted Answer

The stationary phase in TLC is usually a thin layer of silica gel (SiO2) or alumina (Al2O3) coated onto a glass, plastic, or aluminium plate. Silica gel is polar and is used for separating polar compounds. For non-polar compounds, a reversed-phase plate (e.g., C18-bonded silica) may be used. The choice of stationary phase depends on the nature of the compounds to be separated.

Question 6

Can chromatography be used to identify unknown substances?

Accepted Answer

Yes, chromatography can help identify unknown substances by comparing their retention times (GC/HPLC) or Rf values (TLC) with those of known standards under identical conditions. However, conclusive identification often requires additional techniques like mass spectrometry (GC-MS or LC-MS) because different compounds can have similar retention characteristics. In A-level chemistry, you are expected to use retention data and Rf values for identification, but you should be aware of limitations.

Chromatography (A-level only)

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

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Common Mistakes

Frequently Asked Questions

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