📋 Structured Questions
Question 1: Thin-Layer Chromatography (TLC) of Amino Acids
6 marks(a) State two practical precautions that must be taken when setting up and running a TLC plate to ensure accurate results. [2]
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Any two from:
- Draw the starting line in pencil (not ink, as ink dyes will dissolve and run/chromatograph, interfering with the results) [1]
- Ensure the mobile phase solvent level in the beaker is below the pencil start line (so that the sample spots do not dissolve directly into the solvent pool) [1]
- Cover the beaker/tank with a lid (to prevent evaporation of the solvent and keep the tank atmosphere saturated with solvent vapor) [1]
- Do not touch the silica surface of the plate with bare fingers (to avoid transferring skin oils onto the adsorbent layer) [1]
(b) Explain, in terms of intermolecular forces, why different amino acids have different Rf values on a silica TLC plate. [3]
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- The stationary phase (silica) is highly polar, while the mobile phase solvent has a different polarity [1]
- Amino acids with more polar side chains (capable of forming stronger hydrogen bonds or dipole-dipole attractions) will adsorb more strongly to the polar silica stationary phase [1]
- These highly polar amino acids spend less time dissolved in the mobile phase, travel a shorter distance, and thus have lower Rf values than less polar amino acids [1]
(c) Explain why Rf values are useful but cannot be relied on as the sole method of identification. [1]
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Different chemical compounds can have identical Rf values in a particular solvent system [1]
Question 2: Column Chromatography
4 marks(a) Describe the stationary and mobile phases used in column chromatography. [2]
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- Stationary phase: A polar solid adsorbent (usually silica gel or alumina powder) packed inside a vertical glass tube [1]
- Mobile phase: A liquid solvent (or mixture of solvents) called the eluent, which is poured into the top of the column and moves down by gravity [1]
(b) Explain how column chromatography is used to separate and isolate the components of a mixture in bulk. [2]
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- The mixture is loaded onto the top of the packed column and solvent is added continuously [1]
- As the components move down the column at different rates (due to differences in their adsorption/solubility), they exit (elute) at the bottom at different times, allowing them to be collected in separate flasks [1]
Question 3: Gas Chromatography and GC-MS
5 marks(a) State the information that is obtained from the peak areas and the retention times in a gas chromatogram. [2]
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- Retention time: Identifies each compound present (by comparing retention times to database standard values run under identical temperature/flow-rate conditions) [1]
- Peak area: Quantifies the relative amount / concentration of each compound in the mixture [1]
(b) Explain how coupling mass spectrometry (MS) with gas chromatography (GC) enhances the identification of compounds in a mixture. [3]
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- The gas chromatograph (GC) separates the complex mixture into its individual, pure gaseous components [1]
- As each component elutes, it immediately enters the mass spectrometer (MS) where it is ionised and fragmented into smaller ions [1]
- The MS records a unique mass spectrum (containing the molecular ion peak for molar mass and a fragmentation pattern), which acts as a molecular fingerprint that can be compared directly to a computer database for definitive identification [1]
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