4.9.6Unit 4. Rates, Equilibria and Further Organic Chemistry
Chromatography – HPLC and GC — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 56 of 56 Unit quiz
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Section 1 of 5Gas Chromatography (GC)
Mobile phase
- Mobile phase An unreactive carrier gas such as helium or nitrogen sweeps the vaporised sample through the column.
Stationary phase
- Stationary phase A high-boiling liquid coated on a solid support inside a long thin column held in a temperature-controlled oven.
Separation
- Separation Components that dissolve more in the liquid or have higher boiling points spend longer in the column than those staying in the gas.
Detector
- Detector A detector at the exit produces a peak for each component as it emerges, and the trace is recorded against time.
Scope
- Scope Only substances that can be vaporised without decomposing can be analysed, so it suits volatile organic compounds.
Section 2 of 5High Performance Liquid Chromatography (HPLC)
Mobile phase
- Mobile phase A liquid solvent is forced through the column at high pressure by a pump, which speeds up the separation.
Stationary phase
- Stationary phase The column is packed with very fine solid particles, often silica, giving a very large surface area.
Separation
- Separation Components are separated according to how strongly they are attracted to the packing compared with the solvent.
Scope
- Scope It can be used for non-volatile or heat-sensitive compounds such as amino acids, drugs and food additives.
Detection
- Detection Many instruments pass ultraviolet light through the exit stream and record how much each component absorbs.
Section 3 of 5Reading the Results
Reading the Results
Retention time
- Retention time The time from injection to the peak maximum is characteristic of a compound under fixed conditions and is compared with standards.
Peak area
- Peak area The area under a peak is proportional to the amount of that compound, once calibrated against samples of known concentration.
Resolution
- Resolution Well separated peaks give accurate areas, and conditions such as column temperature or solvent mixture can be adjusted to improve this.
GC-MS
- GC-MS Linking a gas chromatograph to a mass spectrometer lets each separated component be identified from its mass spectrum.
| Feature | GC | HPLC |
|---|---|---|
| Mobile phase | Inert gas | Liquid under high pressure |
| Sample must be | Volatile and stable when heated | Soluble in the solvent |
| Output | Retention times and peak areas | Retention times and peak areas |
Section 4 of 5Worked Example
Question
- A GC trace of a mixture shows three peaks with areas 6.0, 9.0 and 15.0 units.
- Assuming the detector responds equally to all three compounds, find the percentage of the second compound.
- Total area = 6.0 + 9.0 + 15.0 = 30.0 units.
- Fraction for the second compound = 9.0 / 30.0 = 0.30.
- Percentage = 0.30 x 100 = 30%.
Answer 30% of the mixture (by the stated assumption)
Section 5 of 5Exam Checklist & Tips
4.9.6 – Key Points for the Exam
- Retention time Time taken for a component to pass through the column to the detector.
- Carrier gas The unreactive gas that acts as the mobile phase in GC.
- Calibration Running standards of known concentration to relate peak area to amount.
- GC-MS Gas chromatography coupled to mass spectrometry for identification.
Common Mistakes
- Saying that all components in a mixture have equal peak heights; it is the area that relates to quantity.
- Using GC for compounds that decompose on heating.
- Thinking the same retention time proves identity under any conditions;
- it is only valid for the same column and settings.
- Describing the carrier gas as taking part in the reaction; it is inert.
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