Edexcel International A Level Chemistry: Spectroscopy and chromatography (YCH11) – Revision Notes

For full explanations and longer worked examples, read the spectroscopy and chromatography study guide first; this page is the short version for final revision.

Included here: Topic 15E: Spectroscopy and chromatography, outcomes 15.17 to 15.23, of the Pearson Edexcel International Advanced Level in Chemistry (YCH11) specification, Issue 1, September 2017. This is International A2 (IA2) material, assessed in Unit 4, whose title is Rates, Equilibria and Further Organic Chemistry. Unit 2, Topic 10D is assumed knowledge; recap it with the Topic 10D notes on mass spectra and IR.

Test yourself afterwards with the spectroscopy and chromatography practice set. For other topics, open the course hub for Edexcel IAL Chemistry or print the chemistry checklist for Edexcel. Unsure where you stand? Try a free 10-minute diagnostic.

Key terms

Term	Meaning
Accurate relative molecular mass	Mr worked out from atomic masses given to four decimal places; it identifies a molecular formula
Chemical shift, δ	Position of an NMR peak in ppm, measured from TMS at δ = 0
Environment	A set of equivalent atoms (related by symmetry); each environment gives one peak
Relative peak area	In ¹H NMR, proportional to the number of H atoms in that environment
(n+1) rule	A ¹H peak with n non-equivalent H atoms on adjacent carbons splits into n + 1 lines
Mobile phase	The phase that moves and carries the mixture (solvent or gas)
Stationary phase	The phase that stays put (water held in paper, a silica or alumina layer, column packing)
Rf value	Ratio of how far a spot travels to how far the solvent front travels, each measured from the baseline
Retention time	Time a component takes to pass through a chromatography column to the detector

15.17 Accurate mass in steps

1. Write the molecular formula.
2. Multiply each atomic mass (four decimal places) by its number of atoms.
3. Add the terms. Keep four decimal places throughout.
4. To identify an unknown: calculate each candidate formula and pick the one closest to the measured m/z.
5. Then suggest structures that fit that formula, using other evidence (IR, reactions, NMR) to choose between isomers.

Reminder: carbon-12 is exactly 12.0000. Values used in these notes: H 1.0078, N 14.0031, O 15.9949.

Isomers have the same accurate mass. Accurate mass gives the formula, never the structure on its own.

15.18 and 15.19 Carbon-13 NMR

• ¹³C NMR shows the positions of ¹³C atoms in a molecule: one peak for each carbon environment.
• Number of peaks = number of different carbon environments, not the number of carbons.
• Symmetry reduces the count. Two CH₃ groups on the same carbon, or the two halves of a symmetrical ketone, count as one environment each.
• The δ value shows the type of carbon. Rough guide used in these notes (always use the data in the question):

Carbon type	Approximate δ / ppm
Alkyl C	5–45
C bonded to O	50–90
Alkene or benzene ring C	110–160
Carbonyl C, acid or ester	160–185
Carbonyl C, aldehyde or ketone	190–220

Justify a peak count in three moves: list each environment, state which carbons are equivalent and why, then give the total.

15.20 Proton NMR

Reading a spectrum, step by step

1. Count the peaks (low or high resolution): number of proton environments.
2. Read δ for each to decide the proton type.
3. Use the area ratio to get the number of H atoms in each environment. Scale the ratio so it adds up to the H count in the formula.
4. Read the splitting (high resolution): lines − 1 = H atoms on adjacent carbons.
5. Assemble fragments and check the total atoms against the formula.

Approximate ¹H δ / ppm	Proton type
0.7–1.7	Alkyl C–H
2.0–2.7	H on a carbon joined to C=O
3.3–4.5	H–C bonded to O or Cl
1.0–5.0 (variable)	Alcohol O–H
4.5–6.5	Alkene C–H
9.0–10.0	Aldehyde CHO
10.0–12.0	Carboxylic acid COOH

Splitting patterns to recognise

Fragment	What you see
CH₃CH₂– (ethyl)	CH₃ triplet (3H) and CH₂ quartet (2H)
(CH₃)₂CH– (isopropyl)	CH₃ doublet (6H) and CH septet (1H)
CH₃ on C=O or on a carbon with no H	singlet (3H)
(CH₃)₃C– (tert-butyl)	singlet (9H)
–CH₂CH₂– between two groups with no H	two triplets (if the CH₂ groups are not equivalent)

Rules that matter:

• Count H on adjacent carbons only.
• Equivalent H atoms do not split each other.
• An O–H peak is normally a singlet and does not split neighbouring peaks.
• High resolution shows splitting; low resolution shows only one peak per environment with its area.

15.21 and 15.22 Paper and thin-layer chromatography

• Separation depends on the balance between attraction to the stationary phase and solubility in the mobile phase.
• Paper: the stationary phase is water held in the paper. TLC: a thin layer of silica or alumina on a plate.
• Rf = distance to spot centre ÷ distance to solvent front, both from the baseline. No units; between 0 and 1.
• A substance held strongly by the stationary phase (for example a polar compound on silica) has a low Rf. A substance very soluble in the solvent has a high Rf.
• Rf changes with the solvent, the stationary phase and the temperature. Compare unknowns and standards on the same plate.

Worked reminder: solvent front 9.0 cm, spot centre 3.6 cm. Rf = 3.6 ÷ 9.0 = 0.40.

15.23 HPLC, GC and mass spectrometry

	HPLC	GC
Type	Column chromatography	Column chromatography
Mobile phase	Liquid under high pressure	Unreactive carrier gas
Stationary phase	Solid particles packed in a column	Liquid on a solid support or column wall
Separates by	Different retention times	Different retention times

• Retention time depends on conditions, so compare with a standard run under the same conditions.
• Linked to a mass spectrometer (GC–MS, HPLC–MS), each component that leaves the column gives a mass spectrum for database matching.
• Applications in the specification: forensics and drug testing in sport.

Must-know distinctions

• Accurate mass vs nominal mass: accurate mass separates formulae; nominal mass does not.
• Peaks vs lines: a peak is one environment; at high resolution that peak may be split into several lines.
• Area vs splitting: area gives H atoms in that environment; splitting gives H atoms on the neighbouring carbons.
• ¹³C vs ¹H: carbon peaks count carbon environments; proton peaks count proton environments, and the two numbers can differ.
• Rf vs retention time: Rf is a ratio from a flat chromatogram; retention time is a time from a column.

Quick self-test

1. Use H 1.0078, N 14.0031 and C 12.0000 to calculate the accurate Mr of C₂H₇N.
2. A molecular ion appears at m/z 59.0372. Using the masses above plus O 15.9949, decide between C₃H₉N and C₂H₅NO.
3. How many peaks are in the ¹³C NMR spectrum of 3,3-dimethylpentane?
4. Which type of carbon gives a ¹³C signal at δ 198?
5. Give the number of peaks and the area ratio in the ¹H NMR spectrum of 2-methylpropan-2-ol, (CH₃)₃COH.
6. Predict the splitting of each peak in the ¹H NMR spectrum of 1,1,2-trichloroethane, CHCl₂CH₂Cl.
7. A peak at δ 9.8 is a triplet. What does this tell you?
8. A spot travels 3.3 cm while the solvent front travels 6.0 cm. Calculate Rf.
9. A compound has Rf 0.36. The solvent front moves 5.5 cm. How far from the baseline is the spot?
10. Name the mobile phase in gas chromatography.
11. Why is GC–MS used in drug testing rather than GC alone?

Answers

1. 2(12.0000) + 7(1.0078) + 14.0031 = 45.0577
2. C₃H₉N = 59.0733; C₂H₅NO = 59.0370. The closer match is C₂H₅NO.
3. Four: the central carbon, the two CH₃ groups on it, the two CH₂ groups, and the two end CH₃ groups.
4. A C=O carbon in an aldehyde or ketone.
5. Two peaks, area ratio 9 : 1 (nine equivalent CH₃ hydrogens, one O–H).
6. The CHCl₂ proton has two neighbours: triplet. The CH₂Cl protons have one neighbour: doublet. Area ratio 1 : 2.
7. An aldehyde CHO proton on a carbon next to a CH₂ group (two neighbours, so three lines).
8. 3.3 ÷ 6.0 = 0.55
9. 0.36 × 5.5 = 1.98, so about 2.0 cm.
10. An unreactive carrier gas (for example helium or nitrogen).
11. Different substances can share a retention time. The mass spectrum of each separated component allows it to be identified by comparison with a database.

Where marks are usually lost

• Writing the accurate Mr to fewer than four decimal places, or rounding each term before adding.
• Picking a molecular formula from whole-number mass when accurate masses are given.
• Claiming accurate mass can tell isomers apart.
• Giving the number of carbons as the number of ¹³C peaks in a symmetrical molecule.
• Stating a peak count without naming the environments when asked to justify it.
• Using the H atoms on the carbon itself in the (n+1) rule.
• Splitting an O–H peak, or splitting a CH₃ that sits next to C=O.
• Not scaling a simplified area ratio to the total number of H atoms.
• Measuring Rf from the bottom edge of the plate, or giving Rf with units.
• Describing retention time as a distance, or saying a matching retention time proves identity.

Official syllabus

Specification followed: Pearson Edexcel YCH11, International Advanced Subsidiary/Advanced Level in Chemistry, Issue 1 of September 2017, Pearson Education Limited. Topic 15E: Spectroscopy and chromatography, outcomes 15.17 to 15.23, assessed in Unit 4.