9701 Chemistry · Topics 22 & 36 · AS + A Level
Analytical Techniques Cheat Sheet — A Level Chemistry 9701
Spectroscopy questions are marks for interpretation, not recall — provided you know which peak means what. This sheet covers reading a mass spectrum including M+1 and M+2 peaks, the infrared wavenumbers worth memorising, proton and carbon-13 NMR with chemical shifts, splitting patterns and the n+1 rule, plus chromatography and the qualitative inorganic tests.
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What’s on this cheat sheet
Analytical Techniques
01 · Mass spectrometry — reading it
x‑axis m/z, y‑axis relative abundance. The peak at the highest m/z is the molecular ion M⁺, and its value is the Mr.
Peaks below it are fragments, formed when M⁺ breaks apart. The base peak is simply the tallest, from the most stable fragment.
02 · Common fragments
| m/z | Fragment |
|---|---|
| 15 | CH₃⁺ |
| 17 | OH⁺ |
| 29 | C₂H₅⁺ or CHO⁺ |
| 43 | C₃H₇⁺ or CH₃CO⁺ |
| 45 | COOH⁺ |
Work out the difference between M⁺ and a fragment — a loss of 15 means a methyl group, 17 an –OH, 29 an ethyl or CHO.
03 · The M+1 and M+2 peaks
M+1 comes from the 1.1 % of carbon that is ¹³C. Its relative height divided by 1.1 gives the number of carbon atoms.
M+2 reveals a halogen: chlorine gives M and M+2 in a 3 : 1 ratio, bromine in a 1 : 1 ratio, because of their two abundant isotopes.
04 · Infrared spectroscopy
Bonds absorb IR at frequencies that make them vibrate — stretching and bending. Each bond has a characteristic wavenumber, so the spectrum shows which groups are present.
The x‑axis runs right to left in cm⁻¹ and the peaks point downwards as transmittance falls. Below 1500 cm⁻¹ is the fingerprint region — unique to a compound, matched against a database rather than interpreted peak by peak.
05 · IR absorptions to recognise
| Bond | cm⁻¹ | Shape |
|---|---|---|
| O–H acid | 2500–3300 | very broad |
| O–H alcohol | 3200–3600 | broad |
| N–H amine | 3300–3500 | sharp, often double |
| C–H | 2850–3100 | sharp |
| C≡N | 2200–2250 | sharp |
| C=O | 1680–1750 | strong, sharp |
| C=C | 1620–1680 | weak |
A very broad O–H with a strong C=O means a carboxylic acid; a C=O with no O–H means an aldehyde, ketone or ester.
06 · Worked example — IR plus MS
Broad O–H + C=O → carboxylic acid
Loss of 15 from 60 → a CH₃ group
45 = COOH⁺
→ ethanoic acid, CH₃COOH
07 · NMR — the principle A2
Nuclei with an odd mass number, ¹H and ¹³C, behave as tiny magnets. In a strong field they absorb radio waves as they flip alignment; the frequency depends on the electron environment around them.
TMS is the reference at δ = 0 — inert, volatile, non‑toxic, and its twelve equivalent protons give one sharp peak. Samples dissolve in CDCl₃, which has no ¹H to interfere.
08 · Reading a ¹H spectrum A2
Number of peaks = number of different proton environments.
Chemical shift δ = the type of environment; electronegative neighbours shift protons downfield.
Integration = the ratio of protons in each environment.
Splitting = how many protons are on the neighbouring carbon.
09 · The n+1 rule A2
| Neighbours | Peak |
|---|---|
| 0 | singlet |
| 1 | doublet |
| 2 | triplet |
| 3 | quartet |
A quartet next to a triplet in a 2 : 3 ratio is the signature of an ethyl group, CH₃CH₂–.
10 · Chemical shifts to know A2
| Proton | δ / ppm |
|---|---|
| R–CH₃ | 0.9 |
| CH next to C=O | 2.0–2.9 |
| CH next to O | 3.3–4.3 |
| aromatic H | 6.5–8.0 |
| CHO | 9.3–10.5 |
| COOH | 10–12 |
Values come from the data booklet — learn the pattern, not the numbers.
11 · O–H and N–H protons A2
These appear as broad singlets at variable shift and do not split their neighbours. Shaking the sample with D₂O exchanges them for deuterium and the peak disappears — the standard way to confirm an –OH or –NH.
12 · Worked example — NMR A2
Triplet + quartet in 3 : 2 → an ethyl group
Broad singlet at 11.5, D₂O exchangeable → COOH
CH₃CH₂ + COOH = C₃H₆O₂ ✓
→ propanoic acid, CH₃CH₂COOH
13 · ¹³C NMR A2
Each peak is one carbon environment, with no splitting and no useful integration. It is the quickest way to count how many distinct carbons a molecule has, and to tell symmetrical isomers apart: propanone gives two peaks, propanal three.
Marks lost here
— Reading the base peak as the molecular ion.
— Trying to interpret individual peaks in the fingerprint region.
— Applying the n+1 rule to protons on the same carbon rather than the neighbouring one.
— Forgetting that an –OH peak is broad, unsplit and removed by D₂O.
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Analytical Techniques — Frequently Asked Questions
What does the n+1 rule mean in NMR?
A proton environment with n equivalent protons on the adjacent carbon is split into n+1 peaks. So a CH₃ next to a CH₂ appears as a triplet, and the CH₂ next to a CH₃ appears as a quartet.
How do you spot a bromine or a chlorine from a mass spectrum?
Look at the M+2 peak. Chlorine gives an M:M+2 ratio of about 3:1 and bromine gives about 1:1, because of the natural abundances of ³⁵Cl/³⁷Cl and ⁷⁹Br/⁸¹Br.
Which infrared absorptions should I memorise?
O–H in an alcohol is a broad band at 3200–3600 cm⁻¹, O–H in a carboxylic acid is very broad at 2500–3300 cm⁻¹, C=O is a sharp strong peak at 1680–1750 cm⁻¹ and N–H appears at 3300–3500 cm⁻¹. The fingerprint region below 1500 cm⁻¹ is used for identification by comparison only.
