Organic Analytical Techniques

Revise Organic Analytical Techniques for Chemistry 4 (IAL) WCH14 (A2 Level) — revision notes and instant AI marking.

📖 Revision notes · preview

Edexcel IAL Chemistry · Unit 4

Organic Analytical Techniques

Every organic molecule leaves behind a unique set of "fingerprints" — in its mass, in how its atoms wobble in a magnetic field, and in how it sticks to different surfaces — and chemists use Mass Spectrometry, NMR, and Chromatography to read those fingerprints and work out exactly what the molecule is.

High-Res Mass Spec ¹³C NMR Low-Res ¹H NMR High-Res ¹H NMR TLC HPLC & GC

Quick Summary — What This Chapter Is Really About

Think of this whole chapter as a detective toolkit. You're given an unknown organic compound and you need to identify it. Each technique gives you a different clue:

High Resolution Mass Spectrometry — tells you the exact molecular formula (to 4 decimal places of mass), letting you rule out "look-alike" formulas that have almost the same mass.
¹³C NMR — counts how many different environments exist, and the chemical shift tells you what's attached to each carbon (C=O, C-O, C-Cl, etc).
Low resolution ¹H NMR — counts how many different environments exist, and the peak area ratio tells you how many H atoms are in each environment.
High resolution ¹H NMR — zooms in further: the splitting pattern of each peak (singlet, doublet, triplet, quartet…) tells you how many H atoms are on the carbon.
TLC (Thin-Layer Chromatography) — separates a mixture into spots and uses the R_f value to identify each component by comparing distance travelled.
HPLC & GC — more powerful, automated versions of chromatography that separate mixtures and use to identify each component, often used in forensics and drug testing.

By the end, you should be able to look at a set of spectra (MS + NMR) and the full structure of an unknown molecule piece by piece — that's the classic exam question in this topic.

1. High Resolution Mass Spectrometry

Why "high resolution" matters

At AS level, mass spectrometry gave you the molecular ion peak (M⁺) rounded to the nearest whole number, which tells you the approximate M_r. The problem is that lots of different molecular formulas can add up to almost the same whole-number mass. For example, both C₄H₁₀ and C₃H₆O round to an M_r of "58" if you're only working to whole numbers.

Think of it like weighing three people on a bathroom scale that only shows whole kilograms — they might all read "70 kg" even though their actual weights are 69.6 kg, 70.1 kg, and 70.4 kg. A more precise scale (high resolution MS) would separate them instantly.

A high resolution mass spectrometer measures mass accurate to 4 decimal places. Because every element's isotope has a very slightly different exact mass (not a nice round number), formulas that look identical at low resolution become distinguishable at high resolution.

The accurate atomic masses you need

Element	Symbol	Accurate A_r
Hydrogen	H	1.0078
Carbon	C	12.0000
Nitrogen	N	14.0031
Oxygen	O	15.9949

Notice carbon is exactly 12.0000 — that's because the entire relative atomic mass scale is against carbon-12. Every other element's mass is ever-so-slightly off a whole number, and that's exactly the "wobble" high resolution MS is designed to detect.

Method

Accurate M_r = Σ (number of each atom × its accurate A_r)