Mass Spectrometry

Learning Objectives (OCR Spec 2.1.2)

• Describe the basic stages of time-of-flight mass spectrometry
• Interpret mass spectra to determine relative isotopic masses and abundances
• Calculate relative atomic mass from a mass spectrum
• Identify the molecular ion peak (M⁺) and use it to deduce Mr
• Explain fragmentation peaks at a qualitative level
• Interpret characteristic isotope patterns for Cl₂, Br₂ and related molecules

What is Mass Spectrometry?

Mass spectrometry is an analytical technique that measures the mass-to-charge ratio (m/z) of ions. It allows chemists to:

• Identify isotopes and calculate relative atomic masses
• Determine the relative molecular mass of a compound
• Deduce structural information from fragmentation patterns
• Detect trace amounts of unknown substances (sensitivity down to 10⁻¹⁵ g)

A mass spectrometer produces a mass spectrum: a plot of relative abundance (y-axis) against m/z (x-axis). Because almost all ions formed have a single +1 charge, m/z is effectively equal to mass.

Mass spectrometry is arguably the most powerful analytical technique in modern chemistry. It is used routinely in:

• Forensic science (drug analysis, trace evidence)
• Anti-doping (detecting performance-enhancing drugs in athletes)
• Pharmaceutical research (confirming the identity and purity of new drugs)
• Space probes (the Cassini mission to Saturn carried a mass spectrometer)
• Archaeology (radiocarbon dating)
• Environmental science (detecting trace pollutants in water and air)

The Four Stages of Time-of-Flight (TOF) Mass Spectrometry

OCR expects students to know the general principle. There are four key stages:

flowchart LR
    A[Sample] --> B[1. Ionisation]
    B --> C[2. Acceleration]
    C --> D[3. Ion Drift]
    D --> E[4. Detection]
    E --> F[Mass Spectrum]

1. Ionisation

The sample is vaporised and ionised. Two methods are commonly used:

Electron impact (EI) ionisation — a high-energy electron beam (around 70 eV) bombards the vaporised sample, knocking an electron from each molecule:

$\text{X(g)} + e^- \rightarrow \text{X}^+\text{(g)} + 2e^-$X(g)+e−→X+(g)+2e−

EI ionisation is energetic and often causes fragmentation — useful for structural information but sometimes the molecular ion peak is weak.

Electrospray ionisation (ESI) — the sample is dissolved in a volatile solvent and sprayed through a fine needle held at high voltage. Each droplet evaporates, leaving protonated molecules:

$\text{X(g)} + \text{H}^+ \rightarrow \text{XH}^+\text{(g)}$X(g)+H+→XH+(g)

ESI is a "soft" ionisation method — it causes little fragmentation and is preferred for biological molecules like proteins.

Whichever method is used, the result is positive ions that can be accelerated.

2. Acceleration

The positive ions are accelerated through a potential difference towards a detector. All ions of the same charge gain the same kinetic energy, so lighter ions move faster than heavier ones.

$E_k = \tfrac{1}{2}mv^2$Ek​=21​mv2

At constant kinetic energy, rearranging gives $v = \sqrt{2E_k/m}$v=2Ek​/m​, i.e. velocity is inversely proportional to √m. A heavy ion moves more slowly than a light ion carrying the same energy.

3. Ion Drift

The ions pass through a field-free region of known length L. Lighter ions cover the distance in less time than heavier ions. The time of flight t is related to mass by:

$t = L\sqrt{\frac{m}{2E_k}}$t=L2Ek​m​​

So the time of flight is proportional to the square root of the mass. By measuring t precisely (nanoseconds), the mass can be determined with very high accuracy.

4. Detection

When an ion strikes the detector, it gains an electron, producing a tiny current. The size of the current gives relative abundance; the time of flight gives m/z. A computer converts this data into a mass spectrum.

The detector is typically an electron multiplier that amplifies each ion hit by a factor of 10⁶ or more, allowing detection of individual ions.

Reading a Mass Spectrum

Each peak represents an ion:

• x-axis (m/z) = mass-to-charge ratio — effectively mass for +1 ions
• y-axis = relative abundance (percentage or arbitrary units)

The base peak is the tallest peak, which is usually assigned 100% relative abundance. All other peaks are expressed as a percentage of the base peak.

Example: Chlorine (Cl₂)

Chlorine gas shows peaks at m/z = 35, 37, 70, 72 and 74. Why?

• 35 and 37 → atomic ions Cl⁺ from the two isotopes ³⁵Cl⁺ and ³⁷Cl⁺ (from fragmentation)
• 70 = ³⁵Cl–³⁵Cl⁺ (both light isotopes)
• 72 = ³⁵Cl–³⁷Cl⁺ (one of each — statistically most likely, so tallest)
• 74 = ³⁷Cl–³⁷Cl⁺ (both heavy isotopes)

The ratio of peaks at 70 : 72 : 74 is approximately 9 : 6 : 1, reflecting the isotope abundances (~75% ³⁵Cl, ~25% ³⁷Cl).

Calculating the 9 : 6 : 1 ratio:

If the abundance of ³⁵Cl is 0.75 and ³⁷Cl is 0.25, the probabilities of each Cl₂ combination are:

• ³⁵Cl–³⁵Cl: 0.75 × 0.75 = 0.5625
• ³⁵Cl–³⁷Cl or ³⁷Cl–³⁵Cl: 2 × 0.75 × 0.25 = 0.375
• ³⁷Cl–³⁷Cl: 0.25 × 0.25 = 0.0625

Ratio = 0.5625 : 0.375 : 0.0625 = 9 : 6 : 1 ✓

Example: Bromine (Br₂)

Bromine has two isotopes at roughly 50% each (⁷⁹Br and ⁸¹Br). So Br₂ shows peaks at m/z = 158, 160 and 162 in a ratio of 1 : 2 : 1. This distinctive "triplet" is a fingerprint for bromine-containing compounds.

Calculating Relative Atomic Mass from a Mass Spectrum

Worked Example 1: Neon

A mass spectrum of neon shows:

m/z	Relative Abundance
20	90.9
21	0.3
22	8.8

$A_r = \frac{(20 \times 90.9) + (21 \times 0.3) + (22 \times 8.8)}{90.9 + 0.3 + 8.8}$Ar​=90.9+0.3+8.8(20×90.9)+(21×0.3)+(22×8.8)​