Mass Spectrometry

Mass spectrometry is a powerful analytical technique used to determine the relative atomic mass of elements, identify isotopes, and determine the molecular mass of compounds. At A-Level, you need to understand the time-of-flight (TOF) mass spectrometer in detail.

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Principles of Mass Spectrometry

A mass spectrometer separates particles based on their mass-to-charge ratio (m/z). The key stages in a time-of-flight mass spectrometer are:

1. Ionisation
2. Acceleration
3. Ion drift (flight tube)
4. Detection

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Stage 1: Ionisation

The sample must be converted into gaseous ions. There are two main methods at A-Level:

Electrospray Ionisation (ESI)

• The sample is dissolved in a volatile solvent.
• The solution is passed through a fine hypodermic needle at high voltage.
• As the solvent evaporates, the sample molecules gain a proton (H⁺) from the solvent to form positive ions: M + H⁺ → MH⁺
• This is a "soft" ionisation technique — it causes minimal fragmentation.
• The ions formed typically have a charge of +1, so m/z ≈ M + 1.

Electron Impact Ionisation (EI)

• The sample is vaporised and then bombarded with high-energy electrons from an electron gun.
• An electron is knocked out of the sample molecule: M(g) → M⁺(g) + e⁻
• This is a "hard" ionisation technique — it can cause the molecule to fragment.
• The ion M⁺ is called the molecular ion (also called the parent ion).

Key Point: Electrospray ionisation produces [M+H]⁺ ions (molecular mass + 1), while electron impact produces M⁺ ions (molecular mass). You must account for this difference when reading spectra.

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Stage 2: Acceleration

The positive ions are accelerated by an electric field. All ions are given the same kinetic energy (KE).

The kinetic energy equation is:

KE = ½mv²

Since all ions have the same KE, lighter ions will have a greater velocity:

v = √(2KE / m)

This means ions with a smaller mass travel faster through the flight tube.

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Stage 3: Ion Drift (Flight Tube)

The ions enter a field-free region (the flight tube or drift region). There is no electric or magnetic field here, so the ions travel at constant velocity. Lighter ions reach the detector first because they are moving faster.

The time of flight is given by:

t = d / v

where d is the length of the flight tube and v is the velocity of the ion.

Substituting v = √(2KE / m):

t = d / √(2KE / m) = d × √(m / 2KE)

Therefore: t² = d²m / (2KE)

Rearranging: m = 2KE × t² / d²

Since KE and d are constants for a given experiment, the mass is proportional to the square of the time of flight: m ∝ t².

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Stage 4: Detection

When ions hit the detector, they generate an electrical signal. The size of the signal is proportional to the number of ions (abundance). The detector records both the time of flight (which gives m/z) and the abundance of each ion.

The data is displayed as a mass spectrum: a bar chart with m/z on the x-axis and relative abundance (or relative intensity) on the y-axis.

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Interpreting Mass Spectra of Elements

Reading a Mass Spectrum

Each peak in the mass spectrum of an element represents a different isotope. The position on the x-axis gives the mass-to-charge ratio (m/z), and the height gives the relative abundance.

For singly charged ions (z = 1), the m/z value equals the isotopic mass.

Worked Example 1: Calculating Aᵣ from a Mass Spectrum

A mass spectrum of neon shows:

• m/z = 20, relative abundance = 90.5
• m/z = 21, relative abundance = 0.3
• m/z = 22, relative abundance = 9.2

Calculate the relative atomic mass of neon.

Step 1: Multiply each mass by its abundance.

20 × 90.5 = 1810.0 21 × 0.3 = 6.3 22 × 9.2 = 202.4

Step 2: Sum the products.

1810.0 + 6.3 + 202.4 = 2018.7

Step 3: Divide by the total abundance.

Total abundance = 90.5 + 0.3 + 9.2 = 100.0

Aᵣ = 2018.7 / 100.0

Aᵣ = 20.2 (to 1 decimal place)

Worked Example 2: Calculating Aᵣ of Magnesium