How are atoms ionised in a mass spectrometer?

Bombardment with high-energy electrons, which knock out an electron to form positive ions.

Why is a mass spectrometer kept under a vacuum?

So that ions do not collide with air molecules, which would stop them reaching the detector.

What does m/z stand for?

Mass-to-charge ratio: the relative mass of an ion divided by its charge.

What does the height of a peak show?

The relative abundance of that ion or isotope in the sample.

Which ions are deflected most in a magnetic field?

Lighter ions, with lower m/z, for the same charge.

Write the equation to calculate relative atomic mass from a spectrum.

Ar = Σ(isotope mass × abundance) ÷ Σ abundances.

A sample has peaks at 20 (90 %) and 22 (10 %). What is Ar?

(20 × 90 + 22 × 10) ÷ 100 = 20.2.

How is the relative molecular mass found from a spectrum?

It is the m/z of the molecular ion, the peak with the highest m/z (ignoring small isotope peaks).

Why are 2+ peaks smaller than 1+ peaks?

Removing two electrons needs more energy, so fewer 2+ ions are formed.

What are the three molecular ion peaks for Cl₂ and their relative heights?

m/z 70, 72 and 74 in the ratio 9 : 6 : 1.

Why is the middle Cl₂ peak (m/z 72) taller than the 74 peak?

³⁵Cl³⁷Cl can form in two ways, and ³⁵Cl is three times as abundant as ³⁷Cl: 2 × 0.75 × 0.25 against 0.25 × 0.25.

Mass spectrometryEdexcel International A Level Chemistry: Flashcards

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List the five stages of mass spectrometry in order.

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List the five stages of mass spectrometry in order.: Vaporisation, ionisation, acceleration, deflection, detection.
How are atoms ionised in a mass spectrometer?: Bombardment with high-energy electrons, which knock out an electron to form positive ions.
Why is a mass spectrometer kept under a vacuum?: So that ions do not collide with air molecules, which would stop them reaching the detector.
What does m/z stand for?: Mass-to-charge ratio: the relative mass of an ion divided by its charge.
What does the height of a peak show?: The relative abundance of that ion or isotope in the sample.
Which ions are deflected most in a magnetic field?: Lighter ions, with lower m/z, for the same charge.
Write the equation to calculate relative atomic mass from a spectrum.: Ar = Σ(isotope mass × abundance) ÷ Σ abundances.
A sample has peaks at 20 (90 %) and 22 (10 %). What is Ar?: (20 × 90 + 22 × 10) ÷ 100 = 20.2.
How is the relative molecular mass found from a spectrum?: It is the m/z of the molecular ion, the peak with the highest m/z (ignoring small isotope peaks).
At what m/z does ²⁴Mg²⁺ appear?: m/z 12, because 24 ÷ 2 = 12.
Why are 2+ peaks smaller than 1+ peaks?: Removing two electrons needs more energy, so fewer 2+ ions are formed.
What are the three molecular ion peaks for Cl₂ and their relative heights?: m/z 70, 72 and 74 in the ratio 9 : 6 : 1.
Why is the middle Cl₂ peak (m/z 72) taller than the 74 peak?: ³⁵Cl³⁷Cl can form in two ways, and ³⁵Cl is three times as abundant as ³⁷Cl: 2 × 0.75 × 0.25 against 0.25 × 0.25.

Exam questions on Mass spectrometry

A forensic laboratory analyses a metal sample using a mass spectrometer. The sample gives two peaks: one at m/z 63 with a relative abundance of 69.2 % and one at m/z 65 with a relative abundance of 30.8 %. All the ions have a charge of 1+.
Explain why the inside of a mass spectrometer is kept under a high vacuum.2 marks
Boron has two isotopes, ¹⁰B and ¹¹B. A mass spectrum of a boron sample gives a relative atomic mass of 10.8.
A second sample of boron contains a greater proportion of ¹⁰B than the first. State and explain how its relative atomic mass compares with 10.8.2 marks
Chlorine consists of two isotopes, ³⁵Cl (75.0 %) and ³⁷Cl (25.0 %). A sample of chlorine gas, Cl₂, is analysed in a mass spectrometer, and molecular ions, Cl₂⁺, are detected.
Identify the molecular ions responsible for the peaks at m/z 70, 72 and 74.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).