What does m/z stand for?

Mass-to-charge ratio of an ion; for a 1+ ion it equals its mass.

How do you find Mr from a mass spectrum?

Read the m/z value of the molecular ion peak, the peak at the highest m/z.

Why do fragment peaks appear?

The molecular ion breaks up into a positive fragment ion and an uncharged radical; only the ion is detected.

What is the base peak?

The tallest peak in the spectrum, the most abundant ion; it is often a fragment, not the molecular ion.

Why can mass spectrometry distinguish isomers?

They have the same molecular ion but break at different bonds, giving different fragment peaks.

Which fragments does propanone give?

[CH₃CO]⁺ at m/z 43 and [CH₃]⁺ at m/z 15, with Mr 58.

Mass spectrometry in organic analysisEdexcel A-Level Chemistry: Flashcards

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What happens to a molecule in the ionisation chamber of a mass spectrometer?

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What happens to a molecule in the ionisation chamber of a mass spectrometer?: It loses one electron to form a positive molecular ion, M → [M]⁺ + e⁻.
What does m/z stand for?: Mass-to-charge ratio of an ion; for a 1+ ion it equals its mass.
How do you find Mr from a mass spectrum?: Read the m/z value of the molecular ion peak, the peak at the highest m/z.
Why do fragment peaks appear?: The molecular ion breaks up into a positive fragment ion and an uncharged radical; only the ion is detected.
What is the base peak?: The tallest peak in the spectrum, the most abundant ion; it is often a fragment, not the molecular ion.
Which ion gives a peak at m/z 15?: [CH₃]⁺
Which ion gives a peak at m/z 29?: [C₂H₅]⁺ or [CHO]⁺
Which ion gives a peak at m/z 43?: [C₃H₇]⁺ or [CH₃CO]⁺
What does a loss of 15 from the molecular ion indicate?: Loss of a CH₃ group.
Which fragment is typical of a primary alcohol such as propan-1-ol?: [CH₂OH]⁺ at m/z 31.
Which fragment is typical of propan-2-ol?: [CH₃CHOH]⁺ at m/z 45.
Why can mass spectrometry distinguish isomers?: They have the same molecular ion but break at different bonds, giving different fragment peaks.
Which fragments does propanone give?: [CH₃CO]⁺ at m/z 43 and [CH₃]⁺ at m/z 15, with Mr 58.

Exam questions on Mass spectrometry in organic analysis

A chemist analyses a liquid that contains carbon, hydrogen and oxygen only. In its mass spectrum the peak with the highest m/z value is at 58, and there are strong fragment peaks at m/z 43 and m/z 15. There are no strong peaks at m/z 29 or 57.
Deduce the identity of the compound, explaining how the peaks at m/z 58, 43 and 15 support your answer.2 marks
Pentan-2-one and pentan-3-one are structural isomers with molecular formula C₅H₁₀O. A student uses mass spectrometry to tell them apart.
Explain how the mass spectra would allow the student to distinguish between the two isomers.2 marks
A student is given an unknown liquid X. Elemental analysis shows that it contains 64.9% carbon, 13.5% hydrogen and 21.6% oxygen by mass. In its mass spectrum the peak with the highest m/z value is at 74 and there is a strong fragment peak at 59. X is not oxidised by warm acidified potassium dichromate(VI). X reacts with sodium metal to give hydrogen gas.
Determine the empirical formula and the molecular formula of X.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).