Which peak in a mass spectrum gives the relative molecular mass?

The molecular ion peak, with the highest m/z (ignoring tiny isotope peaks).

What is the base peak?

The tallest peak in the spectrum; it is not necessarily the molecular ion.

What does a low-resolution spectrometer give?

m/z to the nearest whole number.

What does a high-resolution spectrometer give?

The precise relative molecular mass to several decimal places.

Why are precise atomic masses not whole numbers?

Only carbon-12 is exactly a whole number; the other elements have precise masses such as H = 1.0078.

Why can C₄H₁₀O and C₃H₆O₂ be distinguished only by high resolution?

Both have Mᵣ = 74 to the nearest whole number, but their precise masses differ.

How do you find a molecular formula from a precise mass?

Calculate the precise mass of each candidate formula and match to the measured value.

Calculate the precise Mᵣ of C₂H₄ (C = 12.0000, H = 1.0078).

2 × 12.0000 + 4 × 1.0078 = 28.0312

Can precise mass distinguish butanal from butanone?

No: they are isomers with the same molecular formula and precise mass.

What else is needed to find the structure of a compound?

Other evidence, such as infrared spectra or test-tube reactions.

Mass spectrometry in organic analysisAQA A-Level Chemistry: Flashcards

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What is the molecular ion?

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What is the molecular ion?: The ion M⁺ formed when a molecule loses one electron; its m/z equals the relative molecular mass.
Which peak in a mass spectrum gives the relative molecular mass?: The molecular ion peak, with the highest m/z (ignoring tiny isotope peaks).
What is the base peak?: The tallest peak in the spectrum; it is not necessarily the molecular ion.
What do the peaks at lower m/z represent?: Fragment ions.
What does a low-resolution spectrometer give?: m/z to the nearest whole number.
What does a high-resolution spectrometer give?: The precise relative molecular mass to several decimal places.
Why are precise atomic masses not whole numbers?: Only carbon-12 is exactly a whole number; the other elements have precise masses such as H = 1.0078.
Why can C₄H₁₀O and C₃H₆O₂ be distinguished only by high resolution?: Both have Mᵣ = 74 to the nearest whole number, but their precise masses differ.
How do you find a molecular formula from a precise mass?: Calculate the precise mass of each candidate formula and match to the measured value.
Calculate the precise Mᵣ of C₂H₄ (C = 12.0000, H = 1.0078).: 2 × 12.0000 + 4 × 1.0078 = 28.0312
Can precise mass distinguish butanal from butanone?: No: they are isomers with the same molecular formula and precise mass.
What else is needed to find the structure of a compound?: Other evidence, such as infrared spectra or test-tube reactions.

Exam questions on Mass spectrometry in organic analysis

A pure organic compound is analysed by mass spectrometry. In the mass spectrum, the peak with the highest mass-to-charge ratio (ignoring any very small peaks above it) is at m/z = 74.
The compound is known to contain only carbon, hydrogen and oxygen. Explain why the m/z value of 74 alone cannot be used to find its molecular formula.2 marks
High-resolution mass spectrometry measures relative molecular masses to four decimal places. Precise relative atomic masses: H = 1.0078, C = 12.0000, N = 14.0031, O = 15.9949.
A compound has a precise relative molecular mass of 46.0054. Calculate the precise relative molecular masses of C₂H₆O and CH₂O₂ and decide which is the molecular formula of the compound.2 marks
A pharmaceutical analyst finds that a compound containing only carbon, hydrogen and oxygen has a precise relative molecular mass of 88.0524 by high-resolution mass spectrometry. Precise relative atomic masses: H = 1.0078, C = 12.0000, O = 15.9949.
The compound has a nominal mass of 88. Calculate the precise relative molecular masses of C₄H₈O₂ and C₅H₁₂O and deduce the molecular formula of the compound.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).