Mass spectrometry
Cambridge A-Level Chemistry (9701) · Unit 22: Analytical techniques · 8 flashcards
Mass spectrometry is topic 22.2 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 22 — Analytical techniques , alongside Infrared spectroscopy. In one line: The m/e values represent the mass-to-charge ratio of the ions detected. For singly charged ions (charge = +1), the m/e value directly corresponds to the mass of the ion, which can be used to identify the ion or molecule. It also shows the relative abundance of each detected ion.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 8 flashcards — 2 definitions, 4 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
What information can be obtained from the m/e values in a mass spectrum
The m/e values represent the mass-to-charge ratio of the ions detected. For singly charged ions (charge = +1), the m/e value directly corresponds to the mass of the ion, which can be used to identify the ion or molecule. It also shows the relative abundance of each detected ion.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- analyse mass spectra in terms of m/e values and isotopic abundances (knowledge of the working of the mass spectrometer is not required)
- calculate the relative atomic mass of an element given the relative abundances of its isotopes, or its mass spectrum
- deduce the molecular mass of an organic molecule from the molecular ion peak in a mass spectrum
- suggest the identity of molecules formed by simple fragmentation in a given mass spectrum
- deduce the number of carbon atoms, n, in a compound using the [M + 1]+ peak and the formula n =
- deduce the presence of bromine and chlorine atoms in a compound using the [M + 2]+ peak
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Mass spectrometry
- › Use the Avogadro constant and molar mass accurately; 37.0 g of 37Cl2 is exactly 0.5 moles of molecules.
What information can be obtained from the m/e values in a mass spectrum?
The m/e values represent the mass-to-charge ratio of the ions detected. For singly charged ions (charge = +1), the m/e value directly corresponds to the mass of the ion, which can be used to identify the ion or molecule. It also shows the relative abundance of each detected ion.
How is relative atomic mass calculated from isotopic abundances in a mass spectrum?
The relative atomic mass is calculated as the weighted average of the masses of the isotopes. Multiply the mass of each isotope by its relative abundance, sum these values, and divide by the sum of the abundances (if not already scaled to 100).
What information does the molecular ion peak (M+) provide in a mass spectrum of an organic molecule?
The molecular ion peak (M+) represents the intact molecule that has lost one electron. Its m/e value corresponds to the molecular mass of the organic molecule, which helps in determining the compound's identity.
How can simple fragmentation patterns in a mass spectrum help identify unknown molecules?
Fragmentation patterns reveal information about the structure of the molecule. Specific fragments correspond to the loss of particular groups (
Describe how to determine the number of carbon atoms (n) in a compound using the [M+1]+ peak in a mass spectrum.
The [M+1]+ peak is due to the presence of carbon-13 (¹³C) isotope. The number of carbon atoms (n) can be estimated using the formula: n ≈ ([M+1]+ peak abundance / M+ peak abundance) x 100 / 1.1, where 1.1 is the natural abundance of ¹³C.
What does the presence of an [M+2]+ peak, with an approximate 1:1 ratio to the M+ peak, suggest about the compound?
An [M+2]+ peak with an abundance similar to the M+ peak indicates the presence of a bromine (Br) atom in the molecule. Bromine has two isotopes, ⁷⁹Br and ⁸¹Br, which exist in approximately equal abundance.
What does the presence of an [M+2]+ peak, with approximately one-third the intensity of the M+ peak, suggest about the compound?
An [M+2]+ peak with roughly one-third the intensity of the M+ peak indicates the presence of a chlorine (Cl) atom in the molecule. Chlorine has two isotopes, ³⁵Cl and ³⁷Cl, in an approximate 3:1 ratio.
A mass spectrum shows peaks at m/e = 78, 79 and 80 with relative abundances 100:97:33. What element does this indicate?
Given the high abundance of two peaks separated by 2 m/e values this indicates a compound containing bromine. The peaks at m/e 79 and 81 (100:97 ratio) relate to 79Br and 81Br.
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Mass spectrometry sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Mass spectrometry deck
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