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
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Analyse mass spectra using m/e values and isotopic abundances (you do not need to know how a mass spectrometer works).
- Calculate an element's relative atomic mass from its mass spectrum or from the relative abundance of each of its isotopes.
- Use the M⁺ (molecular ion) peak of a mass spectrum to deduce what the molecular mass of an organic compound is.
- Given a mass spectrum, suggest what molecules are formed by simple fragmentation.
- Use the [M + 1]⁺ peak to deduce n, how many carbon atoms a compound contains, where n = (100 × abundance of [M + 1]⁺) ÷ (1.1 × abundance of M⁺).
- Use the [M + 2]⁺ peak to deduce whether a compound contains bromine or chlorine atoms.
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.
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.
The mass spectrum of a compound shows a molecular ion peak at m/e = 108 and a second peak at m/e = 110 of almost the same height. What element does this indicate?
Bromine. Two peaks 2 mass units apart with roughly equal heights (about 1:1) come from the two isotopes ⁷⁹Br and ⁸¹Br, which have nearly equal natural abundance.
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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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