Ch 2 · Mass spectrometry · AS & A Level Chemistry 9701
Topic 2 of 2 in Analytical techniques — AS & A Level Chemistry 9701 — 57 lessons.
Introduction
A mass spectrometer ionises, accelerates and detects charged particles according to their mass-to-charge ratio, giving the relative atomic mass of elements and, for organic molecules, the molecular mass and structural clues from fragmentation.
Key ideas — Mass spectra, m/e and isotopic abundance
A mass spectrum plots the relative abundance of ions detected against their mass-to-charge ratio, m/e, usually with a single positive charge, so numerically equal to the ion's mass. For an element with several isotopes, each isotope gives a separate peak at its own mass number, and the height of each peak is proportional to the relative abundance of that isotope in the sample.
🧑🏫 Teacher's explanation
Imagine a sorting machine at an airport that scans luggage. Instead of checking for size, it weighs every bag and counts how many bags weigh exactly 10kg, 11kg, 12kg, and so on. A mass spectrum is that sorting machine for molecules. It doesn't tell you what the molecule is directly; it tells you the 'weight' of the pieces it breaks into.
📖 Definitions
🔍 Worked example — Easy
An element has two isotopes. The first isotope has a mass of 10 and an abundance of 20%. The second isotope has a mass of 11 and an abundance of 80%. Calculate the relative atomic mass, A_r.
🔍 Worked example — Standard
A molecular ion peak (M^+) is observed at m/e = 44. A fragment peak is observed at m/e = 28. Suggest a possible fragment ion and the identity of the molecule.
🔍 Worked example — Exam-style
A compound has a molecular ion peak at m/e = 58. The M+1 peak has an abundance of 4.4% relative to the M^+ peak. Calculate the number of carbon atoms in the molecule. (Assume each ^13C atom contributes 1.1% to the M+1 peak abundance).
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Calculating relative atomic mass
The relative atomic mass of an element is the weighted mean of the mass numbers of its isotopes, weighted by their percentage abundance: A_r = Σ(isotope mass × \% abundance)/100
🧑🏫 Teacher's explanation
Imagine you are buying a bag of mixed fruit where most are small grapes and a few are large mangoes. If you want to find the 'average' weight of a single piece of fruit, you can't just add the weights and divide by two. You have to account for the fact that there are many more grapes than mangoes. You multiply each weight by how common it is, add them up, and divide by the total.
📖 Definitions
🔍 Worked example — Easy
An element has two isotopes. Isotope A has a mass of 35 and an abundance of 75\%. Isotope B has a mass of 37 and an abundance of 25\%. Calculate the relative atomic mass.
🔍 Worked example — Standard
A mass spectrum shows two peaks for Neon. The first peak is at m/e = 20 with an abundance of 90\%. The second peak is at m/e = 22 with an abundance of 10\%. Calculate the A_r.
🔍 Worked example — Exam-style
An element has three isotopes with the following data: Isotope 1 (m/e = 63, abundance 69\%), Isotope 2 (m/e = 65, abundance 31\%), and Isotope 3 (m/e = 67, abundance 10\%). Calculate the A_r.
⚠️ Common errors
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🎯 Exam question
Key ideas — The molecular ion peak
When an organic molecule is ionised, it first forms a molecular ion, M^+, by loss of one electron; this ion has essentially the same mass as the whole molecule, so the m/e value of the peak furthest to the right (highest mass) in the spectrum, the molecular ion peak, gives the molecular mass of the compound directly.
🧑🏫 Teacher's explanation
Imagine you are weighing a large, complex Lego structure on a very sensitive scale. If you suddenly pluck one tiny, tiny Lego stud (an electron) away from the structure, the total weight of the structure barely changes. In mass spectrometry, we do exactly this: we blast a molecule with high-energy electrons to knock one electron off. The resulting ion is called the molecular ion, M^+.
📖 Definitions
🔍 Worked example — Easy
A mass spectrum shows a molecular ion peak at m/e = 46. If the compound contains only C, H, and O, suggest a possible molecular formula.
🔍 Worked example — Standard
An organic compound has a molecular ion peak at m/e = 88. It contains only C, H, and O. Determine the molecular formula if the compound is a ketone with the formula $ ext{C}_n ext{H}_{2n} ext{O}$.
🔍 Worked example — Exam-style
A compound has a molecular ion peak at m/e = 58. It contains C and H. Calculate the number of carbon atoms if the M+1 peak has an abundance of 4.4% relative to the M^+ peak.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Fragmentation and fragment ions
The molecular ion is unstable and breaks apart into smaller positive ions and neutral fragments, which are not detected; each fragment ion peak gives structural information, since its m/e value corresponds to the mass of a recognisable part of the molecule. A peak at m/e = 43 in an appropriate spectrum, for example, is often due to loss of CH_3 from the molecular ion, consistent with a CH_3CO^+ fragment remaining.
🧑🏫 Teacher's explanation
Imagine you have a Lego model of a house. If you throw that house against a wall, it doesn't just disappear; it shatters into specific pieces like a door, a window, or a roof. In mass spectrometry, the 'house' is your molecule, and the 'wall' is the high-energy electron beam. The pieces that fly off and carry a charge are the fragment ions we see on the spectrum.
📖 Definitions
🔍 Worked example — Easy
A molecular ion of an organic compound has an m/e of 74. A significant peak is observed at m/e = 45. Identify the mass of the neutral fragment lost.
🔍 Worked example — Standard
An organic molecule has a molecular ion peak at m/e = 58. A peak is observed at m/e = 29. Suggest a possible fragment ion and the neutral piece lost.
🔍 Worked example — Exam-style
A mass spectrum shows a molecular ion peak at m/e = 86. A peak is seen at m/e = 43. If the molecule is a ketone, suggest the structure of the fragment ion and the neutral fragment lost.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Identify the mass of the neutral fragment lost from the molecular ion. (b) Suggest a possible structure for the fragment ion with m/e = 43. [2] [2 marks]
Key ideas — The M+1 peak and number of carbon atoms
Carbon-13 occurs naturally at about 1.1\% abundance alongside the far more abundant carbon-12, so a small peak one mass unit higher than the molecular ion peak, the M+1 peak, appears due to molecules containing one ^13C atom. Because each carbon atom has an independent 1.1\% chance of being ^13C, the height of the M+1 peak relative to the M peak increases with the number of carbon atoms present, and can be used to estimate that number.
🧑🏫 Teacher's explanation
Imagine you have a large bag of white marbles (Carbon-12) and a few red marbles (Carbon-13). If you grab a handful of marbles to form a 'molecule', the more marbles you grab, the higher the chance that at least one of them will be red. In mass spectrometry, the 'M' peak represents the molecules made of the most common isotope, while the 'M+1' peak represents molecules that happen to contain one heavy isotope.
📖 Definitions
🔍 Worked example — Easy
A molecular ion peak has an abundance of 100\% and the M+1 peak has an abundance of 5.5\%. Estimate the number of carbon atoms.
🔍 Worked example — Exam-style
A compound has a molecular ion peak at m/e = 58. The M+1 peak has an intensity of 4.4\% relative to the M peak. Determine the molecular formula of the compound (assume only C and H).
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Determine the number of carbon atoms in the compound. [2] (b) Deduce the molecular formula of the compound, assuming it contains only C and H. [2] [4 marks]
Worked example
Question. A sample of gallium consists of two isotopes: ^69Ga (abundance 60.1%) and ^71Ga (abundance 39.9%). (a) Calculate the relative atomic mass of gallium to 3 significant figures. [2] (b) A separate mass spectrum of an organic compound shows a molecular ion peak at m/e = 74 and a fragment ion peak at m/e = 59. Suggest the identity of the fragment lost, showing your working. [2]
🔍 Worked example — Easy
Calculate the relative atomic mass (A_r) of an element that has two isotopes: ^35Cl (abundance 75.8%) and ^37Cl (abundance 24.2%).
🔍 Worked example — Standard
A mass spectrum of a compound shows a molecular ion peak (M^+) at m/e = 46 and a significant fragment peak at m/e = 31. Identify the fragment lost.
🔍 Worked example — Exam-style
An organic molecule has a molecular ion peak at m/e = 86. A peak is observed at m/e = 87 with an abundance of 18% relative to the M^+ peak. Estimate the number of carbon atoms in the molecule.
⚠️ Common errors
Common mistakes
Practice
Answers