9701 · 22.2
Mass spectrometry flashcards
Revision flashcards for Cambridge 9701 Mass spectrometry (syllabus 22.2). Flip, recall, then mark a real past-paper question.
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What is a mass spectrometer?
An instrument that measures the mass-to-charge ratio ($m/z$) of ions, allowing for the determination of atomic and molecular masses and isotopic abundances.
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What does the mass-to-charge ratio ($m/z$) represent?
The ratio of an ion's mass (in atomic mass units) to its formal charge. In A-Level Chemistry, the charge is almost always +1, so the $m/z$ value is numerically equal to the relative mass of the ion.
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What is the molecular ion peak ($M^+$)?
The peak with the highest $m/z$ value in the mass spectrum of a compound. It represents the intact molecule that has lost one electron. Its $m/z$ value gives the relative molecular mass ($M_r$) of the compound.
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What is the base peak?
The peak with the greatest relative abundance in a mass spectrum (the tallest peak). It is assigned a relative intensity of 100% and corresponds to the most stable positive ion formed in the spectrometer.
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What is fragmentation in mass spectrometry?
The process where the high-energy molecular ion breaks apart into smaller, positively charged fragments and neutral species (like radicals). Only the charged fragments are detected.
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Describe the process of electron impact ionisation.
A vaporised sample is bombarded by a beam of high-energy electrons. These electrons knock one or more electrons off the sample particles, forming positive ions (cations). Example: $CH_4 + e^- \rightarrow [CH_4]^+ \cdot + 2e^-$.
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Why must a mass spectrometer operate under a high vacuum?
To prevent the ions from colliding with air molecules. Such collisions would interfere with their flight path from the ion source to the detector, invalidating the results.
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What does the M+1 peak signify?
A small peak located one mass unit above the molecular ion peak ($M^+$). It arises from the presence of the naturally occurring carbon-13 isotope ($^{13}C$) in an organic molecule.
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How can you identify a compound containing one bromine atom from its mass spectrum?
Look for two peaks of almost equal height (a 1:1 abundance ratio) for the molecular ion, separated by 2 mass units. This is due to the two isotopes, $^{79}Br$ and $^{81}Br$, which have nearly equal natural abundance.
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How can you identify a compound containing one chlorine atom from its mass spectrum?
Look for two peaks for the molecular ion, separated by 2 mass units, with a relative abundance ratio of approximately 3:1. This is due to the isotopes $^{35}Cl$ (c. 75%) and $^{37}Cl$ (c. 25%).
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What two factors determine the degree of deflection of an ion in a magnetic field?
1. The mass of the ion (lighter ions are deflected more). 2. The charge of the ion (more highly charged ions are deflected more). The overall deflection depends on the mass-to-charge ratio, $m/z$.