In simple terms
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The Molecular Weighing Scales
Mass spectrometry is a technique that sorts ions based on their mass. It allows us to find the precise mass of individual atoms and molecules, revealing the different isotopes of an element or the structure of a compound.
Imagine you're at a bowling alley, but instead of one type of ball, you have bowling balls of different weights. You roll them all with the same initial force towards a giant fan blowing from the side. The lightest balls will be pushed far off course, while the heaviest ones will barely deviate. A mass spectrometer does this with ions: it fires them through a magnetic field (the 'fan'), and how much their path bends tells us their mass.
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First, the sample is vaporised and bombarded with high-energy electrons. This knocks an electron off each atom or molecule, creating a positive ion.
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Next, these positive ions are accelerated by an electric field, ensuring they all have the same kinetic energy before the next stage.
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The fast-moving ions then enter a magnetic field, which deflects them. Lighter ions are deflected more than heavier ions.
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Finally, a detector counts the ions at each deflection angle. This data is used to generate a mass spectrum, plotting abundance against mass-to-charge ratio ().
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The Four Stages of Mass Spectrometry
A conventional mass spectrometer operates through four key stages. The sample is first introduced and converted into positive ions, which are then accelerated, separated according to their mass-to-charge ratio, and finally detected.
1. Ionisation: The sample is first vaporised and then injected into the ionisation chamber. Here, it is bombarded with a stream of high-energy electrons from an 'electron gun'. These electrons knock off electrons from the atoms or molecules of the sample, creating positive ions. For a molecule M, this is represented as .
2. Acceleration: The newly formed positive ions are accelerated by a series of negatively charged plates, creating a fine beam of ions all having the same kinetic energy.
3. Deflection: The beam of ions passes into a strong magnetic field, which is perpendicular to their direction of travel. The magnetic field deflects the ions into a curved path. The degree of deflection depends on the mass-to-charge ratio (). Lighter ions are deflected more than heavier ions, and more highly charged ions are deflected more than singly charged ions.
4. Detection: By varying the strength of the magnetic field, ions of different ratios can be directed towards a detector. When an ion hits the detector, it accepts an electron, generating a tiny electrical current. The size of the current is proportional to the number of ions arriving, giving the relative abundance of that ion.
Interpreting Mass Spectra of Elements
When an element is analysed, the mass spectrum shows the different isotopes present. The x-axis represents the mass-to-charge ratio (), which for singly charged ions is simply the isotopic mass. The y-axis shows the relative abundance of each isotope. From this data, we can calculate the relative atomic mass () of the element.
Mass Spectrometry of Molecules and Fragmentation
When a molecule is passed through a mass spectrometer, the peak with the highest value is called the molecular ion peak (). This is formed when the molecule loses one electron but remains intact. The value of this peak gives the relative molecular mass () of the compound. The high energy of the ionisation process often causes the molecular ion to break apart into smaller pieces, a process called fragmentation. This gives rise to other peaks at lower values, creating a unique fragmentation pattern that can be used like a 'fingerprint' to help identify the molecule's structure.
Do not confuse the molecular ion peak with the base peak. The molecular ion peak is the one with the highest value. The base peak is the tallest peak in the spectrum, representing the most abundant (and often most stable) fragment. They can be the same, but often are not.
Characteristic Isotopic Patterns
Some elements have very distinctive isotopic abundances, which lead to characteristic patterns in the mass spectra of molecules containing them. The most important examples for A-Level are chlorine and bromine.
Chlorine (): Has two main isotopes, (~75%) and (~25%). This gives a 3:1 abundance ratio. A molecule containing one chlorine atom will show an M peak and an M+2 peak in a 3:1 ratio. A molecule of chlorine, , will have three possible molecular ions: at , at , and at . The relative abundance ratio of these peaks is approximately 9:6:1.
Bromine (): Has two main isotopes, (~50%) and (~50%). This gives a 1:1 abundance ratio. A molecule containing one bromine atom will show an M peak and an M+2 peak in a 1:1 ratio. A molecule of bromine, , will have three possible molecular ions: at , at , and at . The relative abundance ratio of these peaks is approximately 1:2:1.
Worked examples
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The mass spectrum of a sample of zirconium shows five peaks with the following values and relative abundances:
| Relative Abundance | |
|---|---|
| 90 | 51.5 |
| --- | --- |
| 91 | 11.2 |
| 92 | 17.1 |
| 94 | 17.4 |
| 96 | 2.8 |
Calculate the relative atomic mass of zirconium to one decimal place.
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Calculate the sum of (mass × abundance) for each isotope:
The mass spectrum of propan-1-ol, , is shown. The molecular ion peak is at . Suggest the chemical formula for the fragments responsible for the peaks at , , and .
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First, confirm the of propan-1-ol: . This matches the molecular ion peak.
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What is a mass spectrometer?
An instrument that measures the mass-to-charge ratio () of ions, allowing for the determination of atomic and molecular masses and isotopic abundances.
Key takeaways
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1. Ionisation: The sample is first vaporised and then injected into the ionisation chamber. Here, it is bombarded with a stream of high-energy electrons from an 'electron gun'. These electrons knock off electrons from the atoms or molecules of the sample, creating positive ions. For a molecule M, this is represented as .
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2. Acceleration: The newly formed positive ions are accelerated by a series of negatively charged plates, creating a fine beam of ions all having the same kinetic energy.
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3. Deflection: The beam of ions passes into a strong magnetic field, which is perpendicular to their direction of travel. The magnetic field deflects the ions into a curved path. The degree of deflection depends on the mass-to-charge ratio (). Lighter ions are deflected more than heavier ions, and more highly charged ions are deflected more than singly charged ions.
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4. Detection: By varying the strength of the magnetic field, ions of different ratios can be directed towards a detector. When an ion hits the detector, it accepts an electron, generating a tiny electrical current. The size of the current is proportional to the number of ions arriving, giving the relative abundance of that ion.
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