Mass spectrometry (AQA A-Level Chemistry): Revision Notes
3.6.2 Mass spectrometry
Mass spectrometry is a powerful analytical technique used to determine the molecular mass and formula of compounds. It is particularly useful for identifying unknown compounds, verifying the structure of organic molecules, and detecting isotopes.
How Mass Spectrometry Works
When a sample, whether an element or an organic compound, passes through a mass spectrometer, it is ionised and fragmented. This generates a series of peaks on a mass spectrum, each corresponding to ions with specific mass-to-charge ratios (m/z). These peaks help determine the composition and structure of the sample.
Key Components of Mass Spectrometry
- Ionisation: The sample is bombarded with high-energy electrons, causing it to lose an electron and form positive ions (molecular ions).
- Acceleration: The ions are accelerated using an electric field.
- Deflection: A magnetic field deflects ions based on their m/z ratio.
- Detection: The ions are detected, generating a mass spectrum with peaks representing different fragments of the original molecule.
Interpreting a Mass Spectrum
The Molecular Ion Peak ()
- The molecular ion peak () is the most significant peak, appearing at the highest m/z value in the spectrum. It corresponds to the mass of the entire molecule and is used to determine the relative molecular mass () of the compound.
- Example: If the molecular ion peak appears at m/z = 58, the of the compound is 58.
Fragmentation Peaks
- The other peaks in the mass spectrum represent fragments of the original molecule. These arise when the molecular ion breaks into smaller pieces.
- By analysing these fragment peaks, you can deduce parts of the molecule's structure.
High-Resolution Mass Spectrometry
High-resolution mass spectrometry (HRMS) allows for the measurement of relative atomic and molecular masses with precision up to four decimal places. This level of accuracy is crucial for distinguishing between compounds with similar molecular masses.
Example of High-Resolution Mass Spectrometry
Propane () and Ethanal () both have a rounded of 44.
However, HRMS reveals distinct molecular ion peaks:
- Propane: m/z = 44.0624
- Ethanal: m/z = 44.0302 This difference, detected by HRMS, allows accurate identification of similar compounds.
Determining Molecular Formula from Mass Spectrum
Using the Molecular Ion Peak
To determine the molecular formula:
- Locate the molecular ion peak () in the mass spectrum.
- Use the precise m/z value of this peak to identify the relative molecular mass.
- Compare this value with possible molecular formulas using precise atomic masses.
Example Calculation
Molecular Ion Peak:
Suppose a peak is observed at m/z = 86.
Possible Molecular Formulas:
Consider the precise atomic masses:
- Carbon () = 12.0000
- Hydrogen () = 1.0078
- Oxygen () = 15.9949 A molecule with a molecular mass of 86 could be (hexane) or (pentanone), among others.
High-Resolution Analysis:
Use precise m/z values to distinguish between possible formulas.
Example Application: Analysis of an Organic Compound
Suppose you are given an unknown organic compound, and the mass spectrum shows:
- Molecular Ion Peak at m/z = 74.
- Fragment Peaks at m/z = 29, 45, and 59.
Interpretation:
- The of the compound is 74.
- The peak at m/z = 29 may indicate the presence of an ethyl group ().
- The peak at m/z = 45 could correspond to a fragment containing a hydroxyl group (CH₂OH⁺).
- This information suggests the compound might be an alcohol or an ester with a formula matching the of 74.
Summary of Key Points
- Molecular Ion Peak: Indicates the of the whole molecule; crucial for determining the molecular formula.
- Fragmentation: Peaks represent parts of the molecule, helping deduce its structure.
- High-Resolution Mass Spectrometry: Provides precise measurements to distinguish compounds with similar values.