3.6.2 Mass spectrometry

infoNote

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 ( $M⁺$ )

The molecular ion peak ( $M⁺$ ) 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 ( $M_r$ ) of the compound.
Example: If the molecular ion peak appears at m/z = 58, the $M_r$ 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.

infoNote

Example of High-Resolution Mass Spectrometry

Propane ( $C₃H₈$ ) and Ethanal ( $CH₃CHO$ ) both have a rounded $M_r$ 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 ( $M⁺$ ) 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.

infoNote

Example Calculation

Molecular Ion Peak:

Suppose a peak is observed at m/z = 86.

Possible Molecular Formulas:

Consider the precise atomic masses:

Carbon ( $C$ ) = 12.0000
Hydrogen ( $H$ ) = 1.0078
Oxygen ( $O$ ) = 15.9949 A molecule with a molecular mass of 86 could be $C₆H₁₄$ (hexane) or $C₅H₁₀O$ (pentanone), among others.

High-Resolution Analysis:

Use precise m/z values to distinguish between possible formulas.

infoNote

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 $M_r$ of the compound is 74.
The peak at m/z = 29 may indicate the presence of an ethyl group ( $C₂H₅⁺$ ).
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 $M_r$ of 74.

infoNote

Summary of Key Points

Molecular Ion Peak: Indicates the $M_r$ 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 $M_r$ values.

Mass spectrometry Simplified Revision Notes for A-Level AQA Chemistry

3.6.2 Mass spectrometry

How Mass Spectrometry Works

Key Components of Mass Spectrometry

Interpreting a Mass Spectrum

The Molecular Ion Peak ( $M⁺$ )

Fragmentation Peaks

High-Resolution Mass Spectrometry

Example of High-Resolution Mass Spectrometry

Determining Molecular Formula from Mass Spectrum

Using the Molecular Ion Peak

Example Calculation

Example Application: Analysis of an Organic Compound

Summary of Key Points

500K+ Students Use These Powerful Tools to Master Mass spectrometry For their A-Level Exams.

Other Revision Notes related to Mass spectrometry you should explore

Mass spectrometry Simplified Revision Notes for A-Level AQA Chemistry

3.6.2 Mass spectrometry

How Mass Spectrometry Works

Key Components of Mass Spectrometry

Interpreting a Mass Spectrum

The Molecular Ion Peak (M+M⁺M+)

Fragmentation Peaks

High-Resolution Mass Spectrometry

Example of High-Resolution Mass Spectrometry

Determining Molecular Formula from Mass Spectrum

Using the Molecular Ion Peak

Example Calculation

Example Application: Analysis of an Organic Compound

Summary of Key Points

500K+ Students Use These Powerful Tools to Master Mass spectrometry For their A-Level Exams.

Other Revision Notes related to Mass spectrometry you should explore

The Molecular Ion Peak ( $M⁺$ )