Mass Spectrometry

Full Notes

This builds on Mass Spectrometry as introduced in chapter 7 – here.

Mass Spectrometry – The Basics

• A sample is ionised to form positive ions.
• Ions are accelerated and separated based on their mass-to-charge ratio (m/z).
• The detector records the ions, producing a mass spectrum.
• The molecular ion peak (M⁺) represents the relative molecular mass (M_r) of the compound.

Identifying Molecular Mass and Formula

The molecular ion peak (M⁺) has the highest m/z value on a spectra and we can use precise atomic masses to confirm the molecular formula.

Example:

Determining the molecular formula of a compound with M⁺ = 58.12

Fragmentation

Inside the mass spectrometer, molecular ions can break into fragments. Each fragment forms a smaller ion and shows up as a peak at a lower m/z.

These peaks help identify parts of the molecule.

Example:

Hydrocarbons A and B both have formula C₄H₁₀. Their spectra differ due to fragmentation patterns.

The extra peak at m/z 29 for Hydrocarbon B means it has a C₂H₅ group in its structure (as well as a CH₃ and C₃H₇ group). This would indicate CH₃CH₂CH₂CH₃3 as its structure.

The [M+1]⁺ Peak

This is a small peak 1 unit higher than M⁺, caused by molecules containing one carbon-13 atom.

The [M+2]⁺ Peak – Identifying Cl and Br

Organic compounds that contain chlorine or bromine show an M+2 peak due to isotopes:

This is because chlorine and bromine both have isotopes that differ by a mass number of 2 (Cl-35 and Cl-37 and Br-79 and Br-81).

• Chlorine: ratio of ³⁵Cl to ³⁷Cl is 3:1 → M⁺ and M+2 peaks appear in 3:1 ratio.
• Bromine: ratio of ⁷⁹Br to ⁸¹Br is 1:1 → M⁺ and M+2 peaks appear with equal height.

For example:
The mass spectra for chloromethane shows an M+2 peak. The main molecular ion peak is at 50 (contains the Cl-35 isotope) and 52 (contains the Cl-37 isotope).

Using Accurate Mass Data

High-resolution mass spectrometers measure M_r accurate to 4 decimal places, allowing precise determination of molecular formula.