📊 Part 1: Understanding Mass Spectra

Worked Example: Interpreting a Mass Spectrum of Ethanol (C₂H₅OH)

The mass spectrum of ethanol shows several key peaks:

• Molecular ion peak (M⁺): m/z = 46 (corresponds to C₂H₅OH⁺)

• Base peak: m/z = 31 (corresponds to CH₂OH⁺, loss of CH₃)

• Other fragments: m/z = 45 (loss of H), m/z = 29 (CHO⁺), m/z = 15 (CH₃⁺)

The molecular ion peak confirms the molecular mass, whilst fragment peaks help identify the structure.

1. What information does the molecular ion peak provide?

The most stable fragment of the molecule

The molecular mass of the original compound

The most abundant fragment in the spectrum

The lightest fragment produced

2. The base peak in a mass spectrum represents:

The molecular ion

The fragment with the highest m/z ratio

The most abundant ion (100% relative intensity)

The first peak to appear in the spectrum

3. Which factors can cause fragmentation patterns in mass spectrometry? (Select all that apply)

Electron bombardment energy

Molecular structure and bond strengths

Temperature of the sample

Stability of resulting fragments

🔬 Part 2: Spectrum Analysis

Analyse the following mass spectrum data for an unknown organic compound:
Major peaks: m/z = 58 (molecular ion, 15%), m/z = 43 (base peak, 100%), m/z = 15 (25%)

4. What is the molecular mass of the unknown compound?

Molecular mass = _________ g/mol

5. Calculate the mass of the fragment lost to produce the base peak at m/z = 43.

6. Suggest the identity of the fragment responsible for the base peak (m/z = 43). Explain your reasoning.

7. The peak at m/z = 15 likely corresponds to which fragment?

CH₃⁺

NH⁺

O⁺

CH₂⁺

8. Based on the fragmentation pattern, propose a possible molecular formula for this compound. Justify your answer.

🎯 Part 3: Application Questions

9. Explain why the molecular ion peak is often not the most intense peak in a mass spectrum.

10. A compound shows peaks at m/z = 72, 57, 43, and 29. If the molecular ion is at m/z = 72, determine the mass differences between consecutive major peaks and suggest what molecular fragments might be lost.

11. Compare the advantages and limitations of using mass spectrometry for molecular identification in analytical chemistry.