Mass spectrometry: principles, interpretation
Mass Spectrometry: Principles & Interpretation
📌 Exam Tip: Always write the m/z value next to each peak and note the ion’s chemical formula when you identify it.
1. What is Mass Spectrometry? 🔬
Think of a mass spectrometer as a super‑smart scale that can weigh individual atoms or molecules and tell you exactly what they are. It does this by first turning the sample into tiny charged particles (ions), then separating those ions by their mass‑to‑charge ratio (m/z), and finally counting how many of each ion arrive at the detector.
2. Ionisation Techniques
- Electron Ionisation (EI) – Imagine a tiny bomb that shoots electrons at the molecule, knocking off one electron and creating a radical cation ($M^+$). It’s great for small, volatile compounds.
- Chemical Ionisation (CI) – A gentler method where a reagent gas (like methane) donates a proton, forming $[M+H]^+$ or $[M+Na]^+$. Think of it as giving the molecule a friendly push.
- Electrospray Ionisation (ESI) – Like spraying a fine mist of charged droplets; useful for large, polar molecules such as proteins.
3. Mass Analyzer Types
- Quadrupole – Uses oscillating electric fields to filter ions of a specific m/z.
- Time‑of‑Flight (TOF) – Measures how long ions take to travel a fixed distance; lighter ions arrive faster.
- Orbitrap – Ions orbit a central electrode; the frequency of their motion gives the m/z.
4. Interpreting a Mass Spectrum
A mass spectrum is a graph of intensity (how many ions) versus m/z. Peaks represent ions; the tallest peak is usually the molecular ion ($M^+$). Below are the steps to decode it:
- Identify the base peak (intensity = 100%).
- Locate the molecular ion (often the highest m/z peak). If it’s missing, look for a peak that matches the molecular weight of the compound.
- Check for common fragment ions (e.g., $C_nH_{2n+1}^+$, $C_nH_{2n}^+$).
- Use the nitrogen rule to deduce the number of nitrogen atoms.
- Confirm with the isotopic pattern (e.g., $^{13}C$ peaks at +1 m/z).
5. Common Fragmentation Patterns
| Fragment | m/z (approx.) | Interpretation |
|---|---|---|
| $[M+H]^+$ | Molecular weight + 1 | Protonated molecule (common in ESI) |
| $C_nH_{2n+1}^+$ | (28n + 1) | Alkyl chain fragment |
| $C_nH_{2n}^+$ | (28n) | Alkyl fragment with a double bond |
| $[M-CH_3]^+$ | Molecular weight – 15 | Loss of a methyl group |
6. Practical Example: Ethanol ($C_2H_5OH$)
Let’s walk through the EI spectrum of ethanol. The molecular weight is 46 g mol⁻¹, so the molecular ion appears at m/z = 46.
| Peak | m/z | Ion | Interpretation |
|---|---|---|---|
| Base Peak | 29 | $C_2H_5^+$ | Fragment after loss of $CH_3OH$ |
| Molecular Ion | 46 | $C_2H_5OH^+$ | Intact molecule |
| Other Peak | 15 | $CH_3^+$ | Methyl group fragment |
Exam Tips & Common Mistakes
✔️ Always check the base peak first. ✔️ Use the nitrogen rule: If m/z is odd, the molecule contains an odd number of nitrogen atoms. ✔️ Don’t forget isotopic peaks: For molecules with many carbon atoms, look for a +1 m/z peak that is about 1 % of the base peak intensity. ❌ Common mistake: Assuming the highest peak is always the molecular ion; sometimes the molecular ion is suppressed. ❌ Common mistake: Ignoring the fragmentation pattern; the same m/z can arise from different fragments.
🎓 Practice Question: A compound gives a base peak at m/z = 43 and a prominent peak at m/z = 58. What is the most likely molecular formula?
Hint: 58 is the molecular ion; 43 is a common fragment $C_3H_7^+$.
Revision
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