Interpreting Mass Spectra - A-level Chemistry

Summary

In this tutorial, Mr. Wegg Ferd from Alfea School introduces the fundamentals of mass spectrometry, explaining how samples must be vaporized and ionized for analysis. He walks through the detection of molecular ions, the significance of the M⁺ and M⁺¹ peaks, and how fragmentation patterns arise. Using examples like propanone, butane, ethanoic acid, and benzoic acid, he demonstrates how to calculate molecular masses, identify stable ions, and interpret mass spectra. The video emphasizes understanding bond strengths, predicting fragmentation, and recognizing base peaks, providing practical strategies to quickly interpret spectra and apply this knowledge in exams.

Takeaways

• 😀 Mass spectrometry requires the sample to be in a gaseous state so it can be vaporized and analyzed.
• 😀 Bombarding the sample with electrons ionizes it, forming a positive molecular ion that can be detected.
• 😀 The molecular ion peak (M⁺) represents the heaviest and most important peak on the mass spectrum.
• 😀 The M+1 peak can appear due to the presence of carbon-13 isotopes, slightly increasing the mass.
• 😀 Fragmentation occurs when molecular ions break into smaller ions and free radicals, creating additional peaks.
• 😀 Always clearly indicate ions with a plus sign on spectra to distinguish them from free radicals.
• 😀 The relative abundance on a mass spectrum shows the most stable or common ions relative to the base peak (100%).
• 😀 The base peak is the most abundant fragment ion and often corresponds to the most stable ion formed during fragmentation.
• 😀 Predicting fragmentation involves calculating the masses of fragments based on atomic composition, like CH₃⁺ (15) or COCH₃⁺ (43).
• 😀 Bond strengths influence where molecules fragment, and knowing common patterns for compounds like propanone, butane, and acids helps in analysis.
• 😀 Mass-to-charge ratio (m/z) is the key unit on the spectrum, usually representing the mass of the ion divided by its charge (+1 for typical ions).
• 😀 Larger molecules like benzoic acid fragment similarly, with ring bonds being less likely to break and predictable fragment ions forming from other bonds.

Q & A

What is the first step in preparing a sample for mass spectrometry?

The sample must be vaporized into its gaseous state so it can be introduced into the mass spectrometer.

Why are electrons used to bombard the sample in mass spectrometry?

Electrons are used to knock off an electron from the molecule, converting it into a positive ion that can be detected in the mass spectrometer.

What is the molecular ion (M⁺ peak) in a mass spectrum?

The molecular ion, or M⁺ peak, is the peak representing the heaviest and most abundant ion, corresponding to the intact molecule after losing one electron.

What causes the M+1 peak in a mass spectrum?

The M+1 peak occurs due to the presence of carbon-13 isotopes in the molecule, increasing the mass by one unit.

Why does a molecule fragment in mass spectrometry?

Fragmentation occurs because the molecular ion is unstable after losing an electron, and different bonds break to form smaller ions and free radicals.

How is the base peak identified in a mass spectrum?

The base peak is the most abundant peak in the spectrum, indicating the most stable ion formed during fragmentation.

How can you calculate the expected mass of a molecular ion?

By summing the atomic masses of all atoms in the molecule, since the electron's mass is negligible.

What is the significance of the mass-to-charge ratio (m/z) in a mass spectrum?

The mass-to-charge ratio represents the mass of the ion divided by its charge, usually detecting ions with a single positive charge.

Give an example of how butane fragments in a mass spectrum and the peaks produced.

Butane (C4H10) has a molecular ion peak at 58. It can fragment to produce CH3⁺ (m/z 15), C2H5⁺ (m/z 29), and C3H7⁺ (m/z 43) ions.

How do fragmentation patterns help in identifying molecules?

By analyzing the masses of fragment ions and knowing common fragmentation patterns, we can infer the structure of the molecule.

Why are free radicals important in interpreting a mass spectrum?

Free radicals are formed during fragmentation, and identifying which fragment is the ion versus the radical is crucial for accurate interpretation.

How do the fragmentation patterns of ethanoic acid compare to butane?

Ethanoic acid (CH3COOH) also shows a molecular ion peak (m/z 60) and fragments to CH3⁺ (m/z 15), C2H3O⁺ (m/z 43), and COOH⁺ (m/z 45), reflecting cleavage at predictable bond sites.

What are the challenges in interpreting more complex molecules like benzoic acid?

In complex molecules, certain bonds (like those in aromatic rings) are less likely to break, so interpretation focuses on weaker bonds outside the ring, producing predictable ion peaks.