mass spectrometry

What is the primary function of Mass Spectrometry (MS)?

It is an analytical technique for measuring the mass-to-charge ratio (m/z) of ions in the gas phase.

What are the three main functions a mass spectrometer is designed to perform?

1. Convert neutral atoms/molecules into ions. 2. Separate ions by their m/z ratio. 3. Measure the relative abundance of each ion.

In Electron Ionization Mass Spectrometry (EI-MS), how are sample molecules converted into ions?

The sample is bombarded with a beam of high-energy electrons, causing the sample molecules to lose an electron and form positive ions.

What is a molecular ion (M) in mass spectrometry?

A radical cation formed by the removal of a single electron from a parent molecule.

What is the 'base peak' in a mass spectrum?

The most abundant, tallest peak in the spectrum, assigned an arbitrary intensity of 100.

How is the abundance of other ions reported in a mass spectrum relative to the base peak?

As a percentage of the abundance of the base peak.

What does the term 'resolution' refer to in mass spectrometry?

A measure of how well a mass spectrometer separates ions of different mass.

Why would high-resolution MS be required to distinguish between C

3

H

8

O and C

2

H

4

O

2
?

Because both have the same nominal mass of 60, but their precise masses differ slightly (60.05754 vs 60.02112 amu).

What two elements common in organic compounds give rise to significant M+2 peaks?

Chlorine (Cl) and Bromine (Br).

A ratio of M to M+2 peaks of approximately 3:1 in a mass spectrum indicates the presence of what element?

A single chlorine atom.

A ratio of M to M+2 peaks of approximately 1:1 in a mass spectrum indicates the presence of what element?

A single bromine atom.

What is the 'Nitrogen Rule' in mass spectrometry?

A compound with an even number of nitrogen atoms has an even m/z for its molecular ion, while one with an odd number of nitrogen atoms has an odd m/z.

Why does extensive fragmentation of the molecular ion occur in EI-MS?

Because high-energy electrons (approx. 70 eV) are used, which is enough energy to not only ionize the molecule but also break its bonds.

When a molecular ion fragments, it produces a cation and a radical. Which of these is detected by the mass spectrometer?

Only the cation is detected.

In mass spectrometry, fragmentation patterns are governed by the stability of what species?

The fragmentation mode that gives the most stable carbocation is favored.

What is a characteristic fragmentation pattern for unbranched alkanes?

What is a characteristic fragmentation pattern for unbranched alkanes?

What are two characteristic features of the mass spectra of alkenes?

They typically show a strong molecular ion peak and cleave readily to form resonance-stabilized allylic cations.