Mass spectrometry stands as a cornerstone technique in modern analytical chemistry, offering precise molecular identification through fragmentation pattern analysis. This article explores how these patterns help chemists determine molecular structures.

Fundamentals of Mass Spectrometry

Mass spectrometry operates by:

Key Principles of Molecular Fragmentation

When molecules fragment in a mass spectrometer, two fundamental rules apply:

Common Fragmentation Patterns

Alkanes

Using pentane (C₅H₁₂) as an example:

$\text{C}_5\text{H}_{12}^+ \rightarrow \text{C}_2\text{H}_5^+ + \text{C}_3\text{H}_7^+$

The molecular ion appears at m/z = 72, with a characteristic fragment at m/z = 29 (C₂H₅⁺).

Alcohols

For ethanol (C₂H₆O):

$\text{C}_2\text{H}_6\text{O}^+ \rightarrow \text{CH}_2\text{OH}^+ + \text{CH}_3$

The base peak occurs at m/z = 31 (CH₂OH⁺), with a molecular ion peak at m/z = 45.

Aldehydes and Ketones

Propanone (C₃H₆O) demonstrates:

$\text{C}_3\text{H}_6\text{O}^+ \rightarrow \text{CH}_3^+ + \text{C}_2\text{H}_3\text{O}$

A characteristic peak at m/z = 15 indicates the methyl group (CH₃⁺).

Carboxylic Acids

Propanoic acid (C₃H₆O₂) shows:

$\text{C}_3\text{H}_6\text{O}_2^+ \rightarrow \text{CHO}_2^+ + \text{C}_2\text{H}_5$

Distinctive peaks appear at m/z = 45 and 57, indicating carboxyl group presence.

Advanced Fragment Analysis

Esters

Methyl propanoate exhibits multiple fragmentation patterns:

$\text{C}_4\text{H}_8\text{O}_2^+ \rightarrow \text{C}_3\text{H}_5\text{O}^+ + \text{CH}_3\text{O}$ $\text{C}_4\text{H}_8\text{O}_2^+ \rightarrow \text{C}_2\text{H}_5^+ + \text{C}_2\text{H}_3\text{O}_2$

Amines

Ethanamine demonstrates:

$\text{C}_2\text{H}_7\text{N}^+ \rightarrow \text{CH}_2\text{NH}_2^+ + \text{CH}_3$

The molecular ion peak appears at m/z = 45, with a characteristic fragment at m/z = 30.

Practical Applications

Understanding fragmentation patterns enables: