# Lecture 20: Mass Spectrometry

## Organic Chemistry I

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## Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the basic principles and instrumentation of mass spectrometry
2. Interpret molecular ion peaks to determine molecular weight
3. Recognize the nitrogen rule and its application to molecular formulas
4. Identify common fragmentation patterns for major functional groups
5. Use isotope patterns (M+1, M+2) to identify the presence of specific elements
6. Combine mass spectral data with molecular formula to narrow structural possibilities

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## Lecture Content

### I. Principles of Mass Spectrometry

Mass spectrometry (MS) measures the mass-to-charge ratio (m/z) of ions derived from a sample molecule. Unlike IR, UV, and NMR spectroscopy, mass spectrometry is a destructive technique in which the sample is consumed during analysis.

The basic process involves four stages. First, the sample molecule is ionized to create a charged species. Second, the resulting ions are accelerated by an electric field. Third, the ions are separated according to their m/z ratio using a magnetic field, time-of-flight analyzer, or quadrupole mass filter. Fourth, the separated ions strike a detector, producing a signal proportional to their abundance. The output is a mass spectrum: a plot of relative abundance (expressed as a percentage) versus m/z. The tallest peak in the spectrum is designated the base peak and set to 100% relative abundance. The molecular ion peak and fragment ion peaks together provide structural information about the compound.

### II. Ionization Methods

**Electron ionization (EI)** is the most common method for analyzing small organic molecules. A high-energy electron beam (approximately 70 eV) strikes the gaseous sample molecule, knocking out one of its electrons to produce a radical cation (M+.), known as the molecular ion. The molecular ion has effectively the same mass as the original molecule (the mass of the lost electron is negligible). Because EI delivers substantial excess energy, it often causes extensive fragmentation of the molecular ion.

**Chemical ionization (CI)** is a softer technique that produces less fragmentation. A reagent gas such as methane or ammonia is ionized first, then transfers a proton to the analyte to form [M+H]+ ions, which appear at m/z = molecular weight + 1. CI is useful when EI gives a weak or absent molecular ion peak. **Electrospray ionization (ESI)** and **MALDI** are even softer methods used for large biomolecules such as proteins and nucleic acids. ESI produces multiply charged ions, while MALDI produces singly charged ions, both with minimal fragmentation.

### III. The Molecular Ion (M+) Peak

The molecular ion peak is the most important feature of a mass spectrum because it directly provides the molecular weight of the compound. It appears at the highest m/z value in the spectrum (excluding isotope peaks). However, not all compounds produce a strong M+ peak under EI conditions. Aromatic and conjugated molecules tend to give strong molecular ion peaks because their radical cations are stabilized by resonance. Highly branched alkanes and alcohols often give weak or absent molecular ion peaks because their radical cations fragment readily.

The **nitrogen rule** states that compounds with an odd number of nitrogen atoms have an odd molecular weight, while compounds with zero or an even number of nitrogen atoms have an even molecular weight. This rule provides a quick way to assess whether nitrogen is present in an unknown compound. The degree of unsaturation, calculated from the molecular formula, further helps to narrow the structural possibilities.

### IV. Isotope Patterns

Most elements have naturally occurring isotopes that produce small peaks above the molecular ion in the mass spectrum. The **M+1 peak** is primarily due to 13C, which has a natural abundance of 1.1% per carbon atom. By comparing the intensity of the M+1 peak to the M+ peak and dividing by 1.1, one can estimate the number of carbon atoms in the molecule. Minor contributions also come from deuterium (0.015%) and 15N (0.37%).

The **M+2 peak** is especially diagnostic for chlorine, bromine, and sulfur. Chlorine has two stable isotopes, 35Cl (75.8%) and 37Cl (24.2%), producing M and M+2 peaks in an approximately 3:1 ratio. Bromine has 79Br (50.7%) and 81Br (49.3%), giving M and M+2 peaks in an approximately 1:1 ratio. Sulfur has 32S (95%) and 34S (4.2%), producing an M+2 peak about 4% the size of M+. These isotope patterns are powerful diagnostics: a 3:1 M/M+2 pattern immediately signals one chlorine atom, while a 1:1 pattern signals one bromine. Multiple halogens create more complex patterns, such as the 9:6:1 pattern seen with two chlorine atoms.

<image>Panel A: A mass spectrum of chlorobenzene (C6H5Cl) showing the molecular ion at m/z = 112 and a prominent M+2 peak at m/z = 114 in an approximately 3:1 ratio, labeled to indicate the 35Cl and 37Cl isotope contributions. The base peak at m/z = 77 (loss of Cl to give C6H5+) is labeled. Panel B: A mass spectrum of bromobenzene (C6H5Br) showing the molecular ion at m/z = 156 and an M+2 peak at m/z = 158 in an approximately 1:1 ratio, labeled for 79Br and 81Br. The base peak at m/z = 77 is again labeled. Panel C: A reference table showing the characteristic isotope patterns for common elements: C (M+1), N (M+1), Cl (M+2 in 3:1), Br (M+2 in 1:1), and S (M+2 at ~4%). A caption reads: "Isotope patterns in the molecular ion region are diagnostic for the presence of Cl, Br, and S."</image>

### V. Fragmentation Patterns

After ionization, the molecular ion can break apart into smaller fragment ions following predictable patterns governed by bond strengths and the stability of the resulting ions. Several general rules apply. Fragmentation preferentially produces the most stable carbocation (tertiary > secondary > primary), and resonance-stabilized cations such as allylic, benzylic, and acylium ions are especially favored. Loss of stable neutral molecules is common, including H2O (mass 18), CO (28), CO2 (44), HCN (27), and NH3 (17). Radical cations (odd-electron species) can lose either a radical or a molecule, while even-electron cations typically lose only molecules.

**Alpha-cleavage** is the fragmentation of the bond adjacent to a heteroatom or functional group, and it is a dominant pathway for alcohols, amines, ethers, ketones, and aldehydes. The charge typically remains on the fragment containing the heteroatom because the lone pair stabilizes the positive charge. For ketones, alpha-cleavage produces an acylium ion (R-C=O+) with m/z equal to the mass of R plus 28.

### VI. Fragmentation of Specific Functional Groups

**Alkanes** fragment at branch points, losing CH3 (15), C2H5 (29), C3H7 (43), and so on, often producing clusters of peaks separated by 14 mass units (one CH2 unit). **Alcohols** characteristically lose water (M - 18), and alpha-cleavage adjacent to the OH-bearing carbon gives additional diagnostic fragments. **Amines** undergo alpha-cleavage adjacent to nitrogen as their dominant fragmentation, and fragments containing nitrogen have even mass values according to the nitrogen rule.

**Ketones** fragment by alpha-cleavage on either side of the carbonyl to produce acylium ions. They can also undergo the **McLafferty rearrangement**, in which a gamma-hydrogen migrates to the carbonyl oxygen through a six-membered transition state, producing a neutral alkene and an enol radical cation. **Aromatic compounds** typically show a strong molecular ion peak and may lose a hydrogen atom (M-1). Toluene and alkylbenzenes produce the characteristic tropylium cation (C7H7+, m/z = 91), while the phenyl cation (C6H5+, m/z = 77) is also commonly observed.

<image>A multi-panel figure illustrating common fragmentation mechanisms. Panel A: Alpha-cleavage of 2-pentanone showing the mechanism with curved arrows -- the bond between the alpha-carbon and the carbonyl carbon breaks, forming an acylium ion (CH3CO+, m/z = 43) and an ethyl radical. The mass spectrum below shows the molecular ion at m/z = 86 and a base peak at m/z = 43. Panel B: The McLafferty rearrangement of 2-hexanone, showing the six-membered cyclic transition state, gamma-hydrogen transfer, and the resulting enol radical cation (m/z = 58) and neutral propene. Panel C: Fragmentation of toluene showing loss of H to give tropylium cation (m/z = 91, drawn as a seven-membered ring with delocalized positive charge). A caption reads: "Understanding fragmentation mechanisms allows prediction and interpretation of major peaks in a mass spectrum."</image>

### VII. Determining Molecular Structure from MS Data

A systematic approach to interpreting mass spectral data begins by identifying the molecular ion peak at the highest m/z value (excluding isotope peaks). Next, check for isotope patterns that indicate chlorine, bromine, or sulfur. Apply the nitrogen rule to determine whether nitrogen is present. Calculate the degree of unsaturation from the molecular formula if available. Identify the base peak and major fragment ions, and calculate mass losses from the molecular ion (M minus the fragment mass equals the neutral species lost).

Several **common neutral losses** and their structural significance should be memorized: 15 for loss of CH3, 17 for loss of OH, 18 for loss of H2O (indicating an alcohol), 28 for loss of CO or ethylene, 29 for loss of CHO (aldehyde) or C2H5, 31 for loss of OCH3 (methyl ester or methyl ether), 44 for loss of CO2 (carboxylic acid), and 45 for loss of OC2H5 (ethyl ester). Characteristic fragment ions to watch for include m/z = 91 (tropylium from toluene derivatives), m/z = 77 (phenyl cation), and m/z = 43 (acetyl cation). Mass spectral data achieves its greatest power when combined with other spectroscopic methods, particularly IR and NMR.

### VIII. High-Resolution Mass Spectrometry

Standard (low-resolution) mass spectrometry measures masses to the nearest integer, but **high-resolution mass spectrometry (HRMS)** measures exact masses to four or more decimal places. Because different molecular formulas with the same nominal mass have different exact masses (for example, CO at 27.9949 versus C2H4 at 28.0313 versus HCN at 27.0109), HRMS can distinguish between them and determine the molecular formula unambiguously from the molecular ion. The exact masses of key isotopes are 12C = 12.0000 (by definition), 1H = 1.00783, 16O = 15.9949, and 14N = 14.0031. HRMS is essential in research for confirming the identity of synthetic products and characterizing unknowns.

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