Ligand-Gated Ion Channel Database

Historical Development of Mass Spectrometry

The origins of mass spectrometry date back to the late nineteenth century. In 1897, J. J. Thomson identified the electron and determined its mass-to-charge ratio (m/z), a discovery that later earned him the Nobel Prize in Physics. This work established the fundamental concept underlying mass spectrometric analysis.

In 1912, Thomson constructed the first mass spectrometer, enabling the experimental measurement of the mass-to-charge ratio of charged particles. The technological evolution of mass spectrometry can be divided into three major phases:

• 1912–1960: Development of instruments for elemental analysis and significant improvements in mass resolution.

• 1960–1980: Expansion toward the analysis of organic compounds, increased mass range, and the introduction of accurate mass measurements to determine the molecular formula of ions.

• 1980–present: Development of advanced ionization techniques allowing the analysis of large biological macromolecules, including proteins and nucleic acids.

Definition of Mass Spectrometry

Mass spectrometry is an analytical technique used to determine the molecular mass of compounds with high precision. The method measures the mass-to-charge ratio (m/z) of ionized molecules, allowing the determination of their molecular weight and structural characteristics.

Principle of Mass Spectrometry

The fundamental principle of mass spectrometry is based on the measurement of the mass-to-charge ratio (m/z) of ionized molecules.​

This analytical approach enables precise characterization of molecules by analyzing the behavior of charged particles in electric or magnetic fields.

To accurately measure molecular mass, several experimental conditions are required:

• The molecules must be analyzed in the gas phase, where individual molecules are isolated.

• The molecules must be ionized, allowing them to respond to electromagnetic fields.

• The system exploits relationships between energy, trajectory, and mass of the ions.

• Ion motion is controlled using electric or magnetic fields.

Basic Operation of a Mass Spectrometer

A mass spectrometer measures the mass of isolated ionized molecules through three principal steps:

1. Vaporization

The first stage involves converting the sample from a condensed phase (solid or liquid) to the gas phase. This step separates molecules from one another, allowing them to be analyzed individually.

2. Ionization

In this stage, neutral molecules are converted into charged ions using ionization techniques. Ion formation allows the molecules to be manipulated and detected using electric fields.

3. Mass Analysis

The generated ions are separated according to their mass to charge ratio (m/z). The mass analyzer calculates the molecular mass.​

Information Obtained from Mass Spectrometry

Mass spectrometry provides several types of structural and molecular information, including:

1. Molecular mass determination of the analyzed compound.

2. Detection of fragment ions, which correspond to smaller molecular fragments generated during ionization.

   Composition of a Mass Spectrometer

   A mass spectrometer consists of three principal components: the ion source, the mass analyser, and the detector. Each component performs a specific function that contributes to the measurement of the mass-to-charge ratio (m/z) of ions.

   Ion Source

   

The ion source is responsible for two fundamental processes: vaporization of the sample and ionization of the molecules. Numerous ionization sources have been developed, each based on a different physical principle.

The choice of ionization method depends on the physicochemical characteristics of the analyte molecule. Depending on the ionization technique, the processes of vaporization and ionization may occur either sequentially or simultaneously.

The main criteria for selecting an ion source include:

• Volatility and thermal stability of the analyte

• Chemical functional groups and their ability to undergo ionization

• Molecular size

• Available sample quantity

• Type of sample introduction, either direct injection or chromatographic coupling ( GC or LC)

Ionization sources are generally classified into two categories:

Hard Ionization Sources

Hard ionization methods often generate molecular ions with an odd number of electrons. These ions are highly energetic and tend to undergo extensive fragmentation, sometimes even before exiting the ion source. The resulting fragments provide valuable structural information about the molecule.

Example:

• Electron Impact Ionization (EI)

Soft Ionization Sources

Soft ionization methods typically produce molecular ions with an even number of electrons that are relatively stable. These ions generally survive long enough to pass through the analyzer and reach the detector, enabling accurate mass measurement.

Examples include:

• Chemical Ionization (CI) : moderately soft

• Fast Atom Bombardment (FAB) : moderately soft

• Laser Desorption (LD)

• Electrospray Ionization (ESI)

• Atmospheric Pressure Photoionization (APPI)

• Atmospheric Pressure Chemical Ionization (APCI)

• Matrix-Assisted Laser Desorption/Ionization (MALDI)

Applications of common ionization techniques:

• EI and CI: small volatile molecules that are not thermally sensitive

• FAB and LD: molecules typically below ~6000 Da

• ESI: non-volatile small molecules, frequently coupled with liquid chromatography (LC–ESI)

• MALDI: large biomolecules such as proteins and macromolecular complexes (1–300 kDa)

Mass Analyzer

The mass analyzer is the component responsible for separating ions according to their mass-to-charge ratio (m/z). Different analyzers operate based on distinct physical principles, but all perform the same fundamental function: measurement of m/z values.

Mass analyzers operate under high vacuum conditions to prevent ion collisions with gas molecules.

Common types of mass analyzers include:

• Magnetic sector (B or BE): ion deflection by a magnetic field; one of the earliest analyzer designs

• Quadrupole (Q): ion filtering using oscillating quadrupole electric fields

• Ion Trap (IT): confinement of ions within an electromagnetic trapping field

• Time-of-Flight (TOF): determination of m/z based on ion flight time

• Fourier Transform Ion Cyclotron Resonance (FT-ICR): measurement based on cyclotron motion of ions in a magnetic field

Ions produced in the source are extracted and focused toward the analyzer using electrostatic fields. These fields may range from a few volts ( in quadrupole or ion trap analyzers) to several tens of kilovolts ( in TOF or magnetic sector instruments).

Key performance parameters of a mass analyzer include:

• Resolution (R)

• Mass range (m/z range)

• Scanning speed

• Sensitivity

• Ion transmission speed

Improving one of these characteristics may sometimes reduce another; therefore, analyzer performance often involves optimization within specific operational limits.

Detector

The detector measures and counts the ions after separation by the mass analyzer. Like the ion source and analyzer, multiple detector technologies exist, each based on different physical principles but serving the same function: conversion of ion signals into measurable electrical signals.

Detectors also operate under high vacuum conditions.

Common detector types include:

• Photographic plates

• Faraday cup detectors

• Electron multipliers

• Photon multipliers

These devices convert the impact of ions into electrical or optical signals, which are then processed to generate the mass spectrum.

Fragmentation in Mass Spectrometry

Fragmentation refers to the process of breaking a molecular ion into smaller fragment ions within the mass spectrometer. This phenomenon provides valuable structural information about the analyzed molecule.

Fragmentation analysis can be enhanced by coupling multiple analyzers in sequence, producing tandem mass spectrometry (MS/MS) or multistage mass spectrometry (MSⁿ).

Principle of Tandem Mass Spectrometry (MS/MS)

• First analyzer: selects ions with a specific m/z value, known as the parent ion (precursor ion). This step isolates a target ion from a complex mixture.

• Collision cell: the selected ion undergoes fragmentation through collisions with an inert gas, producing product (daughter) ions.

• Second analyzer: measures the m/z values of the resulting fragments.

The process can be repeated multiple times, leading to MS³, MS⁴, and higher-order fragmentation analyses, which provide deeper structural characterization.

Coupling Mass Spectrometry with Chromatography (LC–MS / GC–MS)

Although mass spectrometry is a highly powerful analytical technique, it may face limitations when analyzing highly complex mixtures, such as natural products or biological matrices. In such cases, problems may arise including:

• Signal suppression due to the presence of numerous compounds

• Reduced sensitivity

• Reduced resolution

To overcome these limitations, mass spectrometry is often coupled with chromatographic separation techniques, such as:

• Liquid Chromatography–Mass Spectrometry (LC–MS)

• Gas Chromatography–Mass Spectrometry (GC–MS)

Advantages of LC/GC–MS Coupling

• • Efficient separation of complex mixtures prior to mass analysis

  • Improved detection sensitivity

  • Universal detection of eluted compounds

  • Enhanced structural information through fragmentation patterns

  • Selective identification of specific target compounds

  • Capability for quantitative analysis of analytes in complex samples.