Mass Spectrometry – Principle, Steps, Instrumentation, Types & Applications

Table of Contents

Introduction

Mass Spectrometry (MS) is a highly precise analytical technique used to identify and measure the amount of chemicals present in a sample. It works by ionizing molecules and separating them based on their mass-to-charge ratio (m/z).

The method is widely used in biochemistry, environmental monitoring, forensic science, pharmaceuticals, and geology because it can detect even tiny quantities of substances and provide structural information about molecules.

The output of mass spectrometry is called a mass spectrum, which is a graph plotting relative abundance of detected ions against their mass-to-charge ratio. From this, scientists can determine the composition, structure, and quantity of molecules present.

Principle of Mass Spectrometry

Mass spectrometry works on the principle of generating charged particles (ions) from the sample and then separating them based on their mass-to-charge ratio.

Ionization: The sample is converted into gas-phase ions, often by electron bombardment.
Separation: Ions are accelerated and passed through an electric or magnetic field, which deflects them differently depending on their mass and charge.
Detection: Ions hit the detector, generating signals that are analyzed to produce the mass spectrum.

Key points about the principle:

Lighter ions are deflected more than heavier ions.
Higher-charged ions are deflected more than singly charged ions.
The m/z value helps identify the ion and, indirectly, the molecule it came from.

Working of Mass Spectrometry – Step-by-Step

Sample Introduction
- Sample (solid, liquid, or gas) is introduced into the system.
- Usually stored in a reservoir and fed at low pressure through a small opening called the molecular leak.
Ionization
- Atoms/molecules are ionized, typically by removing electrons.
- Main ionization methods:
  - Electron Ionization (EI-MS) – High-energy electrons knock off electrons from molecules, creating positive ions.
  - Chemical Ionization (CI-MS) – Uses ion–molecule reactions to produce ions.
  - Fast Atom Bombardment (FAB) – Useful for large biomolecules.
Acceleration
- Positive ions are accelerated using electric fields so that all have the same kinetic energy.
- The ions pass through slits with decreasing voltage until they reach zero potential.
Deflection
- A magnetic field deflects the ions.
- Lighter ions or ions with higher charges are deflected more.
- The degree of deflection depends on mass, charge, and magnetic field strength.
Detection
- Ions reach the detector, where their charge is neutralized.
- The number of ions hitting the detector is measured to give relative abundance.

Instrumentation of Mass Spectrometry

A modern mass spectrometer has the following main components:

1. Sample Inlet System

Controls how the sample enters the instrument.
Must maintain low pressure to avoid collisions between ions and air molecules.

2. Ion Source

Converts sample molecules into ions.
Examples: EI, CI, FAB, MALDI (Matrix-Assisted Laser Desorption/Ionization), ESI (Electrospray Ionization).

3. Mass Analyzer

Separates ions based on m/z ratio.
Types of mass analyzers:
- Magnetic Sector Analyzer – Uses a magnetic field to bend ion paths.
- Double Focusing Analyzer – Combines magnetic and electric fields for high resolution.
- Quadrupole Analyzer – Uses oscillating electric fields to filter ions.
- Time-of-Flight (TOF) Analyzer – Measures the time ions take to reach the detector.
- Ion Trap Analyzer – Traps ions and releases them in sequence.
- Ion Cyclotron Resonance Analyzer – High-resolution, uses strong magnetic fields.

4. Detector

Measures the abundance of ions.
Common types: electron multipliers, Faraday cups, photomultiplier tubes.

5. Data System

Processes and displays the mass spectrum.

Types of Ionization Techniques

Electron Ionization (EI) – Common in gas-phase molecules, produces reproducible fragmentation patterns.
Chemical Ionization (CI) – Softer ionization method, causes less fragmentation.
Electrospray Ionization (ESI) – Ideal for biomolecules like proteins and nucleic acids.
Matrix-Assisted Laser Desorption/Ionization (MALDI) – Uses a laser and matrix for large biomolecules.
Fast Atom Bombardment (FAB) – For polar, thermally labile molecules.

Applications of Mass Spectrometry

In Environmental Science

Detecting soil, water, and air pollutants.
Measuring pesticide residues in food.
Monitoring industrial emissions.

In Geology

Dating rocks and minerals.
Studying soil composition.
Oil and gas exploration.

In Chemistry

Determining molecular weight.
Studying molecular structures.
Quality control in petrochemicals.

In Biology & Medicine

Identifying biomolecules like proteins, carbohydrates, and nucleic acids.
Sequencing proteins and peptides.
Diagnosing metabolic disorders.
Monitoring gases in patient’s breath during surgery.
Detecting drug abuse in blood, urine, and saliva.

In Forensics

Identifying unknown substances in crime scenes.
Detecting toxins or drugs in biological samples.

Advantages of Mass Spectrometry

High sensitivity and precision.
Can analyze small sample amounts.
Provides structural information.
Suitable for a wide range of molecules.

Limitations of Mass Spectrometry

Equipment is expensive.
Requires skilled operation.
Some samples may require complex preparation.

Frequently Asked Questions (FAQ)

Q1. What is the basic principle of mass spectrometry?
It separates ions based on their mass-to-charge ratio using electric or magnetic fields.

Q2. What does m/z mean in mass spectrometry?
It is the mass-to-charge ratio of an ion.

Q3. Can mass spectrometry detect all elements?
Yes, but ionization efficiency may vary for different substances.

Q4. What is a mass spectrum?
A plot showing the relative abundance of ions versus their m/z ratio.

Q5. Is mass spectrometry destructive?
Yes, in most cases the sample is consumed during analysis.