Mass Spectrometry | A-level Chemistry | OCR, AQA, Edexcel

Introduction

This tutorial provides a comprehensive overview of mass spectrometry, a crucial technique in A-level Chemistry. It outlines the principles of mass spectrometry, the process of Time of Flight (TOF) mass spectrometry, and how to analyze mass spectra. Understanding these concepts is essential for identifying unknown compounds, determining isotopic abundances, and gaining structural information about molecules.

Step 1: Understanding Mass Spectrometry

Mass spectrometry is a molecular analysis technique that operates on three fundamental principles:

• Ion Formation: Ions are generated from the sample.
• Ion Separation: Ions are separated based on their mass-to-charge (m/z) ratio.
• Ion Detection: The ions are detected to provide information about their composition.

Applications of Mass Spectrometry

• Structural elucidation of compounds
• Identification of unknown substances
• Determination of isotopic abundances

Step 2: Time of Flight Mass Spectrometry Process

TOF mass spectrometry consists of several key steps:

Step 2.1: Ionisation

• Dissolve the sample in a volatile solvent.
• Vaporize the solvent and force it through a hollow needle connected to the positive terminal of a high voltage supply.
• This process creates tiny positively charged droplets that lose electrons.
• As the solvent evaporates, the droplets shrink to form single positively charged ions.

Step 2.2: Acceleration

• The positive ions move toward a negatively charged electric plate.
• This attraction causes the ions to accelerate, ensuring they all have the same kinetic energy.
• Heavier ions will move more slowly compared to lighter ions.

Step 2.3: Ion Drift

• The ions pass through a hole in the negatively charged plate, forming a beam of ions.
• This beam travels along a tube, known as the Flight Tube.

Step 2.4: Detection

• Upon reaching the detector, the flight times of the ions are recorded.
• Positive ions capture an electron, resulting in a flow of current.

Step 2.5: Data Analysis

• The detector sends a signal to a computer, which generates a mass spectrum.
• The mass spectrum provides valuable information about the mass and abundance of the ions.

Step 3: Low Resolution Mass Spectrometry

• Understand that isotopes, such as chlorine (Cl-35 and Cl-37), have the same number of protons but different numbers of neutrons, leading to different m/z ratios.
• Low resolution mass spectrometry can resolve masses to one decimal place, while high resolution can achieve precision up to five decimal places.
• This precision allows for the distinct detection of isotopes, enabling accurate analysis of elemental compositions.

Conclusion

Mass spectrometry is a powerful tool in chemistry for analyzing the composition and structure of molecular compounds. By understanding the steps involved in TOF mass spectrometry and how to interpret mass spectra, you can effectively identify unknown substances and determine isotopic ratios. Consider applying this knowledge in practical laboratory settings or further studies in analytical chemistry.