Atomic Structure - Time of Flight Mass Spectrometry｜AQA A Level Chemistry Revision

Summary

This lesson provides an in-depth overview of Time of Flight (TOF) Mass Spectrometry, covering the key stages required for the AQA specification. It explains the process of ionization (both electrospray and electron impact), the subsequent acceleration and drift of ions, their detection, and the analysis of data. Key equations for calculating kinetic energy and time of flight are also discussed, with emphasis on the importance of understanding units and conversions. The lesson concludes with active recall and practice questions to help students solidify their understanding and improve their exam performance.

Takeaways

• 😀 The Time-of-Flight Mass Spectrometer (TOF MS) identifies elements by determining isotopes present in a sample.
• 😀 TOF MS involves five key stages: Ionization, Acceleration, Ion Drift, Ion Detection, and Data Analysis.
• 😀 A high vacuum is essential for TOF MS to prevent ions from colliding with air molecules and disrupting results.
• 😀 Electrospray Ionization (ESI) is used for large organic molecules, where the sample is dissolved in a polar solvent and ionized without fragmentation.
• 😀 Electron Impact Ionization (EI) is used for smaller molecules and causes fragmentation, which is helpful for analyzing low mass substances.
• 😀 In the Acceleration stage, positive ions are accelerated towards a negatively charged plate, with lighter ions traveling faster than heavier ones.
• 😀 The velocity of ions in the Acceleration stage is dependent on their mass: lighter ions travel faster, and heavier ions travel slower.
• 😀 During Ion Drift, ions move towards the detector. Their time of flight depends on their mass, with lighter ions reaching the detector quicker.
• 😀 In Ion Detection, the current produced is proportional to the abundance of the ion species detected, giving a measure of their relative abundance.
• 😀 Data Analysis in TOF MS involves interpreting the mass spectrum, which shows the mass-to-charge ratio and the relative abundance of the detected ions.

Q & A

What is the primary function of Time-of-Flight Mass Spectrometry (TOF-MS)?

The primary function of TOF-MS is to identify elements by determining the isotopes present within a sample.

Why is it important for the mass spectrometer to be kept under a vacuum?

The mass spectrometer must be kept under a vacuum to prevent the ions from colliding with molecules in the air, which would interfere with the results and distort the mass spectrum.

What is the difference between Electrospray Ionization (ESI) and Electron Impact Ionization (EI)?

In Electrospray Ionization (ESI), the sample is dissolved in a volatile and polar solvent and ionized by gaining a proton, making it ideal for large organic molecules. In Electron Impact Ionization (EI), the sample is vaporized and ionized by a high-energy electron gun, causing fragmentation, which is suitable for small molecules.

How does the ionization process in Electrospray Ionization (ESI) prevent fragmentation of large organic molecules?

In ESI, the sample is ionized by gaining a proton, which avoids fragmentation, making it ideal for analyzing large organic molecules.

What happens during the acceleration stage in TOF-MS?

During the acceleration stage, positive ions are accelerated towards a negatively charged plate by an electric field, achieving constant kinetic energy. Lighter ions travel faster than heavier ions due to their smaller mass.

How does the mass-to-charge ratio (m/z) influence the time it takes for ions to reach the detector?

Ions with a lower mass-to-charge ratio (m/z) travel faster than those with a higher m/z, so they reach the detector sooner in the time-of-flight stage.

What is the role of the negatively charged detection plate in TOF-MS?

The negatively charged detection plate attracts the positive ions, allowing them to gain an electron and produce a current, which is then measured. The amount of current is proportional to the abundance of the ion.

What two key pieces of information are used in data analysis during TOF-MS?

Data analysis in TOF-MS involves the current produced by the ions and their time of flight (T), which together help generate the mass spectrum.

What is the relationship between ion mass and velocity in the acceleration stage of TOF-MS?

In the acceleration stage, lighter ions travel faster than heavier ions due to their lower mass, as their velocity is inversely proportional to their mass.

Why is it important to convert mass from grams to kilograms when performing TOF-MS calculations?

It is important to convert mass from grams to kilograms because the kinetic energy equation in TOF-MS calculations requires mass to be in kilograms to ensure the correct unit consistency.