Time of Flight Mass Spectrometry

Mass spectrometry is a technique used to determine the atomic or molecular mass of substances by converting them into ions and analysing their mass-to-charge ratio .

As the charge of the ions is typically just 1+, the mass to charge ratio is typically equal to the mass of the ion

Time of Flight (TOF) Mass Spectrometry is a specific type of mass spectrometry that determines the mass of ions based on the time taken to travel through a flight tube.

The relative atomic mass displayed on the periodic table represents a weighted average of all the naturally occurring isotopes of an element. This value is calculated based on the mass and relative abundance of each isotope. This information comes from the results of time of flight (TOF) mass Spectrometry.

A mass spectrum displays the different isotopes of an element and their relative abundances (percentage or ratio). The peaks on the graph correspond to the mass-to-charge ratio of each isotope, with taller peaks representing more abundant isotopes.

For example, the mass spectrum for Boron shows the following isotope abundances:

More detail on relative atomic mass calculations can be found in the ‘Isotopes and Relative Atomic Mass’ note

Vacuum Requirement:

A vacuum ensures that the ions travel freely through the machine without colliding with air molecules, preventing loss or deflection of particles.

The process involves four main stages:

Stage 1: Ionisation

Two methods are commonly used:

(i) Electron Impact (used for elements and small molecules):

1. The sample is vaporised and bombarded with high-energy electrons.

2. This removes an electron, forming positive ions.

Equation:

Net equation:

(ii) Electrospray Ionisation (used for larger molecules (e.g., proteins, pharmaceuticals).

1. The sample is dissolved in a polar solvent and passed through a fine needle at high voltage.

2. It gains a proton from the solvent to form ions.

Equation:

Stage 2: Acceleration

• Positive ions are accelerated by an electric field, ensuring they have constant kinetic energy.

• kinetic energy

• mass of particle

• velocity of particle

If we rearrange this for v we get:

Stage 3: Ion Drift (Flight Tube)

• Ions move through a drift region where no forces act on them.

• Lighter ions move faster, while heavier ions move more slowly.

• Formula for Time of Flight (TOF):

Where:

• time

• distance

• velocity

Combining (2) and (3) we get:

Stage 4: Detection

Since KE is constant for all ions, ions with lower mass take less time to reach the detector.

When the ions hit the detector, they gain an electron, producing a current.

The size of the current is proportional to the abundance of the ion.