Time of Flight Mass Spectrometry Flashcards

Vocabulary flashcards covering principles, equations, stages, and mass spectra interpretation for time of flight (ToF) mass spectrometry.

Time of Flight (ToF) Mass Spectrometry

A form of mass spectrometry where particles of a substance are ionised to form $1+$ ions and accelerated so that they all have the same kinetic energy, allowing the time taken to travel a fixed distance to be used to find the mass of each ion.

Electron Impact Ionisation

An ionisation technique used for elements and low formula mass substances where vaporised sample particles are fired at by high-energy electrons from an electron gun, knocking off one electron to form a $1+$ ion via $\text{X}_{(g)} + e^- \rightarrow \text{X}^+_{(g)} + 2e^-$.

Electron Gun

A hot wire filament with an electric current running through it that emits high-energy electrons used to ionise vaporised samples in electron impact ionisation.

Molecular Ion

The $1+$ ion formed when a molecule loses an electron during electron impact ionisation.

Electrospray Ionisation

A soft ionisation technique where a sample is dissolved in a volatile solvent and injected through a fine hypodermic needle attached to a high-voltage power supply, gaining a proton to form $\text{XH}^+$ ions with mass $M_r + 1$ via $\text{X}_{(g)} + \text{H}^+ \rightarrow \text{XH}^+_{(g)}$.

Stage 1 – Ionisation Methods

A diagram contrasting electron impact ionisation (top), where high-energy electrons knock off an electron from sample particles, with electrospray ionisation (bottom), where particles gain a proton at a high-voltage hypodermic needle.

Stage 2 – Acceleration

The stage in time of flight mass spectrometry where positive ions are accelerated using an electric field so that all ions acquire the exact same kinetic energy.

Kinetic Energy Equation

The formula $KE = \frac{1}{2}mv^2$, where $KE$ is kinetic energy in Joules ($\text{J}$), $m$ is particle mass in kilograms ($\text{kg}$), and $v$ is particle velocity in meters per second ($\text{m}\text{s}^{-1}$).

Ion Velocity Equation

The equation $v = \text{sqrt}\frac{2KE}{m}$, showing that the velocity of an ion in time of flight mass spectrometry depends inversely on the square root of its mass when kinetic energy is constant.

Stage 3 – Flight Tube

The drift region where positive ions travel through a negatively charged plate hole into a tube, separating based on speed as lighter ions travel faster than heavier ones.

Time of Flight Equation

The expression $t = d\text{sqrt}\frac{m}{2KE}$, where $t$ is time of flight in seconds ($\text{s}$), $d$ is flight tube length in meters ($\text{m}$), $m$ is particle mass in kilograms ($\text{kg}$), and $KE$ is kinetic energy in Joules ($\text{J}$).

Stage 4 – Detection

The final stage where positive ions hit a negatively charged electric plate, gain electrons to become discharged, and generate an electric current proportional to the number of ions hitting the plate.

Mass Spectrum

A plot produced by a computer displaying the mass-to-charge ($m/z$) ratio against the relative abundance of each ion reaching the detector.

Mass-to-Charge Ratio ($m/z$)

The ratio of an ion's mass to its charge, which is effectively equal to the mass of the ion when the ion carries a $1+$ charge.

Relative Atomic Mass ($A_r$) Formula

The formula $\text{relative atomic mass } (A_r) = \frac{\text{combined mass of all isotopes}}{\text{combined abundance of all isotopes}}$ used to calculate the mean mass of an element's isotopes.

Soft Ionisation

An ionisation method such as electrospray ionisation used for high molecular mass biological molecules (like proteins) where fragmentation rarely takes place.

Propane Mass Spectrum Molecular Ion Peak

Following electron impact ionisation of propane, the peak with the greatest $m/z$ value at $44$ represents the molecular ion, indicating a relative molecular mass of $44$.

Protein Protonation Peak ($MH^+$)

In electrospray ionisation of a protein, a peak at $m/z = 521.1$ for $MH^+$ indicates that the relative molecular mass of the protein is $520.1$.