mass spectronomy

What is mass spectrometry?

An analytical technique used to identify isotopes or molecules and determine their relative masses and abundances.

What does m/z mean?

Mass-to-charge ratio.

What does TOF stand for?

Time of Flight.

What are the four main stages of TOF mass spectrometry?

Ionisation, acceleration, ion drift, detection.

Why must particles be ionised?

Only charged particles can be accelerated by an electric field and detected.

What two ionisation methods are required for AQA A-level Chemistry?

Electron impact ionisation and electrospray ionisation.

What happens during electron impact ionisation?

High-energy electrons collide with gaseous particles and remove an electron, forming positive ions.

What is the equation for electron impact ionisation?

X(g) + e⁻ → X⁺(g) + 2e⁻.

Why can electron impact cause fragmentation?

The high-energy electron collisions can break covalent bonds.

What happens during electrospray ionisation?

The sample is dissolved in a volatile solvent and passed through a fine needle at high voltage, forming positively charged droplets that lose solvent to produce gaseous ions.

What ion is commonly formed during positive electrospray ionisation?

[M + H]⁺.

What does M represent in [M + H]⁺?

The original molecule.

Why is electrospray useful for large molecules?

It causes less fragmentation than electron impact ionisation.

What happens during acceleration?

Positive ions are accelerated by an electric field so they have the same kinetic energy.

What is the kinetic energy equation?

KE = ½mv².

What happens in the flight tube?

Ions travel through a vacuum towards the detector.

Why is the flight tube under vacuum?

To prevent ions colliding with air particles.

Which ions reach the detector first?

Ions with the lowest m/z.

Why do lighter ions travel faster?

For the same kinetic energy, a smaller mass gives a greater velocity.

What happens when an ion reaches the detector?

The ion gains an electron, causing a current to flow.

What determines the size of the detector current?

The number of ions reaching the detector.

What does the x-axis of a mass spectrum show?

m/z.

What does the y-axis of a mass spectrum show?

Relative abundance or relative intensity.

What is the base peak?

The most abundant ion peak, assigned an intensity of 100%.

Why do isotopes form different peaks?

They have different masses and therefore different m/z values.

For a singly charged ion, what does m/z equal?

The mass of the ion.

What happens to m/z for an ion with a 2+ charge?

m/z is approximately half the ion's mass.

How is relative atomic mass calculated from mass spectrometry data?

Σ(isotopic mass × abundance) ÷ Σ(abundance).

What is relative atomic mass, Ar?

The weighted mean mass of an atom compared with one-twelfth of the mass of a carbon-12 atom.

Why is Ar usually not a whole number?

Most elements contain several isotopes with different abundances.

What is the molecular ion peak?

The peak produced by an ion representing the whole molecule without fragmentation.

How can the molecular ion peak be used to find Mr?

For a singly charged molecular ion, its m/z value corresponds approximately to Mr.