Mass Spectrometry – TOF Instrument & Data Interpretation

These flashcards review the core principles and processes of TOF mass spectrometry, including ionisation methods, instrument stages, spectrum interpretation, and an isotopic example for lithium.

What information can a mass spectrometer provide about a sample?

Relative atomic mass, relative isotopic mass, relative molecular mass, and the relative abundance of each isotope present.

Name the four main stages in a time-of-flight (TOF) mass spectrometer.

1) Ionisation 2) Acceleration 3) Ion drift 4) Detection

What happens during the ionisation stage of a TOF mass spectrometer?

The sample is converted into positive ions by either electrospray ionisation or electron-impact ionisation.

Describe electrospray ionisation.

The sample is dissolved in a solvent, forced through a fine nozzle at high pressure, and a high voltage is applied so each molecule gains an H⁺; the solvent then evaporates, leaving positive ions.

Describe electron-impact ionisation.

The sample is vaporised and bombarded with high-energy electrons, each particle loses one electron and forms a 1⁺ ion.

What occurs during the acceleration stage in a TOF mass spectrometer?

Positive ions are accelerated by an electric field so that they all gain the same kinetic energy.

What occurs in the ion-drift region of a TOF mass spectrometer?

Ions travel through a field-free region; lighter ions drift faster than heavier ones.

How are ions detected in a TOF mass spectrometer?

Ions strike the detector, causing a small current; the time taken to arrive is measured and converted into a mass-to-charge (m/z) value.

What does a mass spectrum display?

A chart of relative isotopic masses (x-axis: m/z) against their relative abundances (y-axis).

How can an element be identified from its mass spectrum?

Each element has a unique isotopic pattern—specific m/z values and abundances—that acts as a fingerprint for comparison with known data.

How many naturally occurring isotopes does lithium have and what are their relative abundances?

Two isotopes: ⁶Li at 7.6 % and ⁷Li at 92.4 % (peaks at m/z = 6 and m/z = 7, respectively).