What are the four stages in a TOF mass spectrometer?

Ionisation, acceleration, ion drift, detection (then data analysis).

Write an equation for electron impact ionisation of X.

X(g) + e⁻ → X⁺(g) + 2e⁻

Describe electrospray ionisation.

The sample is dissolved in a volatile solvent and forced through a fine needle at high voltage; each particle gains H⁺ to form XH⁺(g).

Why are ions accelerated to the same kinetic energy?

So that their speed depends only on their mass, allowing ions of different mass to be separated by time of flight.

Which ions reach the detector first in a TOF instrument?

The lighter ions, because at constant kinetic energy (½mv²) they travel faster.

What happens to ions at the detector?

They gain electrons, producing a current that is proportional to their abundance.

What does the height of a peak show?

The relative abundance of that isotope.

What does the peak with the highest m/z show for a molecule?

The molecular ion, which gives the relative molecular mass.

Define relative atomic mass.

The weighted mean mass of an atom of an element compared with 1/12 of the mass of an atom of ¹²C.

Write the formula for calculating Ar from isotope data.

Ar = Σ(isotopic mass × abundance) ÷ Σ(abundance), or ÷ 100 if abundances are percentages.

Why is relative atomic mass often not a whole number?

It is a weighted mean of the masses of the isotopes, which occur in different proportions.

Mass spectrometry and relative atomic massAQA A-Level Chemistry: Flashcards

Card 1 of 120 of 12 known

Question

What does a mass spectrometer measure?

Tap or press Space to reveal

Tap card or press Space to flip

See all 12 cards

What does a mass spectrometer measure?: The relative isotopic mass and relative abundance of the isotopes in a sample, as m/z and peak height.
What are the four stages in a TOF mass spectrometer?: Ionisation, acceleration, ion drift, detection (then data analysis).
Write an equation for electron impact ionisation of X.: X(g) + e⁻ → X⁺(g) + 2e⁻
Describe electrospray ionisation.: The sample is dissolved in a volatile solvent and forced through a fine needle at high voltage; each particle gains H⁺ to form XH⁺(g).
Why are ions accelerated to the same kinetic energy?: So that their speed depends only on their mass, allowing ions of different mass to be separated by time of flight.
Which ions reach the detector first in a TOF instrument?: The lighter ions, because at constant kinetic energy (½mv²) they travel faster.
What happens to ions at the detector?: They gain electrons, producing a current that is proportional to their abundance.
What does the height of a peak show?: The relative abundance of that isotope.
What does the peak with the highest m/z show for a molecule?: The molecular ion, which gives the relative molecular mass.
Define relative atomic mass.: The weighted mean mass of an atom of an element compared with 1/12 of the mass of an atom of ¹²C.
Write the formula for calculating Ar from isotope data.: Ar = Σ(isotopic mass × abundance) ÷ Σ(abundance), or ÷ 100 if abundances are percentages.
Why is relative atomic mass often not a whole number?: It is a weighted mean of the masses of the isotopes, which occur in different proportions.

Exam questions on Mass spectrometry and relative atomic mass

A chemist analyses a sample of copper in a time of flight (TOF) mass spectrometer. The sample is vaporised and ionised by electron impact to form ions with a 1+ charge. The ions are accelerated by an electric field, drift along a flight tube and then reach a detector. Copper has two isotopes, ⁶³Cu and ⁶⁵Cu.
Explain why the ions must be accelerated to the same kinetic energy in a TOF mass spectrometer.2 marks
A sample of neon gas from the air is analysed in a mass spectrometer. The sample contains three isotopes, with these relative abundances: ²⁰Ne 90.5%, ²¹Ne 0.3% and ²²Ne 9.2%.
A second sample of neon is enriched in the lighter isotope. It contains only ²⁰Ne and ²²Ne, with relative abundances of 80.0% and 20.0%. Calculate the relative atomic mass of this sample to 3 significant figures.2 marks
A mass spectrometer is used to analyse a sample of an unknown metal. The ions detected are all 1+ ions. The mass-to-charge ratios (m/z) of the peaks and their relative abundances are: m/z 54, 5.8%; m/z 56, 91.8%; m/z 57, 2.1%; m/z 58, 0.3%.
Calculate the relative atomic mass of the metal to 1 decimal place, and use the periodic table to identify the element.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).