What are the 4 stages of time-of-flight mass spectrometry?

The 4 stages of time-of-flight mass spectrometry are:

  1. Ionisation
  2. Ion acceleration
  3. Ion drift
  4. Ion detection

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How is a sample ionised using electron impact?

A beam of high-energy electrons is fired at a vaporised sample.

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How is a sample ionised using electrospray ionisation?

A sample is dissolved in a volatile solvent and injected through a charged needle. Molecules are ionised by gaining a proton from the solvent as they leave the needle. 

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How are positive ions accelerated in a time-of-flight mass spectrometer?

An electric field is used to attract ions towards a negatively charged plate. 

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How are ions of different masses separated by time-of-flight mass spectrometry?

Ions have the same kinetic energy as they travel through a flight tube. Lighter ions travel faster and reach the detector sooner.

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How are ions detected in a time-of-flight mass spectrometer?

Ions gain an electron at the detector which creates an electric current.

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How does a mass spectrometer measure the relative abundance of each isotope?

By the size of the current produced - a more abundant isotope generates a larger current in the detector.

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How is the molecular ion (M+) identified in a mass spectrum?

The molecular ion (M+) is the peak with the highest m/z value. 

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How is the relative molecular mass of a molecule deduced from its mass spectrum?

By identifying the m/z value of the molecular ion peak

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How can the number of isotopes of a particular element be determined from a mass spectrum?

By counting the number of peaks.

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How can relative isotopic abundance by determined from a mass spectrum?

By the relative height of each peak.

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What is the formula for time of flight (t), in terms of mass (m), length of flight tube (d), and kinetic energy (Ek)?

t=d×√2Ek​m​​


t = time of flight (s)

d = length of flight tube (m)

Ek = kinetic energy (J)

m = mass of ion (kg)

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