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1.1.3Physical Chemistry

Time of Flight Mass Spectrometry — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 3 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6The Four Stages

Ions travel from the sample to the detector, and the time taken depends on their mass.

1
Ionisation: the sample is turned into positive ions, by electron impact or by electrospray ionisation.
2
Acceleration: the positive ions are accelerated by an electric field so that all of them have the same kinetic energy.
3
Ion drift: the ions pass through a field-free flight tube, where lighter ions travel faster and arrive sooner.
4
Detection: the ions hit a detector that records the time of flight and produces a current signal.
5
Data processing: a computer converts the flight times to m/z values and the signals to relative abundances in a mass spectrum.

Part 2 of 6Ionisation Methods

Electron impact
  • Electron impact High-energy electrons knock an electron from each atom or molecule: X(g) + e⁻ → X⁺(g) + 2e⁻.
Electrospray
  • Electrospray The sample is dissolved in a volatile solvent and sprayed through a charged needle, forming ions by gaining a proton.
Electrospray equation
  • Electrospray equation X(g) + H⁺ → XH⁺(g), so the measured m/z is one unit greater than the relative molecular mass.
Vacuum
  • Vacuum The instrument is kept under vacuum so ions do not collide with air particles on their way to the detector.

Part 3 of 6Why Mass Affects Flight Time

Why Mass Affects Flight Time
Equal kinetic energy
  • Equal kinetic energy After acceleration, ions have the same kinetic energy, KE = ½mv².
Speed
  • Speed Rearranging gives v = √(2KE ÷ m), so lighter ions have a higher speed.
Flight time
  • Flight time The time to cross a flight tube of length d is t = d ÷ v = d√(m ÷ 2KE), so t rises with mass.
m/z
  • m/z The mass spectrum plots abundance against mass-to-charge ratio, and for ions with a 1+ charge m/z equals the mass number.

Part 4 of 6Using the Spectrum

Peak position
  • Peak position Each peak’s position gives the m/z of an isotope; its height gives the relative abundance.
Molecular ion
  • Molecular ion For a molecule, the peak with the highest m/z is usually the molecular ion, giving the relative molecular mass.
Electrospray correction
  • Electrospray correction If the spectrum was recorded by electrospray, subtract 1 from the highest m/z to find Mr.
Calculation
  • Calculation Ar is found by multiplying each m/z by its abundance, adding, and dividing by the total abundance.

Part 5 of 6Worked Example

Question

  • A sample of magnesium gave peaks at m/z 24, 25 and 26 with abundances 78.6 %, 10.1 % and 11.3 %.
  • Calculate the relative atomic mass of magnesium to 3 significant figures.

  1. Multiply
    • 24 × 78.6 = 1886.4;
      • 25 × 10.1 = 252.5;
      • 26 × 11.3 = 293.8.
  2. Add1886.4 + 252.5 + 293.8 = 2432.7.
  3. Divide by the total abundance of 100: 2432.7 ÷ 100 = 24.327.
  4. Round to 3 significant figures.

Answer Ar of magnesium = 24.3

Part 6 of 6Exam Checklist & Tips

1.1.3 – Key Points for the Exam

  • m/z
    • The mass-to-charge ratio of an ion;
      • for a 1+ ion it equals the ion’s relative mass.
  • Time of flight The time an ion takes to travel the length of the flight tube.
  • Electrospray ionisation A soft ionisation method in which the sample is sprayed in solution and ionised by gaining H⁺.
  • Molecular ion The ion formed when a molecule loses one electron, whose m/z equals the relative molecular mass.

Common Mistakes

  • Saying heavier ions travel faster; at equal kinetic energy lighter ions are faster and arrive first.
  • Forgetting to subtract 1 from the m/z of the molecular peak when electrospray ionisation has been used.
  • Omitting the vacuum and stating that ions would be deflected by air in the tube.
  • Treating the height of the peak as the m/z value rather than the abundance.

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