AQA AS and A level
Time of flight mass spectrometry calculations
Practise ion speed, flight time and mass calculations with the generator, or write a complete answer before checking the worked solution.
Ionisation and detection: TOF process questions.
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TOF 0.1.1 · site 3.62.1
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Made by Dr T. Smith · Chemistryhelp.co.uk
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A reliable TOF calculation route
- Start with v = d ÷ t when distance and time are given.
- Use KE = ½mv² to connect that speed to the mass of one ion.
- Convert between the mass of one ion and the mass of one mole using 6.022 × 10²³ mol⁻¹.
- For electrospray, the detected ion is usually [M + H]⁺.
Electron impact and electrospray
Electron impact
A gaseous particle loses an electron to form a positive ion, usually M⁺.
Electrospray
A molecule gains H⁺, so the peak for [M + H]⁺ is one unit above M.
Common slips
- Using centimetres in v = d ÷ t; convert the distance to metres.
- Leaving the mass of one ion in grams; convert it to kilograms.
- Stopping at v²; take the square root to obtain v.
- Multiplying or dividing by the Avogadro constant in the wrong direction.
- Forgetting to subtract 1 after electrospray ionisation.