A mass spectrometer is able to very accurately determine the relative atomic mass of an element, by measuring the mass and abundance each of the isotopes of an element in a sample (and this may vary depending on where the element sample has come from).
If you are following the AQA A level syllabus, you need to know how the mass spectrometer works in some detail. AQA has published an excellent set of notes (and also questions) which you should download.
If you are following either the OCR A or OCR B syllabus you don’t need to learn this, but I would encourage you to watch the video so you have an understanding of where a mass spectrum comes from, as you do need to be able to interpret the data in a variety of ways.
Time of flight calculations
Once the sample has been ionised (all of the atoms of different isotopes are now 1+ ions), the ions are accelerated so that they have the same kinetic energy.
We can rearrange this equation to find the velocity of an ion.
The time taken for an ion to travel the length of the flight tube depends on its velocity and the distance travelled (as in the length of that flight tube).
Combining these equations gives us:
If you are mathematically minded and can show how the equations are derived, brilliant! You need to be able to do this in examination questions.
If you are, like me, mathematically challenged, then spend some time studying how one equation comes from another, and memorise it – it’s far easier in the long run 😅.
E.g. Calculate the time taken for a 24Mg+ ion to travel the length of a flight tube that was 1.2m long, given that the ions were accelerated to have 1.0 x 10-16 J of kinetic energy.
It’s good practice to set out your calculations clearly – I always start with the equation and then annotate it with information from the question. That way I can see clearly the steps I need to take to get to the correct answer (and the examiner can also follow your logic which means if you slip up, you are still likely to pick ups few marks).
Just be aware, there’s a mistake in the video 😟 – the mass of the ion in question 1 should be 3.9854 x 10-26kg but the final answer is correct :
Practice questions
- Calculate the time taken for a 39K+ ion to travel the length of a flight tube that was 0.9m long, given that the ions were accelerated to have 2.71 x 10-16 J of kinetic energy.
- Calculate the length of a flight tube given that a 47Ti+ ion was accelerated to 2.0 x 10-16 J of kinetic energy and it took 1.89 x 10-5 s to travel to the detector.
- In a time of flight mass spectrometer the ions are accelerated to have 1.0 x 10-16 J of kinetic energy. The time taken for a 69Ga+ ion to reach the detector is 7.020 x 10-5 s. Calculate the time of flight for a 71Ga+ ion.
Answers
