Download Time of Flight Mass Spectrometry: Ionization, Acceleration, and Detection and more Exams Chemistry in PDF only on Docsity!
Student guide: Time of flight mass spectrometry
This guide relates to section 3.1.1.2 of our AS and A-level Chemistry specifications. We have produced it to supplement the specification and the Teaching Notes that are already available. On a separate document we have given you a range of summary questions on the topic with example marking guidance.
Time of flight mass spectrometry
Mass spectrometry is a powerful instrumental method of analysis. It can be used to:
find the abundance and mass of each isotope in an element allowing us to determine its relative atomic mass
find the relative molecular mass of substances made of molecules.
A common form of mass spectrometry is time of flight (ToF) mass spectrometry. In this technique, particles of the substance are ionised to form 1+ ions which are accelerated so that they all have the same kinetic energy. The time taken to travel a fixed distance is then used to find the mass of each ion in the sample.
Stage 1 – Ionisation
The sample can be ionised in a number of ways. Two of these techniques are electron impact and electrospray ionisation (which are simplified here for AS/A level).
Electron impact (also known as electron ionisation)
The sample being analysed is vaporised and then high energy electrons are fired at it. The high energy electrons come from an ‘electron gun’ which is a hot wire filament with a current running through it that emits electrons. This usually knocks off one electron from each particle forming a 1+ ion.
X(g) + e–^ X+(g) + 2e–
(also written as X(g) X+(g) + e–)
The 1+ ions are then attracted towards a negative electric plate where they are accelerated.
This technique is used for elements and substances with low formula mass (that can be inorganic or organic molecules). When molecules are ionised in this way, the 1+ ion formed is known as a molecular ion.
eg methane CH 4 (g) + e–^ CH 4 +(g) + 2e–
(also written as CH 4 (g) CH 4 +(g) + e–)
The molecular ion often breaks down into smaller fragments some of which are also detected in the mass spectrum. (Fragmentation of molecular ions is not included on the specification and is only included here as useful background information).
Electrospray ionisation
The sample X is dissolved in a volatile solvent (eg water or methanol) and injected through a fine hypodermic needle to give a fine mist (aerosol). The tip of the needle is attached to the positive terminal of a high-voltage power supply. The particles are ionised by gaining a proton (ie an H+^ ion which is simply one proton) from the solvent as they leave the needle producing XH+^ ions (ions with a single positive charge and a mass of M r + 1).
X(g) + H+^ XH+(g)
The solvent evaporates away while the XH+^ ions are attracted towards a negative plate where they are accelerated.
Fine mist
Hypodermic needle attached to positive terminal of high- voltage power supply
Sample in volatile solvent
Particles gain a proton as they leave the needle
Positive ions are accelerated by a negative electric plate
Gaseous sample
Positive ions are accelerated by a negative electric plate
An electron is knocked off each particle by the high- energy electrons to form 1+ ions
Electron gun (hot wire filament)
High-energy electrons
The time of flight along the flight tube is given by the following expression:
(students would be given this equation if expected to use it in an exam)
t = time of flight (s)
d = length of flight tube (m)
𝑣 = velocity of the particle (m s–^1 )
m = mass of the particle (kg)
KE = kinetic energy of particle (J)
This shows that the time of flight is proportional to the square root of the mass of the ions. Therefore lighter ions travel faster and reach the detector in less time than the heavier particles that move slower and take longer to reach the detector.
eg Ions of the three isotopes of magnesium (^24 Mg+, 25 Mg+, 26 Mg+) will travel at different speeds through the flight tube and separate, with the lightest ion (^24 Mg+) reaching the detector first.
Flight tube
Ions set off along the flight tube at the same time^ Detector
24
26
25
Detector
24
26
25 Lighter ions travel faster and start to separate out
Detector
24
26
25
The lightest ions reach the detector first
Stage 4 – Detection
The positive ions hit a negatively charged electric plate. When they hit the detector plate, the positive ions are discharged by gaining electrons from the plate. This generates a movement of electrons and hence an electric current that is measured. The size of the current gives a measure of the number of ions hitting the plate.
The mass spectrum
A computer uses the data to produce a mass spectrum. This shows the mass to charge ( m/z ) ratio and abundance of each ion that reaches the detector. Given that all ions produced by electrospray ionisation and most of the ions by electron ionisation have a 1+ charge, the m/z is effectively the mass of each ion.
In the following example, the mass spectrum of magnesium is shown. Ions with mass to charge ratio 24.0, 25.0 and 26.0 reach the detector. This shows that magnesium is made up of three isotopes: 24 Mg, 25 Mg and 26 Mg.
The relative atomic mass of an element can be found by calculating the mean mass of these isotopes.
relative atomic mass ( A r) = combined mass of all isotopes combined abundance of all isotopes
eg for magnesium:
relative atomic mass ( A r) = (79.0 × 24.0) + (10.0 × 25.0) + (11.0 × 26.0) = 24. 79.0 + 10.0 + 11.
For molecules that are ionised by electron impact, the signal with the greatest m/z value is from the molecular ion and its m/z value gives the relative molecular mass. However, there may be some other small peaks present around the molecular ion peak due to molecular ions that contain different isotopes.
20.0 21.0 22.0 23.0 24.0 25.0 26.0 27.0 28.0 29.0 30.
Relative abundance
100 90 80 70 60 50 40 30 20 10 0
mass / charge ratio
79.0%
10.0% 11.0%
