In 2005, the Huygens probe landed on Titan, Saturn’s largest moon. On its descent through the moon’s atmosphere, it took samples of the gases present, and once landed, it managed to vaporise some of the frozen hydrocarbons that make up Titan’s surface. Scientists wanted to know exactly what elements, isotopes and molecules the samples contained, and to do this, they used a process called mass spectrometry.
Mass spectrometry is an analytical technique used to determine the mass of various molecules by finding out the mass-to-charge ratio of their ions. It can also give information about the relative molecular mass of a compound, and the relative abundance and masses of isotopes of an element.
What is time of flight mass spectrometry?
Time of flight (TOF) mass spectrometry is a form of mass spectrometry that accelerates positively charged ions to the same kinetic energy. Scientists can calculate the mass of the ions using this kinetic energy and their time taken to travel a fixed distance down the flight tube.
What are the stages of TOF spectrometry?
Kinetic energy? Flight tubes? TOF spectrometry may sound like a complicated process, but it has just four simple stages:
- Ionisation.
- Acceleration.
- Flight.
- Detection.
Let’s explore those steps in more detail.
Ionisation
When particles first enter the mass spectrometer, they are neutrally charged. This isn’t very useful for scientists, as neutral particles aren’t attracted or repelled by positive or negative electrical fields. However, if we ionise them, they become much easier to manipulate. We can turn the particles in our sample into positive ions using two different methods:
- In electron impact, the sample is vaporized and high energy electrons are fired at it using an electron gun. An electron gun isn’t as scary as it sounds - it is just a hot wire filament with a current running through it. This knocks off one electron, forming a +1 ion known as the molecular ion. The equation is given below: This technique is used for low mass elements and compounds, and can cause fragmentation of the molecular ion into smaller particles.
The process of electron impact. An electron is knocked off each particle to form ions with a charge of +1.Vaia Originals
- In electrospray ionisation, we dissolve the sample and force it through a fine needle. The needle is attached to the positive terminal of a high-voltage power supply, and so this causes each particle to gain a proton, forming a +1 ion. This method is known as ‘soft’ ionisation as fragmentation rarely occurs. The equation is shown below:
Why is one technique preferable over the other? Well, electron impact is quite a harsh process and can cause the molecular ion to split into smaller particles, which is known as fragmenting. This can be helpful for working out the structure of the ion, as you’ll explore in "NMR spectroscopy", but it also can only be used for low mass molecules. However, electrospray ionisation is a gentler technique and rarely causes fragmentation, making it much easier to identify the molecular ion.
Acceleration
The positive ions are attracted to a negative electrical plate in the spectrometer, causing them to accelerate to the same kinetic energy. You may know the equation linking energy (KE), velocity (v) and mass (m):
We can rearrange this to make velocity the subject. Velocity is proportional to the square root of kinetic energy divided by mass:
This means that if kinetic energy stays the same for all particles but mass increases, velocity must decrease. Therefore, heavier ions have a lower velocity than lighter ions.
Flight
The ions pass through a hole in the negative plate and travel along a long cylinder called the flight tube. During their flight, they spread out according to their velocities and masses.
Detection
When the positive ions reach the end of the tube, they hit a negatively charged electrical plate and each gain an electron. This generates a current. The current is proportional in size to the number of ions hitting the plate, so a stronger current means there are more of that type of ion - it is more abundant. Remember that smaller, lighter ions have a greater velocity than heavier ones, which means that they will travel faster than, and reach the electrical plate before, the heavier ions.
Flight. Lighter ions have a greater velocity than heavier ions and pass through the flight tube faster. This means that they reach the detector first. Vaia Originals
TOF mass spectrometry is done in a vacuum. This is to prevent the ions colliding with air particles. The whole process is summarised below.
An illustration showing the overall process of TOF mass spectrometry.Vaia Originals
What is relative atomic mass?
If you take a look at the periodic table, you’ll see that a lot of the mass numbers of the elements are not whole numbers. Why is this? Atoms can’t have half a proton or neutron! In fact, the mass number shown on the periodic table is the element’s relative atomic mass.
Relative atomic mass is the average weight of all the isotopes of an element in a sample, compared to 1/12 of the mass of a 12C atom.
Relative atomic mass takes into account each isotope’s abundance and so is rarely a whole number. For example, if 50% of an imaginary element had an atomic mass of 32 and 50% had an atomic mass of 33, the relative atomic mass would be 32.5. We’ll explore how you calculate this figure in just a second, but first let’s look at how we can get data from TOF spectrometry.
When the ions are detected in TOF spectrometry, a computer produces a mass spectrum using the information. This shows the mass-to-charge (m/z) ratio of the ion along the x-axis and its relative abundance along the y-axis. Because the ions should all have a charge of +1, the mass-to-charge ratio simply represents their mass. We can easily find the mass and abundance of each ion by reading off the graph. If the ions are all isotopes of the same element, we can then work out relative atomic mass.
An example of a TOF spectrum for boron.Vaia OriginalsWorking out relative atomic mass
Let’s go through an example:
A sample of neon gives the following data.
Time of flight spectrometry data for neon.Vaia OriginalsWe can see two peaks - one at 20 and one at 22. These represent isotopes that have relative abundances of 90% and 10% respectively.
To calculate the relative atomic mass, we convert the abundance of each isotope to a decimal, multiply it by the isotope’s mass, and add all these values together. So for this sample:
20 x 0.9 = 18
22 x 0.1 = 2.2
18 + 2.2 = 20.2
The relative atomic mass is 20.2.
Mass spectroscopy exam questions
Sometimes you may not be asked to find the abundance of ions or molecules, but instead information such as their velocity. There are two equations we can use to find either the mass, velocity, time of flight, kinetic energy or distance travelled of the ions, provided the other values are known:
Where:
- t = time of flight (s)
- d = length of flight tube (m)
- v = velocity of the particle (m/s )
- m = mass of the particle (kg)
- KE = kinetic energy of particle (J)
These can look a little complicated, especially in practice as you are working with very small numbers. However, if you follow the process methodically and lay out your workings neatly, you’ll easily be able to manage the calculations.
Let’s take a look at the values that we know. We have mass, velocity, and time of flight. If we rearrange the first equation, we get:
We can then substitute this into the second equation to get:
Which we can rearrange into:
Substituting the values in gives:
Giving an answer of 2.13 m, to 2 decimal places.
Let’s work through another example together.
If we rearrange the first equation, we get:
We can then substitute this into the second equation to get:
Which can be further rearranged to:
If we substitute our values in, we get:
Which equals 1.549 m to 3 decimal places.
Mass Spectrometry - Key takeaways
- Mass spectrometry gives information about relative abundance of isotopes in a sample.
- Time of flight spectrometry involves 4 stages: ionisation, acceleration, flight and detection. It can be used to calculate relative atomic mass.
Explanations 
Textbooks 
Exams 
Magazine 
kg are accelerated to
J and their time of flight is 
seconds. Calculate the distance travelled.
J. Their time of flight is 
and the mass of each ion is 
kg. Calculate the length of the flight tube.
