TOF mass spectrometer

Time of flight mass spectrometer

A mass spectrometer is used to identify unknown substances, to determine the composition of mixtures and to accurately measure the relative amounts of substances present. It has many uses in the chemical, engineering and space industries, the Rovers which NASA sent to the planet Mars all had mass spectrometers on board to analysis Martian rocks, minerals and gases in the atmosphere. In the A-level chemistry specifications there are two main types of mass spectrometers that you will need to know about. If you are studying the AQA specification you will need to be able explain the working of a time of flight mass spectrometer and carry out some calculations based on this method of mass spectroscopy. So let's start here. For details on the other type of mass spectrometer, the single beam mass spectrometer and its operation click here or use the link above.

Time of flight mass spectrometer (TOF-MS)

The time of flight mass spectrometer was developed during the 1950. To analyse the sample of the unknown substance under examination we need to consider what happens in each of 4 separate stages in the operation of this type of mass spectrometer. These 4 stages are:

Stage 1- ionisation.

The sample under test is injected at the sample point (see diagram below) under low pressure. This will vapourise the sample and turn it into a gas. If the sample is a solid it will be heated to vapourise it.

Ionisation is one of the key steps that happen inside the mass spectrometer. There are 2 common methods used to ionise the sample, these are two methods are: The image below shows how electron impact ionisation works. Here an electron gun, which is essentially just a wire that is heated up to a very high temperature, at this high temperature it emits a steady stream of high energy fast moving electrons. These electrons are attracted to a positively charged plate opposite the electron gun. The positively charged plate will attract and accelerate the electrons given off by the electron gun.

Time of flight mass spectrometer with an electron gun which is used to ionise the sample

If we imagine the electrons as being like fast moving solid particles or balls/bullets; then when the molecules or atoms in the injected sample meet these fast moving electrons, they will have 1 of their outer electrons removed or knocked off, this means they will have been ionised. This will leave a positively charged ion behind (this is shown in the image opposite).

fast moving electrons from the electron gun knock electrons off the 
atoms in the sample to form positively charged ion If we call the injected sample X, then we can show this as:

X(g) → X+(g) + e

Where e represents the electron being knocked off the sample X. You may also see this equation written as:

X(g) + e → X+(g) + 2e

Here the first e represents the electron from the electron gun which impacts the sample X, forming a positively charged ion, X+(g) and the 2e on the product side represent the one electron lost by the sample X and also the electron from the electron gun.

For example the equations below represent the ionisation of of neon gas and hydrogen gas:

Ne(g) + e Ne+(g) + 2e
or simply:
Ne(g) Ne+(g) + e

And for hydrogen gas we have:

H2(g) + e H2(g) + + 2e
or simply:
H2(g) H2(g)+ + e

Electro-spray ionisation

The second method of ionising the sample uses a method called electro-spray ionisation. This method is often preferred for larger molecules which may be blasted to pieces by an electron gun! Here the sample is dissolved in a volatile polar solvent (volatile means it evaporates easily) such as water or methanol, next it is injected into the sample point under high pressure and through a very fine needle which has a high voltage (around 4000v) applied to it. This results in the sample being ionised and forming fine droplets or a mist containing the ionised sample. The solvent being volatile quickly evaporated leaving the positively charged sample. This is shown in the diagram below

details of how electro-spray ionisation in a TOF mass spectrometer works

The sample X becomes ionised not by having electrons knocked removed as is the case in electron impact ionisation, instead the sample gains a hydrogen ion, H+, from the solvent . We can show this as:

X(g) + H+ → XH+(g)

All this takes place in the needle on the end of the injection point. This is also shown below:

TOF mass spectrometer uses electro-spray ionisation to ionise the sample

Step2 - acceleration

The next key step in the mass spectrometer is acceleration of the ions formed. There is one vital assumption that we will make: we will assume that all the ions formed in the ionisation step have a charge of +1, that is only 1 single electron has been removed. The positively charged ion will be now be accelerated up the tube by an electric field, they will be accelerated towards the negatively charged plates.

One of the main pieces of information that the mass spectrometer provides is the Ar or Mr of the substance under analysis. If the sample contains a mixture of substances it would be very useful if we could obtain the Mr for each substance present. Now after the ionisation step we may have a range of different substances all with a charge of +1. The mass spectrometer will give you a read out of the mass to charge ratio, often called m/z ratio of any substance it detects. Now if the charge (z) is simply equal to +1, then the mass to charge ratio, m/z, is simply the mass.

In the acceleration stage in the mass spectrometer would you expect the heavy or light ions to be accelerated quickest up the tube? Or put another way, if you pushed a golf ball or a bowling ball, which would move with the greatest velocity or speed? Obviously the lightest ions will be accelerated quickest up the tube towards the charged plates. Now you may remember from your physics lessons that the kinetic energy of an object can be calculated from the formula:

Kinetic energy (KE) = 1/2 mv2

Where: m =mass in kilograms (kg), v= velocity in metres per second (ms-1) and the KE = kinetic energy in joules (J).

A key feature of this type of mass spectrometer is that all ions, no matter their mass will all gain the same amount of kinetic energy during the acceleration stage. The kinetic energy of the ions will depend on their mass and velocity, so lighter faster moving ions will have the same kinetic energy as heavier slower moving ions.

the kinetic energy of the ions is the same in a TOF spectrometer

In the exam your exam you could be asked to calculate any one from the list below. It is often necessary to rearrange formulae, I have given some examples below but I would try the practice questions and complete as many of the examples as possible to ensure you are completely confident with these questions.

Stage 3 - ion-drift

The accelerating ion now enter a portion of the mass spectrometer called the flight tube. The flight tube is of known length and the time it takes the ion to reach the detector is carefully recorded. There is no electric field in the flight path and so this part of the spectrometer is often called ion-drift. I always find this name a bit unhelpful since drift implies a slow lumbering motion, whereas in reality the flight times to the detector can often be less than a nanosecond as the ions are moving with very high velocities.

Stage 4- detection

The detector plate has a negative charge due to excess electrons being placed on it from an external source. When the positively charged ions arrive at the detector plate they gain an electron and are reduced. This involves the movement of electrons, or a flow of current. The more electrons that flow the more ions are present, that is their abundance is high, this will be recorded by a computer connected to the detector. Remember the ions will arrive at the detector plate at various times, since their flight time down the flight tube will obviously depend on how fast they are travelling and this as previously mentioned depends on their mass to charge ratio, or simply their mass if the charge is +1.

Key Points

Practice questions

Check your understanding - Questions on TOF mass spectroscopy