What is Mass Spectrometry?
Mass spectrometry is a type of chemical analysis that measures the mass of ions, and calculates the relative abundance of each of those ions within the sample.
It’s a kind of instrumental analysis, which means the sample is processed and measured within a device, or instrument, called a mass spectrometer. A sample is introduced into the instrument and submitted to processes that create ions, before directing the ions at a detector.
Mass Spectrometry Units
As you would expect from its’ name, the mass spectrometer measures the mass of ions. The processing of the data is designed so that the output shows mass in atomic mass units (amu).
Relative Abundance
Relative abundance is a measure of the proportion of a sample that has a particular mass. If the sample is an element we can use the information to work out the proportion of the sample made up from each isotope.
The measurement is usually percent and, naturally, the total of the relative abundances for all ions present is 100%
How Does a Mass Spectrometer Work?
Earlier we mentioned that the spectrometer processes the sample, and now we’re going to look at that process in more detail.
The first step is getting the sample into the spectrometer, or sample introduction. The simplest method is to inject the sample through a special seal using a syringe needle.
Any solvent used is then evaporated away within the instrument.
The sample proceeds into the ionisation process. A high voltage is passed through the sample in order to remove electrons and create positive ions. If the sample is molecular, such as Cl2, the process breaks bonds in some of the molecules too.
In the acceleration phase the ions are accelerated toward a negatively charged plate electrode. The ions are focused into a beam that is directed through a hole in the plate and down the flight tube with a specific kinetic energy.
The flight tube is kept under high vacuum throughout the experiment so that air molecules do not obstruct or interfere with the passage of the ions towards the detector, or give a rogue signal at the detector.
Detection occurs when ions strike the detector. A signal is amplified and processed to obtain a mass spectrum, showing mass and relative abundance of the ions detected.
Actually, to be more accurate, instead of mass, the spectrum shows mass/charge (m/z) value on the x axis. Some of the ions created at the ionisation stage will have lost more than one electron, hence they have a higher charge.
How Does the Mass Spectrometer Measure Mass?
We’ve described the process that happens to the sample from injection into the instrument through to detection but we haven’t yet described how this enables the spectrometer to distinguish and measure the mass of individual ions. Exactly how this happens depends on what type of mass spectrometer is used, what principle it uses to determine mass.
There are two fundamental types of mass spectrometer and the one you need to know about depends on which syllabus you are studying. Both types use all the processes outlined earlier, but apply different use of the flight tube to determine mass.
If you are studying AQA A-Level Chemistry you will need to understand the principles of the Time of Flight (TOF) mass spectrometer.
If you are studying any other syllabus of A-Level Chemistry, or Advanced Higher Chemistry, you will need to know the magnetic deflection type of mass spectrometer.
Time of Flight (TOF) Mass Spectrometer – AQA
As its name suggests, the principle of this spectrometer is a measurement of the time taken for an ion to travel down the flight tube to the detector. The flight tube of a flight tube is straight and, like any mass spectrometer, operated under high vacuum to avoid interference from air molecules.
Each ion is given the same kinetic energy when it is accelerated down the flight tube. Because we know that energy, we know the length of the flight tube, and we know the time taken to reach the detector we can calculate the mass of the ion. Actually, this is done by the instrument’s software according to the following equation, but you are expected to be able to make these calculations in your exam questions:
K.E. = ½mv²
K.E. is kinetic energy (in joules)
m is mass of the ion (in kilograms)
v is the velocity of the ion (in metres per second)
You aren’t expected to remember this equation as it is given if you need to do related calculations in your exam.
However, you are expected to remember the effect of an ion’s mass on it’s velocity. As kinetic energy is the same for each ion you can conclude that the velocity of a lighter ion is greater than the velocity of a heavier ion. In other words, the lightest ions reach the detector first, and the heaviest ions reach the detector last.
Examiners love to ask about this. For example, a question will state that a particular element has particular isotopes, and ask you which ion reaches the detector first. Quick tip: choose the isotope with the lightest mass, and remember to include the positive charge of the ion!
Other common question types ask you to calculate the mass of an ion, the length of the flight tube, or the time taken for an ion to reach the detector.
You will have noticed that neither time nor length is directly shown in the equation we have been using. You will need to first calculate the velocity, v. Of course, velocity is a measure of distance travelled divided by the time taken (or v = d/t), and you can rearrange this to calculate the time or distance. The distance, in this case, is the length of the flight tube.
Mass Spectrometer for all other A-Level and Advanced Higher
If you’re studying Chemistry A-Level syllabus other than AQA, or you’re studying Advance Higher, the mass spectrometer you study uses the principle of magnetic deflection to measure the mass of ions.
In this spectrometer, the flight tube is curved and an electromagnet is positioned around that curve. A magnetic field is applied as the ions pass through the curved section. The process is operated under high vacuum to remove the possibility of air molecules interfering with the flight of ions.
The ions are deflected from their path as they pass through the magnetic field, and that deflection is greater for lighter ions. So the lightest ions are deflected from their path to a greater degree than heavier ions with the same charge.
When the ions strike the detector it is the position on the detector that is recorded. This indicated how much deflection the ion experienced due to the magnetic field. Because the kinetic energy of ions and the applied magnetic field are both constant, the instrument software is able to calculate the mass or m/z of ions.
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