THE MASS SPECTROMETER This page describes how a mass spectrum is produced using a mass spectrometer. In fact, there are several different designs of mass spectrometer which differ in detail - this page looks at one which is easy to understand. How a mass spectrometer works The basic principle If something is moving and you subject it to a sideways force, instead of moving in a straight line, it will move in a curve - deflected out of its original path by the sideways force. Suppose you had a cannonball travelling past you and you wanted to deflect it as it went by you. All you've got is a jet of water from a hose-pipe that you can squirt at it. Frankly, its not going to make a lot of difference! Because the cannonball is so heavy, it will hardly be deflected at all from its original course. But suppose instead, you tried to deflect a table tennis ball travelling at the same speed as the cannonball using the same jet of water. Because this ball is so light, you will get a huge deflection. The amount of deflection you will get for a given sideways force depends on the mass of the ball. If you knew the speed of the ball and the size of the force, you could calculate the mass of the ball if you knew what sort of curved path it was deflected through. The less the deflection, the heavier the ball. | |
| Note: I'm not suggesting that you personally would have to do the calculation, although the maths isn't actually very difficult - certainly no more than A'level standard! | |
You can apply exactly the same principle to atomic sized particles. An outline of what happens in a mass spectrometer Atoms and molecules can be deflected by magnetic fields - provided the atom or molecule is first turned into an ion. Electrically charged particles are affected by a magnetic field although electrically neutral ones aren't. The sequence is : Stage 1: Ionisation The atom or molecule is ionised by knocking one or more electrons off to give a positive ion. This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon, for example). Most mass spectrometers work with positive ions. | |
| Note: All mass spectrometers that you will come across if you are doing a course for 16 - 18 year olds work with positive ions. Even if a few atoms in a sample of chlorine, for example, captured an electron instead of losing one, the negative ions formed wouldn't get all the way through the ordinary mass spectrometer. But it has been pointed out to me that there is work being done on negative ion mass spectrometers, although they use a different ionisation technique. My thanks to Professor John Todd of the University of Kent for drawing this to my attention. | |
Stage 2: Acceleration The ions are accelerated so that they all have the same kinetic energy. Stage 3: Deflection The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected. The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected. Stage 4: Detection The beam of ions passing through the machine is detected electrically. A full diagram of a mass spectrometer
Understanding what's going on The need for a vacuum It's important that the ions produced in the ionisation chamber have a free run through the machine without hitting air molecules. Ionisation
The vaporised sample passes into the ionisation chamber. The electrically heated metal coil gives off electrons which are attracted to the electron trap which is a positively charged plate. The particles in the sample (atoms or molecules) are therefore bombarded with a stream of electrons, and some of the collisions are energetic enough to knock one or more electrons out of the sample particles to make positive ions. Most of the positive ions formed will carry a charge of +1 because it is much more difficult to remove further electrons from an already positive ion. These positive ions are persuaded out into the rest of the machine by the ion repeller which is another metal plate carrying a slight positive charge. | |
| Note: As you will see in a moment, the whole ionisation chamber is held at a positive voltage of about 10,000 volts. Where we are talking about the two plates having positive charges, these charges are in addition to that 10,000 volts. | |
Acceleration
The positive ions are repelled away from the very positive ionisation chamber and pass through three slits, the final one of which is at 0 volts. The middle slit carries some intermediate voltage. All the ions are accelerated into a finely focused beam. Deflection
Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on:
These two factors are combined into the mass/charge ratio. Mass/charge ratio is given the symbol m/z (or sometimes m/e). For example, if an ion had a mass of 28 and a charge of 1+, its mass/charge ratio would be 28. An ion with a mass of 56 and a charge of 2+ would also have a mass/charge ratio of 28. In the last diagram, ion stream A is most deflected - it will contain ions with the smallest mass/charge ratio. Ion stream C is the least deflected - it contains ions with the greatest mass/charge ratio. It makes it simpler to talk about this if we assume that the charge on all the ions is 1+. Most of the ions passing through the mass spectrometer will have a charge of 1+, so that the mass/charge ratio will be the same as the mass of the ion. | |
| Note: You must be aware of the possibility of 2+ (etc) ions, but the vast majority of A'level questions will give you mass spectra which only involve 1+ ions. Unless there is some hint in the question, you can reasonably assume that the ions you are talking about will have a charge of 1+. | |
Assuming 1+ ions, stream A has the lightest ions, stream B the next lightest and stream C the heaviest. Lighter ions are going to be more deflected than heavy ones. Detection Only ion stream B makes it right through the machine to the ion detector. The other ions collide with the walls where they will pick up electrons and be neutralised. Eventually, they get removed from the mass spectrometer by the vacuum pump.
When an ion hits the metal box, its charge is neutralised by an electron jumping from the metal on to the ion (right hand diagram). That leaves a space amongst the electrons in the metal, and the electrons in the wire shuffle along to fill it. A flow of electrons in the wire is detected as an electric current which can be amplified and recorded. The more ions arriving, the greater the current. Detecting the other ions How might the other ions be detected - those in streams A and C which have been lost in the machine? Remember that stream A was most deflected - it has the smallest value of m/z (the lightest ions if the charge is 1+). To bring them on to the detector, you would need to deflect them less - by using a smaller magnetic field (a smaller sideways force). To bring those with a larger m/z value (the heavier ions if the charge is +1) on to the detector you would have to deflect them more by using a larger magnetic field. If you vary the magnetic field, you can bring each ion stream in turn on to the detector to produce a current which is proportional to the number of ions arriving. The mass of each ion being detected is related to the size of the magnetic field used to bring it on to the detector. The machine can be calibrated to record current (which is a measure of the number of ions) against m/z directly. The mass is measured on the 12C scale. | |
| Note: The 12C scale is a scale on which the 12C isotope weighs exactly 12 units. | |
What the mass spectrometer output looks like The output from the chart recorder is usually simplified into a "stick diagram". This shows the relative current produced by ions of varying mass/charge ratio. The stick diagram for molybdenum looks lilke this:
You may find diagrams in which the vertical axis is labelled as either "relative abundance" or "relative intensity". Whichever is used, it means the same thing. The vertical scale is related to the current received by the chart recorder - and so to the number of ions arriving at the detector: the greater the current, the more abundant the ion. As you will see from the diagram, the commonest ion has a mass/charge ratio of 98. Other ions have mass/charge ratios of 92, 94, 95, 96, 97 and 100. That means that molybdenum consists of 7 different isotopes. Assuming that the ions all have a charge of 1+, that means that the masses of the 7 isotopes on the carbon-12 scale are 92, 94, 95, 96, 97, 98 and 100. | |
| Note: If there were also 2+ ions present, you would know because every one of the lines in the stick diagram would have another line at exactly half its m/z value (because, for example, 98/2 = 49). Those lines would be much less tall than the 1+ ion lines because the chances of forming 2+ ions are much less than forming 1+ ions. If you want to go straight on to how you use these mass spectra to calculate relative atomic masses you can jump straight to that page by following this link rather than going via the menus below. | |
Questions to test your understanding If this is the first set of questions you have done, please read the introductory page before you start. You will need to use the BACK BUTTON on your browser to come back here afterwards. questions on how a mass spectrometer works answers
© Jim Clark 2000 (last modified March 2019) | |
FAQs
How does the mass spectrometer work? ›
Mass spectrometers detect the speed at which positively charged ions move through a vacuum chamber toward a negatively charged plate. The speed of the ions is determined by the weight. This process allows researchers to apply analytical techniques to determine the composition of the sample.
What is mass spectrometer step by step? ›- Stage 1: Ionization: The atom is ionised by knocking one or more electrons off to give a positive ion. ...
- Stage 2: Acceleration: The ions are accelerated so that they all have the same kinetic energy.
- Stage 3: Deflection: The ions are then deflected by a magnetic field according to their masses.
A mass spectrometer can measure the mass of a molecule only after it converts the molecule to a gas-phase ion. To do so, it imparts an electrical charge to molecules and converts the resultant flux of electrically charged ions into a proportional electrical current that a data system then reads.
How does a mass spectrometer work for kids? ›A mass spectrometer ionizes atoms and molecules with a high-energy electron beam and then deflects the ions through a magnetic field based on their mass-to-charge ratios ( ). The mass spectrum of a sample shows the relative abundances of the ions on the y-axis and their ratios on the x-axis.
How does a spectrometer work? ›The beam of light strikes the diffraction grating, which works like a prism and separates the light into its component wavelengths. The grating is rotated so that only a specific wavelength of light reaches the exit slit. Then the light interacts with the sample.
How does mass spectrometer detect charge? ›Contrary to conventional MS, charge-detection mass spectrometry (CDMS) is a method that determines the mass of ions directly. In addition to the m/z ratio, which can be inferred by the frequency of ion oscillations, the charge can be determined through the amplitude of the image current generated by a single ion.
What are the 4 stages of the mass spectrometer? ›Now you know has a mass spectrometer works – just remember the four stages in order – ionisation, acceleration, deflection, and detection.
What is the basic principle of mass spectrometry? ›Principle of MS
Mass spectrometry (MS) is an analytical technique that separates ionized particles such as atoms, molecules, and clusters by using differences in the ratios of their charges to their respective masses (mass/charge; m/z), and can be used to determine the molecular weight of the particles.
Mass spectrometry is a complex analytical technique where the components of a sample are separated by their mass and electrical charge. It's used for both quantitative and qualitative chemical analysis of sample compounds.
What are the 5 steps of mass spectrometry? ›In a mass spectrometry experiment, the process sequence of analysis works in five stages, including sample introduction, analyte ionization, mass analysis, ion detection, and data processing.
How do you explain spectrometry? ›
Spectrometry is the measurement of the interactions between light and matter, and the reactions and measurements of radiation intensity and wavelength. In other words, spectrometry is a method of studying and measuring a specific spectrum, and it's widely used for the spectroscopic analysis of sample materials.
How does a mass spectrometer function in general terms? ›Basic Principle. A mass spectrometer generates multiple ions from the sample under investigation, it then separates them according to their specific mass-to-charge ratio (m/z), and then records the relative abundance of each ion type.
How does a mass spectrometer work step by step? ›The four stages of mass spectrometry are – ionization, acceleration, deflection, and detection. Ionization: The sample is vaporized before being passed into an ionization chamber where it is bombarded by a stream of electrons emitted by an electrically heated metal coil.
What is the basic operation of a mass spectrometer? ›Most standard mass spectrometers work with positive ions. Once the ions are generated, they are all accelerated to ensure they all have the same kinetic energy, and they are then deflected by a field according to their mass-to-charge ratio to filter the ions into a detector.
How does a mass spectrometer work at a level? ›Mass spectrometry works by ionizing a sample, usually by bombarding it with electrons, and then measuring the mass-to-charge ratio of the resulting ions. This information can be used to determine the molecular weight of the substance and identify its components.
How does mass spectrometry work in MCAT? ›Mass spectrometers work on samples in a gaseous state. An ion source ionizes the gaseous samples. Mass analyzers separate ionized samples according to their mass-to-charge ratio.
How does a mass spectrometer separate particles by mass? ›In scanning mass spectrometry, one starts with a mixture of ions having different mass-to-charge ratios and different relative abundances. Electromagnetic fields separate the ions according to their mass-to-charge ratios, and a slit is used to select which mass-to-charge ratio reaches the detector.
How does mass spectrometry imaging work? ›Imaging MS obtains the spatial distribution of molecules by detecting ions pixel-by-pixel, a pixel at a time. The intensity plot of a specific ion is aligned with its spatial coordinate and rendered as a false-color heatmap image.