How the Mass Spectrometer Works (2024)

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    This page describes how a mass spectrum is produced using a mass spectrometer.

    How a mass spectrometer works

    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 traveling 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 traveling 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. You can apply exactly the same principle to atomic sized particles.

    An outline of what happens in a mass spectrometer

    Atoms can be deflected by magnetic fields - provided the atom 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: Ionization: The atom 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). Mass spectrometers always work with positive ions.
    • 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

    How the Mass Spectrometer Works (2)

    Understanding what's going on

    The need for a vacuum

    It's important that the ions produced in the ionization chamber have a free run through the machine without hitting air molecules.

    Ionization

    The vaporized sample passes into the ionization chamber. The electrically heated metal coil gives off electrons which are attracted to the electron trap which is a positively charged plate.

    How the Mass Spectrometer Works (3)

    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.

    Acceleration

    How the Mass Spectrometer Works (4)

    The positive ions are repelled away from the very positive ionization 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

    How the Mass Spectrometer Works (5)

    Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on:

    • the mass of the ion. Lighter ions are deflected more than heavier ones.
    • the charge on the ion. Ions with 2 (or more) positive charges are deflected more than ones with only 1 positive charge.

    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. 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.

    How the Mass Spectrometer Works (6)

    When an ion hits the metal box, its charge is neutralized 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.

    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 like this:

    How the Mass Spectrometer Works (7)

    You may find diagrams in which the vertical axis is labeled 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.

    Contributors and Attributions

    How the Mass Spectrometer Works (2024)

    FAQs

    How the Mass Spectrometer Works? ›

    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 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? ›

    The sequence is :
    1. Stage 1: Ionization: The atom is ionised by knocking one or more electrons off to give a positive ion. ...
    2. Stage 2: Acceleration: The ions are accelerated so that they all have the same kinetic energy.
    3. Stage 3: Deflection: The ions are then deflected by a magnetic field according to their masses.
    Aug 29, 2023

    How does mass spectrometry work for dummies? ›

    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 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.

    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.

    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 are the 5 stages of operation of a mass spectrometer? ›

    • Ionization. Ionization refers to the production of gas-phase ions to allow for mass analysis. ...
    • Acceleration. In the mass analyzer, the ions are accelerated through a potential difference and focused into a beam. ...
    • Deflection. ...
    • Detection. ...
    • Data Processing.
    Feb 9, 2023

    How do you use a spectrometer step by step? ›

    B. Measuring Absorbance with Spectrophotometer
    1. Turn on the spectrophotometer. Let it warm up for 15 minutes.
    2. Select wavelength scan.
    3. Fill a cuvette 2/3 full with DI water to serve as the “BLANK” cuvette.
    4. Calibrate the Spectrometer.
    Sep 9, 2021

    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 important is mass spectrometry? ›

    Importance of Mass Spectrometry:

    Its high sensitivity and wide dynamic range make it an indispensable tool for determining the concentration of target analytes. This is particularly valuable in clinical diagnostics, pharmaceutical quality control, and environmental monitoring.

    How difficult is mass spectrometry? ›

    Mass spectrometry is a pretty complex science. In fact, it is so complex that some scientists even try to stay away from it and turn to basic molecular techniques to get answers to their questions.

    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 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.

    How is mass determined in mass spectrometry? ›

    Relative atomic mass can be calculated from the mass spectrum: Multiply the relative isotopic abundance (%) on the y-axis by the m/z on the x- axis for each isotope to get total mass. Add up the total masses of each isotope, and divide by 100.

    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.

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