What is mass spectrometry?
Mass spectrometry (MS) is an analytical technique, used extensively to deduce the chemical structure of chemical and biological molecules as well as to identify the composition of complex mixtures. This is done by separating molecules based on their mass-to-charge ratio (m/z) on a mass spectrometer.
Something very important for you to note is that - unlike what its name suggests, a mass spectrometer does not directly measure the mass of molecules (it does not function as a scale). Instead, it first converts the molecules in a sample to charged particles - ions, enabling them to interact with electric and magnetic fields. The mass spectrometer is then able to measure the mass-to-charge (m/z) ratio of these ions based on their motion in electric and magnetic fields. The results, in turn, are recorded on a diagram called a mass spectrum, which shows the different ions detected from the sample with their corresponding m/z, and the relative amount of each ion.
Box 1: The mass-to-charge ratio of a Ca2+ ion. Note that mass spectrometers are usually used to analyse more complex molecules than metal ions.
The key question you may be wondering is - what can the m/z value tell us and why is mass spectrometry a powerful technique?
Knowing the m/z ratio and the ion charge, the mass for the corresponding substance can be easily deduced (see Box 2). As you will see later, the charge is preset for each mass spectrometry experiment and can be adjusted on some mass spectrometers.
Box 2: Calculation of molecular mass from the m/z ratio and the charge of a molecule.
The type of compounds that can be analysed by mass spectrometry ranges from small chemical molecules to large biological macromolecules, such as proteins. The sensitivity, which is the least amount of material required for analysis, can now reach femtomole (1x 10^-15 mol) to sub-femtomole (< 1x 10^-15 mol) level, meaning that even a tiny amount of substance can be detected. Therefore, one of the greatest advantages of mass spectrometry is that it does not require extensive sample purification and can effectively analyse complex mixtures.
Applications of mass spectrometry
Because of the high sensitivity and the rich molecular information it provides, mass spectrometry is widely used in both scientific research and solving real-life problems. Here we will look at a few classic applications of mass spectrometry; but bear in mind, this is by no means the full extent of its powers.
Molecular identification
The most straightforward use of mass spectrometry is determining molecular weight and detecting certain compounds within a sample.
Moreover, sample molecules are often broken into smaller fragment ions during the ionisation process. Therefore, piecing together the m/z of the fragment ions can provide clues to predict the molecular formula and structure of unknown compounds.
Isotope studies
Besides that, m/z can also help study isotopes. For example, the isotope ratio of a given sample can be calculated from the relative amount of m/z corresponding to each isotope.
Examining the ratio of 12C/13C in bones or seashells is a commonly used method in palaeobiology to investigate the food source and living habits of the study object.
Other applications
Combining the types of molecular information mentioned above, mass spectrometry is a key technique for analysing compounds for clinical, environmental, industrial and forensic studies. For example, using mass spectrometry to generate profiles of abnormal proteins in cancer cells, can be used as an early diagnostic tool for cancer. Furthermore, mass spectrometry is used to analyse forensic materials helps collect information about crimes, establishing a ‘golden standard’ for forensic scientists and ensuring that conclusions are made justly. Similarly, by detecting harmful materials such as pesticides, heavy metals and toxins, mass spectrometry provides safety control for food industries.
A brief history of mass spectrometry
Before we dive into the principles of mass spectrometry, let’s first take a brief look at its history.
The birth of the mass spectrometer can be traced back to the early 20th century when Sir J. J. Thomson, a physicist at the University of Cambridge, discovered electrons and observed their deflection in electric and magnetic fields. This led to Thomson’s invention of an instrument called 'Parabola spectrograph’ to measure the mass-to-charge ratio of charged particles, which is now recognised as the first mass spectrometer.
Between 1918 and 1919, Thomson’s students F.W Aston and A.J. Dempster further improved the resolution of the instrument, allowing ions with similar m/z ratios to be effectively captured as separated signals. In one experiment, Aston calculated two different masses from the mass spectrum of Ne: 20 Da and 22 Da. Moreover, these two types of Ne had an abundance ratio of 10: 1. By calculating the average mass from this, he found that this could explain the atomic mass Ne at 20.2:
Box 3: The average mass of the two types of Ne agrees with its atomic mass. Note again that the mass (in Da) could not be directly measured, instead it would have been converted from the m/z using the simple calculation in Box 2.
Therefore, there should be two types of Ne atoms in nature. Aston’s discovery of 20Ne and 22Ne proved, for the first time, the existence of isotopes, for which he was awarded the Nobel Prize in 1922.
Moving forward, mass spectrometry technology continued to improve and its application became increasingly diverse. Notable examples include Alfred Nier’s design of a mass spectrometer-based device for detecting the fissionable isotope, 235U. This was used in the Manhattan Project during World War II, marking the beginning of the nuclear age. During the 1980s, new mass spectrometry techniques called MALDI and ESI were invented, providing robust tools for analysing large biomacromolecules. Moreover, after entering the 21st century, high-performance mass spectrometers have greatly improved the sensitivity, resolution and accuracy of m/z measurements. As a result, mass spectrometry has also boosted some new areas of studies, such as large-scale studies of proteins (proteomics) and metabolites (metabolomics).
Components of a mass spectrometer
With all these great achievements in the past century, you must be wondering how a modern mass spectrometer works to produce such powerful data. We shall start by introducing its three basic components:
The ion source, which converts the sample into gas-phase ions.
The mass analyser, which is used to focus and separate the ions based on their m/z.
The detector, which detects and quantifies the ions with different m/z.
Apart from these three basic components, it also includes a computer system to record and process the data into a mass spectrum. It is also worth noting that the interior of the mass spectrometer is kept under a high vacuum by a vacuum pump. Vacuum conditions are crucial for the precision of a mass spectrometer as collisions between ions and air molecules would lead to a change in the ion path, loss of charge and undesired reactions, resulting in inaccurate m/z values.
Figure 1: Schematic illustration of the basic components of a mass spectrometer. Figure modified from Finehout & Lee, 2006.
Now that we know the general setup of a mass spectrometer, we will discuss each of the components in more detail in the following sections, starting with the ion source.
Inside the mass spectrometer - the ion source
In order to measure m/z, the sample needs to be first converted into gaseous ions. The ion source provides an energy input for the ionisation process, as well as the gas-phase conversion if the sample's starting state is not gaseous.
Depending on the amount of energy input, the ionisation process is classified into “soft” and “hard” ionisation. During “soft” ionisation, very little residual energy is left after converting the sample to gas-phase ions. “Hard” ionisation, by contrast, inputs a greater amount of energy and the excess energy causes additional fragmentation of the ions, as we shall see later.
Here, we will look at the three most common ionisation methods, electron impact ionisation (EI), matrix-assisted laser desorption ionisation (MALDI) and electrospray ionisation (ESI).
Electron impact ionisation
Electron impact ionisation (EI) produces gaseous ions through interactions between the sample and an electron beam (see Figure 2). Because of the high energy input, which fragments the sample ions, EI is classified as “hard” ionisation.
The first step is to achieve the gas phase, which is done by heating the sample through a probe tip until it evaporates. Meanwhile, in another part of the instrument called the EI chamber, an electron beam is produced by thermionic emission. This is done by heating up a wire made of rhenium or tungsten; as the metal absorbs the heat, its electrons will eventually escape the solid surface. The electrons produced are then accelerated by a voltage to achieve an energy of typically 70 eV, which is required for producing the gas-phase sample.
To produce ions, the vapourised sample is introduced to the EI chamber towards the path of the electron beam. This way, the electron beam repels the gas-phase molecule’s outer shell electrons, removing one electron from the outer shell, producing a radical cation.
Figure 2: Ionisation process inside the EI chamber. A weak magnetic field is used to focus the electron beam produced via heating the wire filament. The electrons are collected by an electron trap. After gas-phase ions are produced, an ion repeller helps the ions leave the EI chamber where they are focussed by a series of electric fields. Figure modified from Rockwood et al., 2018.
You may be wondering why the energy needs to be 70 eV.
The short answer is: 70 eV ensures the maximum energy transfer from the electron beam to the sample, thus ensuring successful ionisation. More specifically, it is because the de Broglie wavelength, a parameter describing the ion motion in quantum mechanics, of the electrons at 70 eV matches the typical bond length in organic molecules. In fact, the amount of energy applied in EI is usually much higher than the actual amount of energy required for producing gaseous ions. As a result, the excess energy input often fragments the sample molecule. The overall ionisation process by EI can be summarised in the following way:
Box 4: Ionisation and fragmentation of molecule M. A portion of molecule R is lost during fragmentation. The unpaired electron (radical) can either be lost with R· (1) or remain part of the fragment ion (2).
The m/z ratios of the fragmented ions add extra information to the mass spectrum. Since most compounds follow defined fragmentation pathways, this information can be used in determining the molecular structure of the sample.
Considering the ionisation process, EI is best suited for compounds that meet the following criteria: Firstly, samples need to be thermal resistant in order to remain stable during the vaporisation; secondly, they need to be relatively small in size (below 1000 Da) because the energy required to vaporise large molecules would be too high to be achieved by heating.
Matrix-assisted laser desorption ionisation (MALDI)
Another ionisation method for mass spectrometry is Matrix-Assisted Laser Desorption Ionisation (MALDI). Different from the initial vaporisation process in EI, ionisation by MALDI starts from a solid phase instead of a gaseous phase.
To prepare the solid phase for MALDI, a small amount of the sample is co-crystallised with a matrix. The matrix does not only provide the base of the solid phase, but as we will discuss later, it also aids the ionisation process of the sample. The matrix material is typically organic molecules with low molecular weight and strong optical absorption in the UV (ultraviolet light) or IR (infrared light) range, such as 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid.
When ionisation starts, a short laser pulse irradiates the sample-matrix crystal. The wavelength of the laser beam is tuned to match the matrix’s maximum absorption range, ensuring maximum energy transfer from the laser beam to the matrix. After absorbing the energy, some matrix molecules absorb too much energy to stay stable in the solid phase. Therefore, they leave the crystal surface with the embedded sample into the gas phase. To re-stabilise themselves, the matrix molecule transfers a proton to the sample molecule, losing energy in the process. At this point, the sample is in the gas phase and positively charged. Finally, the matrix molecules gradually evaporate away from the sample (see Figure 3).
Figure 3: Ionisation process by MALDI. Excited matrix molecules leave the crystal surface together with its embedded samples. The matrix molecules re-stabilise themselves by transferring a proton to the sample, producing positively charged sample ions and thus finishing the ionisation process. Figure adapted from MALDI Mass Spectrometry - Creative Proteomics.
You may be wondering why a matrix is used in the first place. This is because the matrix acts as a buffer, which prevents the sample from absorbing too much energy directly from the laser beam. Therefore, unlike EI, MALDI is considered to be a ‘soft’ ionisation technique, where the sample only absorbs enough energy to be ionised but not enough to be fragmented. Furthermore, by directly initiating the ionisation from the solid phase, MALDI is able to analyse much larger molecules including biomacromolecules like proteins with molecular weight up to 500 kDa.
However, it is worth pointing out that the ionisation process itself could constitute an error source for MALDI mass spectrometry. When the laser pulse is focused on the sample-matrix crystal, samples at the surface will be ionised before those buried inside the crystal. This means that the same type of ions in the crystal interior will arrive at the mass analyser later than those on the surface, leading to poor resolution. Modern MALDI has successfully solved this problem via an additional step called ‘delayed extraction’ (see Figure 4).
Figure 4: The process of delayed extraction. Ions are coloured based on ionisation time (ions produced earlier are red, newly produced ions are in cantaloupe orange). The arrow length for each ion represents the actual velocity of the ion. Figure modified from Introduction to MALDI-TOF (slideserve.com).
Electrospray ionisation (ESI)
Like MALDI, electrospray ionisation (ESI) is also a ‘soft’ ionisation technique. Because of the low energy input, ESI also does not cause the sample to fragment usually. Currently, ESI can be used to analyse large molecules with a molecular weight higher than 500 kDa. Hence, ESI is widely used in biological studies to analyse large molecules. For example, by measuring the amount of biomolecules that are biomarkers for a certain disease, ESI is used clinically for disease diagnosis. In terms of the ionisation process, the gas-phase ions are produced from the liquid rather than the solid phase, as compared with MALDI. ESI ionisation process can be summarised in two main steps (see Figure 5):
1. Ionisation – formation of charged droplets
In ESI, ionisation occurs while the sample is still in solution. A capillary tube with a gold coat at the tip is connected to a counter electrode, and a high voltage of several kV is across the tip and the counter electrode, making them oppositely charged. The sample solution is pumped through this capillary tube, and due to the high voltage, it sprays out of the tip as charged droplets.
You may be wondering how exactly does the sample become charged? The analyte usually undergoes some electrochemical reactions. This could be either redox reactions that happen at the liquid-metal interface of the capillary tip (2H2O → 4H+ + 4e- + O2) or acid-base reactions between the analyte (the dissolved sample) and the solvent. Depending on the pH of the solution and the salts present, ESI can produce either positive and negative ions with single or multiple charges. In positive mode, the analyte (M) is most commonly associated with a proton produced by redox reaction forming a [M + H]+ molecular ion; but other molecular ions involving metal cations, such as [M + Na]+ and [M + K]+, can also be formed. On the other hand, in negative mode, the analyte can lose a proton forming [M − H]−, or associate with chloride to form [M + Cl]−.
2. Gas-phase conversion – desolvation of the charged droplets
Charged droplets emerging from the capillary tip still contain solvent. To evaporate the solvent, drying nitrogen gas is blown over the droplets. As the solvent evaporates, the droplets begin to sink, and the now charged analytes inside begin to move closer together. Eventually, the Coulombic repulsion between the charges overcomes the surface tension holding the droplet together. As a result, the droplet explodes into smaller droplets. As this process repeats, all the solvent will evaporate, leaving only gas-phase ions of the sample.
Figure 5: Ionisation process by ESI. The sample solution is introduced into a capillary tube and sprayed out from it as tiny charged droplets. These charged droplets are then accelerated towards an oppositely charged counter electrode, where it exits through a small opening. The solvent is then dried by N2 gas leaving only the gas-phase ions. These gas-phase ions are focused by a set of skimmers before they enter the mass analyser. Figure modified from Mass Spectrometry Facility | ESI (bris.ac.uk).
Additionally, since ESI directly accepts liquid samples, it is often connected to liquid chromatography, a purification technique. In this special type of experiment known as LC-mass spectrometry, different compounds in a sample are first separated before entering the mass spectrometer, which greatly improves the efficiency of analysing complex mixtures.
Typically, the sample is usually introduced to the capillary tube with a flow rate of 10-20 µl/min. However, more recently, a new ESI nano-electrospray technique has been developed which reduced the flow rate requirement to 10-30 nl/min. This means that smaller volumes of sample are needed, providing an opportunity in analysing rare and valuable materials.
Inside the mass spectrometer - the mass analyser
The gas-phase ions are then separated by the mass analyser according to their m/z. There are different types of mass analysers, but they share the same basic principle: electric and magnetic fields can affect the motion of ions, the extent of which depends on the ion's m/z ratio and the electric/magnetic field strength. Here we will introduce the five most common mass analysers.
Quadrupole
The quadrupole is the mass analyser invented the earliest amongst the five. It is composed of four parallel metal rods with the opposite two being connected together. A direct current (DC) is applied to one pair and a radio frequency (RF) alternate current (AC) to the other, generating an electric field inside the quadrupole. As the AC oscillates, the electric field also oscillates.
Box 5: A comparison of AC and DC. (a) The amplitude of AC, represented here as the voltage (V), oscillates over time. RF-AC simply means that the frequency of oscillation is within the range of radio-frequency. (b) In contrast, the amplitude of DC is constant over time.
The electric field interacts with charged sample ions inside it. More specifically, only ions with a certain m/z will be in resonance with a particular oscillating field. This way, the electric field acts as a ‘mass filter’ where only ions that are in resonance travel through the quadrupole to the detector, whilst other ions hit the rots and get annihilated along the journey. The mass spectrum of a sample is recorded by varying the DC and RF to produce suitable electric fields for ions with different m/z to pass through.
Figure 6: Diagram of a quadrupole mass analyser. As ions produced from the ion source are injected into quadrupole, ions with the m/z that resonate with the applied electric field (in red) will pass through the quadrupole to the detector, other ions (in blue) will crash onto the quadrupole rods. Figure modified from Santoiemma, 2017.
Two important performance factors to consider for any mass analyser are resolution, the ability to distinguish between two similar m/z ratios; and sensitivity, the ability to detect small amounts of sample. For the quadrupole, these two parameters are relatively low as some ions are lost during analysis. Additionally, it can only analyse ions with an m/z ratio of up to 4000. However, quadrupole is still widely used today as a cheap and robust technique for studying relatively small molecules.
Ion trap
The ion trap has a similar mechanism as the quadrupole but with a different instrumental setup. Three electrodes are arranged in a sandwich geometry with a ring electrode in between two cap electrodes. Upon entering the ion trap, ions will perform circular motions inside the analyser (i.e. become trapped), the radius of which will depend on their m/z. As the DC and AC are varied to adjust the strength of the electric field, some ions will achieve the correct radius to be ejected from the cap electrode. This way, instead of annihilating ions with the ‘incorrect’ m/z as seen in quadrupole, all ions are stored inside the ion trap.
Figure 7: Ion trap mass analyser. The top diagram shows the overall arrangement of the instrument from the outside. The bottom diagram shows an inside cross-section of an ion trap instrument. Figure modified from Georgiou & Danezis, 2015.
Time-of-flight (TOF)
Ion separation by TOF is based on the fact that ions with the same kinetic energy but different m/z ratios will travel at different velocities. Ions are accelerated into the TOF tube by a constant electric field where all of the electrical potential energy is converted into kinetic energy. This way ions with the same charge will receive the same kinetic energy from an electric field (see Box 6). As they travel along the tube, ions with different m/z ratios will travel at different speeds and therefore take different amounts of time to pass through to reach the detector. For a given charge, the lighter the ion, the faster it travels. Finally, the amount of time each ion takes will be used to calculate the m/z ratio.
Box 6: Potential energy an ion receives from the electric field (1) is converted to kinetic energy (2). Ions with the same charge (q) will have the same potential energy in a field with uniform voltage (V). For ions with the same kinetic energy, the higher the mass (m) the lower the velocity (v).
Hence, an accurate m/z measurement requires ions with the same m/z to reach the detector at the same time. However, this is often not the case in practice as the starting location and the electric field experienced by ions might be slightly different, leading to poor resolution. This problem is solved by adding an extra part called reflectron (see Figure 8) at the end of the TOF tube. The reflectron provides an electric field where the direction of the electric field is opposite to the direction of the travelling ions. In this case, as ions enter the reflectron, their movement will at first slow down gradually until their speed reaches zero. At this point, they will also reach the furthest distance they could penetrate the reflectron. After that, the ions will regain the kinetic energy from the electric field provided by the reflectron, but at this time, their travelling direction will be in the same direction as the electric field of the reflectron (which is opposite to their initial travelling direction). Now, when considering the individual ion movement, ions with slightly higher kinetic energy will firstly travel deeper into the reflectron before their travelling direction is inverted. In this way, ions with higher kinetic energy would travel a longer distance, which gives ions with the lower kinetic energy some time to catch up. This refocuses the ions with the same m/z onto the detector (ions with the same m/z regardless of their actual kinetic energy, will arrive at the detector at the same time.)
Figure 8: Schematic illustration of TOF mass analyser. Ions injected in will first travel towards the end of TOF, where their path will be reflected back by the reflectron. By varying the distance ions with the same m/z ratio but slightly different velocity travel, the reflectron corrects this systematic error, allowing them to reach the detector at the same time. The total time each type of ion takes from injection to the detector will be used to calculate the m/z ratio. Figure modified from Several types of mass analyser (creative-proteomics.com).
Fourier Transformation Ion Cyclotron Resonance (FT-ICR)
Fourier Transformation Ion Cyclotron Resonance (FT-ICR) is the best performing mass analyser with the highest resolution and accuracy so far. Unlike other mass analysers, FT-ICR also serves as a detector.
Ions are trapped by a strong magnetic field inside a cell. As they interact with the magnetic field, ions will undergo circular motions with a frequency, which is dependent on their m/z and the magnetic field strength. To modulate the motion of the ions, a radio frequency (RF) voltage is applied creating an oscillating electric field. If the frequency of the electric field matches the frequency of the orbiting ions, ions can gain energy from the electric field and expand their circular path. In this expanded circular path, ions can come in contact with the walls of the cell, creating an oscillating current as they do so. By varying the RF voltage, different ions will enter the path with a larger diameter and generate a corresponding electric signal. The frequency of the current generated is related to the ion’s m/z, and the intensity of the current is proportional to the number of ions for that m/z. Finally, the signals from the current are converted into a mass spectrum using Fourier transformation.
Figure 9: Overall mass analysing process in FT-ICR. Ions are introduced in the direction of the red arrow to FT-ICR. The strong magnetic field makes these ions circulating inside the instrument cell with a diameter corresponding to the strength of the magnetic field. An RF voltage excites ions circulating the cell is excited to a path with a higher diameter, with the ions touching the upper and bottom part of the cell. This completes the circuit and generates an oscillating current, represented as a sinusoidal signal in the time domain. The sinusoidal signal is converted into the frequency domain by the Fourier transform. m/z ratios are calculated from each of the frequencies recorded and displayed in the mass spectrum. Figure modified from Mass Spectrometry Facility | FTICR-MS (bris.ac.uk).
Orbitrap
The Orbitrap works in a very similar way to an FT-ICR, it also has the detector components along with the mass analyser part. It retains a high resolution and accuracy comparable to FT-ICR, but is much smaller and cheaper, offering wider accessibility than FT-ICR. Instead of using strong magnetic fields, ions are trapped by an electrostatic field, where they move around the central electrode. Ions experience different forces that direct their motion in electrostatic and magnetic fields. Therefore, electrodes in an orbitrap are arranged differently from the magnets in FT-ICR.
Inside an orbitrap, ion motions can be separated into two parts:
The oscillating frequency at the z-axis depends on the ions' m/z ratio. Similar to FT-ICR, this is detected by sensing the oscillating current generated as the ions come in contact with the outer electrode. Once again, the current frequency and intensity are then Fourier transformed to produce a mass spectrum (see Figure 10).
Figure 10: Mass analysing process by orbitrap. Figure modified from Savaryn et al., 2016.
Inside the mass spectrometer – the detector
The detector is the last analysing component of a mass spectrometer (except for FT-CIR and orbitrap, which both have incorporated the detector in their mass analyser). It detects the m/z and the abundance of each ion type.
The earliest mass spectrometer detector was a photographic plate, in which different ions hit the plate at different locations, leaving a black spot. The darkness of the spot reflected the ion abundance. Nowadays, detectors are digitised and provide much more accurate measurements. The process often includes amplifying the ion signal before detection, which offers more sensitive detection for rarer ion types.
Photomultiplier (Figure 11A) and electron multiplier (Figure 11B) are two common detectors in mass spectrometers.
In an electron multiplier, the ions (for a given same m/z) strike a special electrode called dynode. This results in a burst of secondary electrons. These electrons continue initiating more electron emission as they travel along the instrument, resulting in a series of amplifications. The final amount of electron emitted at the end will be measured, which is used to reflect the ion abundance.
In contrast to directly detecting electrons, a photomultiplier converts those secondary electrons into photon signals before detection. After secondary electrons are released by ions striking at the dynode, they emit a burst of photons by striking a phosphorus screen. The photons then go through a similar amplification process as in an electron multiplier before the final detection. This additional conversion can help keep the multiplier in a vacuum and reduce the chance of contamination, significantly improving the lifetime of the multiplier.
Apart from detecting different ions sequentially, another detector named micro-channel plate array detector (MCP; see Figure 11C) can detect all types of ions simultaneously. It essentially combines many detectors in an array, where they work together to analyse different ions at the same time. In this way, by referring to the specific location of the detector where a given ion type is detected, MCP provides an additional spatial resolution of ion detection. It is often the detector used for mass spectrometry-imaging techniques, which helps obtain spatial information of the sample.
Figure 11: Different types of detectors in mass spectrometry. (a) In an electron multiplier, signals are exaggerated by ions striking the dynode multiple times, resulting in an increasing amount of ions being released after each strike. (b) In a photon multiplier, secondary ions are released by ions striking the dynode. These secondary ions then strike a phosphorescent screen, resulting in a burst of photon signals which will be used as the final signal for obtaining the sample result. (c) A microchannel plate array detector consists of several individual detectors arranged in a plate, where each detector functions similar to a multiplier. By incorporating all the signals collected by each detector, spatial information of the sample can be obtained. Figure modified from Mass Spectrometry Facility | Detectors (bris.ac.uk) & Microchannel Plates (dmphotonics.com).
A quick recap...
Here we have introduced the three main components of a mass spectrometer: the ion source, the mass analyser and the detector. Complex mixtures are ionised and converted to the gas phase at the ion source. These gas-phase ions then travel through the mass analyser, where they are separate based on their mass-to-charge (m/z) ratio. Finally, when each ion species reaches the detector, a signal is produced to plot the mass spectrum.
Below is a summary of the different types of hardware that are currently in use. Bear in mind that mass spectrometers are built with different combinations of ion sources, mass analysers and detectors to best suit the samples under study.
Figure 12: Summary of the main hardware components of a mass spectrometer.
From mass spectrometry to solving real-world problems
With all the fascinating developments in mass spectrometers over the last 150 years, mass spectrometry has been helping us enormously to cope with a diverse range of challenges. Apart from the applications in criminal science and food industries we discussed at the beginning, mass spectrometry has become an essential technique for biomedical research and drug discovery.
For example, following the recently developed real-time mass spectrometry technique is used to monitor brain metabolites in mice during drug toxicology studies. Furthermore, mass spectrometry is also applied during surgeries to provide real-time guidance for the precise removal of cancerous tissues. In dealing with the emerging SARS-CoV-2 virus, scientists recently established a mass spectrometry-based detection method, which can provide results within 10 mins and has nearly 90% accuracy. This could potentially help expand the detection capacity and improve the efficiency of suppressing viral transmissions.
Nowadays, innovation in mass spectrometry has by no means reached its limit. Its future is just as exciting and worth expecting as the development of newer mass spectrometry equipment and applications were in the past.
Author
Lingyi Wang
BSc Biotechnology with a Year in Industry
Imperial College London