To oversimplify, a mass spectrometer is a rather large scale used to measure something very small \u2013 individual molecules. This is not only a gross oversimplification but also a lie. Mass spectrometry (MS) is a technique that enables us not only to determine the mass of molecules but also to detect trace amounts of them in highly complex matrices, as well as to delve into the structure of chemical compounds. And although today the technique is mainly used for analytical purposes, in the past it was used to enrich uranium during the Manhattan Project.<\/p>\n<\/div>\n\n\n\n
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<\/figure>\n<\/div><\/div>\n<\/div>\n\n\n\nWhat exactly does a mass spectrometer measure?<\/h2>\n\n\n\n
Contrary to common, simplified terminology, a mass spectrometer does not directly measure molecular mass (M). The instrument measures the ratio of an ion\u2019s mass to its charge<\/strong>, denoted by the symbol m\/z<\/strong> (where m is the ion\u2019s mass in daltons and z is the number of elementary charges).<\/p>\n\n\n\n For a measurement to be possible, the molecule must undergo two processes:<\/p>\n\n\n\n The result of the measurement is a mass spectrum \u2013 a two-dimensional graph in which the x-axis represents the m\/z value and the y-axis represents the signal intensity, corresponding to the abundance of ions with a given m\/z value. By knowing the charge (z) and having a precise m\/z measurement, the researcher is able to calculate the molecular mass of the analyte under investigation with great accuracy.<\/p>\n\n\n\n Mass spectrometry is based on the ability of electrically charged particles to move in an electric field. Electrophoresis <\/a>also makes use of this same property. In mass spectrometry, there are even more interactions involved, as magnetic interactions are also utilised.<\/p>\n\n\n\n In short, the physical basis of mass spectrometry is the interaction of charged particles with electromagnetic fields. When an ion of mass m and charge z moves through a vacuum, it is subject to the Lorentz force:<\/p>\n\n\n Where:<\/p>\n\n\n\n By adjusting the electric field strength and the magnetic field induction, the movement of ions can be controlled as follows:<\/p>\n\n\n\n This enables the mass spectrometer to separate ions based on their mass-to-charge ratio (m\/z). The separated ions reach the detector individually, which counts a signal directly proportional to their quantity.<\/p>\n\n\n\n Most molecules are electrically neutral. The first step is therefore to impart an electric charge to them. This process takes place in the part of the spectrometer known as the ion source. There are many types of ion sources, which will be discussed separately. They are not universal and are suitable for specific types of molecules. It is in the ion source that neutral molecules acquire a charge (z). Depending on the structure of the molecule, there may be more than one charge. Another process also takes place in the ion source. Ions transition to the gaseous phase, which enables them to move within an electromagnetic field.<\/p>\n\n\n\n In the initial phase of the process, the ions are accelerated in an electric field. The kinetic energy (Ek) acquired by the ion depends on the applied voltage (V):<\/p>\n\n\n Separation<\/strong><\/p>\n\n\n\n At this stage, the ions that have been set in motion separate. This process takes place in what are known as ion analysers or ion separators. As with ion sources, there are many types of analysers, each designed for different molecules. Their operating principles also differ. For this reason, they will be discussed in a separate article.<\/p>\n\n\n\n Mass spectrometry is a technique that is over 100 years old.<\/p>\n\n\n\n The history of MS is a chronicle of Nobel Prize-winning discoveries, which began with a desire to understand the structure of matter.<\/p>\n\n\n\n Regardless of its complexity, every mass spectrometer consists of four main modules that work in close coordination:<\/p>\n\n\n\n The mass analyser and detector must be operated under high-vacuum conditions (10\u207b\u2075 to 10\u207b\u00b9\u2070 Torr)<\/strong>. This ensures that the moving ions do not collide with air molecules and their motion remains undisturbed<\/p>\n\n\n\n The choice of ion source depends on the polarity, mass and thermal stability of the compound under investigation.<\/p>\n\n\n\n Electron Ionisation (EI)<\/strong><\/p>\n\n\n\n This is a \u2018hard\u2019 technique, used mainly for small, stable molecules (e.g. in toxicological studies). The sample is bombarded with a beam of electrons, leading to severe fragmentation. This makes it possible to obtain a \u2018molecular fingerprint\u2019 of the molecule.<\/p>\n\n\n\n Electrospray ionisation (ESI)<\/strong><\/p>\n\n\n\n A key technique in biology. A sample in solution flows through a capillary under high voltage, forming a spray of charged droplets. As the solvent evaporates, the droplets shrink until the ions are \u2018pushed\u2019 into the gas phase.<\/p>\n\n\n\n MALDI (Matrix-Assisted Laser Desorption\/Ionization)<\/strong><\/p>\n\n\n\n The sample is co-crystallised with a so-called template (a small organic molecule) and then irradiated with a laser pulse. The template absorbs the energy and transfers it to the analyte, causing its desorption and ionisation.<\/p>\n\n\n\n The analyser determines the resolution (the ability to distinguish between ions of similar masses) and the accuracy of the measurement.<\/p>\n\n\n\n Why is mass spectrometry so important? Because it has revolutionised the way we study cells.<\/p>\n\n\n\n Mass spectrometry is a technology that combines the rigorous laws of physics with the infinite complexity of biology. From Thomson\u2019s identification of isotopes to the modern mapping of the human proteome, MS remains the most precise \u2018molecular scale\u2019 available to science. For biology students and teachers, understanding m\/z, ionisation mechanisms and the differences between analysers is key to comprehending modern scientific publications and the direction of development in personalised medicine.<\/p>\n\n\n\n Literature and soiurces<\/strong><\/p>\n\n\n\n Graphics<\/p>\n\n\n\n Photo by Bas van Breukelen<\/a> on Unsplash<\/a><\/p>\n\n\n\n\n
The measurement mechanism in mass spectrometry<\/h2>\n\n\n\n
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Stages of analysis in a mass spectrometer<\/h3>\n\n\n\n
Ionisation<\/h4>\n\n\n\n
Acceleration (Electric Field)<\/h4>\n\n\n\n
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<\/figure>\n<\/div>\n\n\nA brief history of mass spectrometry<\/h2>\n\n\n\n
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The construction of a modern mass spectrometer<\/h2>\n\n\n\n
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Sources of ions: How do you \u2018charge\u2019 molecules?<\/strong><\/h3>\n\n\n\n
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\n\n\n\nIon analysers: Filters and traps<\/strong><\/h3>\n\n\n\n
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Applications in the life sciences (Proteomics and Metabolomics)<\/strong><\/h2>\n\n\n\n
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\n\n\n\nSummary<\/strong><\/h2>\n\n\n\n
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