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What is mass spectrometry?
Identifying a molecule or an atom by its mass is easy with a mass spectrometer. Find out how these fascinating instruments work.
Molecules like ascorbic acid are roughly one nanometer in size, which is a mere 0.000001 millimeters. It’s impossible to characterize such tiny molecules with the naked eye or even the most sensitive laboratory balances. But mass spectrometers can. These instruments manage to recognize molecules by fragmenting multi-atomic particles, determining the mass of each individual component and creating a characteristic pattern from this.
The history of mass spectrometry goes back to 1918.
At that time, the Canadian-US physicist Arthur Jeffrey Dempster laid the foundation for the construction principle behind today's instruments. In 1950, the American Roland Gohlke, another pioneer of mass spectrometry, succeeded in combining a mass spectrometer with a gas chromatograph, which allowed him to separate and identify substance mixtures using a single assembly of instruments.
But how do mass spectrometers work? These days, they exist in a wide range of variants. However, the basic components are usually identical. To identify molecules and atoms, the instruments need an ion source, an analyzer and a detector.
The role of the ion source is to ionize the molecules of the sample. Several ionization methods exist, of which the most common is electron impact ionization (EI). Sample molecules are initially vaporized. The gaseous sample is then transferred into a chamber where it is bombarded by an electron beam from a hot cathode. The energy can be varied, but in most cases it is set to around 70 electron volts. Under this bombardment, the beam knocks electrons out of the molecules, creating monovalent, positively charged ions. If more energy is transferred in this ionization phase fragmentation, i.e. the degradation of the molecular ion, occurs. The generated fragments are composed of fragment ions and neutral particles (radicals). In the subsequent steps, the mass spectrometer is able to determine the mass of the fragment ions and display it in a mass spectrum. This creates characteristic ion patterns for each substance.
In addition to EI, other ionization techniques exist, such as chemical ionization (CI), field ionization (FI) or photoionization (PI). Liquids and solids can also be bombarded with fast ions or atoms for ionization. The methods are called fast atom bombardment (FAB) and secondary ion mass spectrometry (SIMS).
In the next stage, the ions leave the ion source as a focused beam and fly into the so-called mass analyzer. Its task is to ensure that only ions of identical velocity reach the downstream detector, because only then can the mass of the particles be determined with precision. Again, a variety of techniques exists, and magnetic sector field instruments are frequently used. Their ion separator is a velocity filter, named Wien filter after its developer Wilhelm Wien. It consists of an upper, positively charged capacitor plate and a negatively charged lower plate. When ions fly through the Wien filter towards a pinhole, the Lorentz force directs them upwards and the electric force downwards. Only ions for which both forces are in balance fly straight through the pinhole, while ions that are too slow or too fast are deflected upwards or downwards and cannot pass. Other ion separation techniques use time-of-flight (TOF), quadrupole or cyclotron resonance analyzers.
After the sorted ions fly through the pinhole, they enter another magnetic field of the detector. The Lorentz force effective there deflects the ions from their straight path into a curved one. This deviation leads the ions to the detector. This is where Faraday cups, photomultiplier tubes or photographic plates come into play. The greater the mass, the greater the radius of the curved path. When flying through the magnetic field, high-mass fragment ions end up on an outer track, smaller fragments on an inner one. The extent of deflection is decisive for evaluation. Mass spectrometers do not measure the ions’ mass as such, but the mass-to-charge ratio (m/z), which is plotted on the abscissa of the mass spectrum. In this spectrum, the isotopes of an element appear as separate peaks. For monovalent ions, the m/z value corresponds to the atomic mass unit u, while for ions with a higher valence, the mass is z times higher. In this way, each molecular compound creates a pattern that is as characteristic as a fingerprint. A comparison with mass spectra stored in databases subsequently lets almost any organic compound be reliably identified.
Mass spectrometry is an established technique in numerous industries. In chemistry the method is used to analyze chemical elements and molecules. It is so sensitive that very small amounts of substances down to one trillionth of a milligram (i.e., a femtogram) can be detected. The method is also popular with toxicologists who can identify poisons and drugs in blood, while environmental scientists find pollutants in soil samples. Biologists, on the other hand, use mass spectrometry for proteomics, i.e. to investigate proteins in living organisms. Physicists determine the mass of atomic nuclei. And archaeologists analyze the ratios of isotopes in bones to draw conclusions about the diet of the human or animal. Meanwhile, the technology is even used at airports to detect residues of explosives or drugs on passengers or luggage. That’s how multi-talented mass spectrometry is!
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