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The magic of mass spectrometry: How can collision-activated molecular fragmentation reveal hidden structures?
In the world of mass spectrometry, the possibilities are endless, especially through collision-activated molecular fragmentation (CID). This technology allows scientists to dig deep into the structure and properties of molecules, seeing through the fragments of a molecule to reveal its complexity hidden beneath the surface. CID technology mainly accelerates ions and causes them to collide with neutral gases, causing energy changes within molecules and ultimately causing molecular breakage.
"Through collision-activated reactions, we can not only confirm the presence of a molecule, but also guess its potential structure."
Basic principles of collision activation
Collision-activated fragmentation works by accelerating selected ions to a high energy state such that when they collide with neutral molecules, part of their energy is converted into internal energy, resulting in bond breaking and the generation of small fragments. These fragments can then be further analyzed by mass spectrometry to unravel the mysteries of the molecular structure.
Low Energy and High Energy CID
Low-energy CID is primarily performed below 1 kiloelectronvolt (1 keV), and while it is highly efficient in producing molecular fragments, the type of fragmentation observed is strongly affected by the kinetic energy of the ions. When the ion kinetic energy is very low, most of the segments are converted into structural rearrangements, while the probability of direct bond breakage increases with increasing ion kinetic energy.
Compared to low energy CID, high energy CID uses ions with kinetic energies typically ranging from 1 keV to 20 keV. This method can generate some fragments that cannot be observed by low-energy CID, such as charge-remote fragmentation that occurs in molecules containing hydrocarbon structures.
Triple quadrupole mass spectrometer
A triple quadrupole mass spectrometer consists of three quadrupoles, the first quadrupole (Q1) acts as a mass filter, selectively passing ions and accelerating them to the second quadrupole (Q2). Q2 acts as a collision cell. In a high-pressure environment, the selected ions collide with neutral gas and CID occurs. The generated fragments are then accelerated into Q3 for mass analysis, the results of which can be used to obtain detailed information on the molecular structure.
Fourier transform ion cyclotron resonance
In a Fourier transform ion cyclotron resonance mass spectrometer, particles are trapped in an ICR cell and their kinetic energy is increased by applying a pulsed electric field at their resonant frequency. A short burst of collision gas is introduced during this process to promote collisions between excited ions and neutral molecules, thereby producing the desired fragments. In addition, by continuous non-resonant irradiation, alternating excitation and de-excitation can be achieved, which allows ions to undergo multiple collisions at low collision energies.
High Energy Collision Fracture
High-energy collisional fragmentation (HCD) is a CID technique specific to orbitrap mass spectrometers. Its characteristic is that the fragmentation occurs outside the trapping chamber, and this process is not limited by the mass cutoff of resonant excitation, so it is very suitable for quantitative analysis based on isotope labeling. Despite its name, HCD collision energies are typically below 100 eV.
Fracturing mechanism
In the CID process, the fragmentation mechanism is divided into homolytic fragmentation and heterolytic fragmentation. The fragments produced by homolytic fracture retain their original bonding electrons, while heterolytic fracture causes the bonding electrons to move with one fragment. More specifically, remote charge cleavage is a covalent bond cleavage process that occurs in the gas phase, where the bond being cleaved is not adjacent to the charge site.
Future Discussions
The development of mass spectrometry technology may bring more unprecedented possibilities, especially the potential in identifying and analyzing complex molecular structures. Through advances in collisional activation techniques, we will be able to uncover more molecular mysteries, leading to a new round of exploration in chemistry and biology. Looking to the future, have you ever thought about how more precise structural analysis will change our scientific understanding?
