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Exploring the mysteries of low- and high-energy CID: How do these two techniques affect mass spectrometry results?
In the field of mass spectrometry, collision-induced dissociation (CID) technology has proven its irreplaceability in molecular structure analysis. CID technology relies on the collision of selected ions with neutral gas molecules in the gas phase, which causes the energy-driven fragmentation of these molecules to generate fragment ions of different sizes, which can then be further analyzed.
The choice of low-energy CID and high-energy CID will directly affect the accuracy and sensitivity of the analysis results.
Low energy CID and high energy CID
Low-energy CIDs typically operate in the energy range below about 1 kiloelectronvolt (1 keV). This technique is extremely efficient at fragmenting selected precursor ions, but the type of fragmentation observed is strongly dependent on the energy of the ion's motion. As the energy increases, the internal energy of the ion increases, and the probability of direct bond breaking also increases, leading to the generation of fragments with different structures.
Relatively speaking, high-energy CID (HECID) usually operates in a higher energy range, typically between 1 keV and 20 keV. This energy setting can generate certain special fragments that cannot be formed in low-energy CID, including the charge-distant fragmentation observed in molecules with hydrocarbon side chains.
High-energy CID not only reveals the complexity of molecules, but also provides unprecedented structural elucidation capabilities.
Triple quadrupole mass spectrometer
Triple quadrupole mass spectrometer is a common mass spectrometry instrument that contains three quadrupole. The first quadrupole, called "Q1," acts like a mass filter, selectively transporting specific ions and accelerating them toward the second quadrupole, "Q2." The gas pressure of Q2 is higher, where selected ions collide with neutral gas and dissociate through CID technology. The resulting fragment ions are then accelerated into the third quadrupole Q3, where a mass range scan is performed to analyze the results.
Many experiments using CID on triple quadrupole can further determine the origin of specific fragments, rather than just the fragments produced.
Fourier transform ion cyclotron resonance mass spectrometry
In Fourier transform ion cyclotron resonance mass spectrometry, ions can be excited by a pulsed electric field. As the energy of the excitation is different, the kinetic energy of the ions also changes. However, since a long time is required for excited ions to collide with neutral molecules at low pressures, a pulse valve is often used to briefly introduce the collision gas. In this process, specific experimental techniques, such as sustained non-resonant radiation collision-induced dissociation technology (SORI-CID), also enable mass spectrometry to obtain more refined data.
Higher energy collision dissociation
Higher energy collisional dissociation (HCD) is a CID technique used exclusively in orbitrap mass spectrometers, in which fragmentation occurs outside the cavity. HCD is efficient in running and data analysis and is not affected by the low mass cutoff of resonance excitations, making it suitable for quantitative analyzes that rely on reporter ions.
Although HCD technology is called high-energy impact, its actual collision energy is usually less than 100 electron volts.
Fragmentation mechanism
During the CID process, there are two main mechanisms of cleavage: homolytic and heterogeneous. Homolysis causes each fragment to retain one of its original bonding electrons, while heterolysis causes the bonding electrons to remain on only one product. In addition, charge-distant fragmentation is a more specialized form of fragmentation, in which the broken bond is not in the vicinity of the charged site, which gives it additional significance in mass spectrometry analysis.
Through these unique fragmentation mechanisms, scientists can obtain rich structural information that facilitates deeper molecular analysis.
Today, with the help of low-energy and high-energy CID technology, mass spectrometry is opening a new chapter for scientific research. In the future, what other unrevealed molecular structures and chemical reactions will be discovered and understood through these technologies?
