CID process

Ion trap MS is particularly suited for chemical structure elucidation, as it allows for simultaneous ion storage, ion activation and fragmentation, and product ion analysis. The fragmentation pathway of selected ions and the fragmentation products provide information on the molecular structure. Compared with triple-quadrupole and especially with sector instruments, the ion trap instrument provides more efficient conversion of precursor ion into product ions. However, the CID process via resonance excitation, although quite efficient in terms of conversion yield, generally results in only one (major) product ion in the product-ion mass spectrum. MS/MS with a quadrupole ion trap offers a number of advantages ... 

At low extraction voltages, the in-source CID process is greatly inhibited and the spectra display intense signals for the protonated molecular ions. By raising the extraction voltage, in-source CID spectra were obtained. Neutral losses of the carboxylated ethoxy chain and... 

Vekey, K. Czira, G. Translational Energy Loss and Scattering in CID Processes. Org. Mass Spectrom. 1993,28,546-551. 

An overall view shows the CID process as a sequence of two steps. The first step is very fast (10 14 to 10 16 s) and corresponds to the collision between the ion and the target when a fraction of the ion translational energy is converted into internal energy, bringing the ion into an excited state. The second step is the unimolecular decomposition of the activated ion. The collision yield then depends on the activated precursor ion decomposition probability according to the theory of quasi-equilibrium or RRKM. This theory is explained elsewhere. Let us recall that it is based on four suppositions ... 

In addition to kinetic shifts, the threshold for a CID process can be delayed by competition with another decomposition channel. Consider the bis-ligated sodium cation, Na+(H20)(NH3), which can decompose by losing either ligand, Eq. (6) ... 

Indeed, these methods were largely developed for such systems and then applied to CID processes. One key advantage of bimolecular reactions is the absence of a third body, such that the energy available to the reaction intermediate, ABC+, is the entire kinetic energy of the collision. In such a case, the parameter n no longer describes the energy deposition process, but rather the efficiency of the reaction, such that Eq. (3) is still an appropriate means to determine the threshold for such reactions. 

Unlike the other species in the mixture, 2-DBT and 4,6-DBT showed both the parent ion and a fragment missing a hydrogen. It is suspected that the methyl groups on these two compounds lose one hydrogen during the ionization and collision-induced dissociation (CID) process as depicted in Scheme 5. 

The CID process in the ion trap is fundamentally different from CID in the collision cell of a triple-quadrupole instmment ... 

R.D. Voyksner. T. Pack, Investigation of CID process and spectra in the transport region of an ESI single-quadrupole MS, Rapid Commun. Mass Spectrom., 5 (1991) 263. 

There are three main types of mass analyzers in ESTMS-MS instruments triple quadrupole, ion traps, and quadrupole-time-of-flight (Q-TOF). There are several differences between the mass analyzers in MALDI-TOF and in ESI-MS-MS. Unlike in MALDI-TOF-MS, in ESTMS-MS two mass analyzers are used in tandem to increase the sensitivity of the technique. The peptide ions produced by the ESI sources are carried to the first mass analyzer and only peptides of a set miz ratio are selected. The selected ions are then carried to a collision cell where they are subjected to additional fragmentation to produce smaller amino acid ions using a process called as collision induced dissociation (CID). The CID process employs inert gases such as argon for the dissociation of peptides. These smaller amino acid ions are then resolved in the second mass analyzer before sending to the detector. This process essentially enables highly sensitive detection of actual amino acid sequence of the peptides based on the mIz ratios of individual amino acids. 

In the case of the ESTMS-MS data, actual amino acid sequence can be deduced. This is possible due to the CID processes, which breaks the peptides further into amino acid ions. Each amino acid ion has a specific mass and by calculating masses of specific amino acid from the MS spectra, the exact sequence of the peptide and in turn the protein can be deduced. The workhorse of such analysis is a program called Sequest . Since the ESTMS-MS analysis provides information about the actual amino acid sequence, it is also useful to obtain information about protein modifications (such as phosphorylation) and toxicant-induced protein adducts. This has become even easier with the advent of new software tools and highly intelligent algorithms such as SALSA . 

Two general types of reactions can be considered collision-induced dissociation (CID) processes, reaction (3),... 

A generalized method for determining both relative and absolute bond strengths to an ion center by CID has been developed by Cooks and co-workers. In extensive studies on X - (56) and Ag" " (57), they demonstrated that under appropriate conditions, if AX is more intense than BX in the competitive CID processes 16 and 17, then D (X -A) > D (X -B). This relative bond energy information can be... 

In comparison to Hnear quadrupole instruments, the CID process in an ion trap is more complex since the selected ionic species are resonantly exdted at their se-... 

Using a constant collision energy in absolute terms is experimentally very difficult to realize and one should be pragmatic about this issue. In any mass spectrometer, the CID process is crucially dependent on the actual inert gas pressure in the collision cell and this inevitably varies from day to day (in particular after maintenance). 

Jackson et al. [42] have proposed another interesting CID method in which the CID process is accomplished during a mass acquisition scan. The method is very similar to that of axial modulation except that the amplitude of the AC dipole field is tuned to fragment precursor ions rather than ejecting precursor ions. They have shown this method to be fast, efficient, and highly energetic CID process. 

The CID process (in positive ion mode) is effective only for protonated molecules. If the mIz selected in the first analyzer corresponds to a sodiated species, [M -i- Na]+, then the stability of these ions results in either no fragmentation, regardless of how much energy is added, or that a point is reached where the acquired energy leads to the complete disintegration of the ion. 

Improved signahnoise ratios at the selected miz are observed in Q3 because the CID process suppresses the chemical noise. 

Q1 scanned, Q3 scanned. Both Q1 and Q3 are scanned at the same rate, but the scans are offset from each other by an mJz value corresponding to a specific, selected, component that is lost as a neutral species during the CID process in the collision cell (q2). When a particular neutral loss is known to occur in a class of compounds, this type of MS/MS analysis will identify the components of mixtures that release the neutral fragment e.g., carboxylic acids tend to lose CO2, and so the Q1 and Q3 scans are offset by 44 Da. 

Ion activation is also accomplished by collisions of the fast-moving precursor ions with a solid surface [12,13]. This ion-surface collision technique, known as surface-induced dissociation (SID), can be implemented on a variety of tandem mass spectrometry systems, such as magnetic sector, TOF, quadrupoles, ion traps, and FT-ICR-MS, by placing a solid surface in the path of the ions [13]. The surface can be a bare metal (e.g., a stainless-steel plate) or a metal covered with self-assembled monolayers [14]. Ion-surface collisions are more efficient in terms of internal energy conversion because of the greater mass of the colliding surface [see Eq. (4.4)]. Consequently, compared to the CID process, in which a serious decrease in the dissociation efficiency is observed, ions of much higher mass can... 

All four types of scan laws discussed in Section 4.2 can be implemented with a triple-quadrupole instrument. For example, to acquire a product-ion spectrum, Qi is set to transmit ions of a specified miz value into Q2, where they undergo a CID process. Q3 is scanned to mass-analyze the products formed in Q2. A precursor-ion spectrum is acquired by reversing this procedure that is, Q3 is set to transmit just the m/z value of a desired product ion, and Qi is scanned to transmit all precursors of this chosen product ion. As compared to the magnetic sector-based tandem instruments, a simple scan law is used in the triple-quadrupole instruments to monitor the loss of a neutral. The fields of Qi and Q3 are both scanned in tandem, but with an offset value related to the mass of the neutral. 

In contrast, the fragmentation patterns that are observed under low-energy CID are qualitatively similar to the conventional mass spectra and produce primarily b- and y- ions. The products formed in a low-energy process are due principally to charge-directed fragmentations. Also, w ,v , and d sequence-specific ions are absent in the low-energy CID process. 

It will be clear that the process outlined here is less formal than a traditional ID process. Subject-matter characteristics as well as the cognitions and expertise of the author are taken into account from the first moment on. However, the process is a very systematic one. Although all the different parameters are not considered simultaneously, they are all taken into account by gradually plugging them into the process. Finally, whereas the traditional ID process can be described to be linear, this CID process is cyclical and elaborative. 

---