Pseudomolecular ions, formation

Pseudomolecular ion formation is an artifact common to most ESEAPCI-MS analysis. Although these ions cause the complicating of the mass spectra, they were also found to be useful for the confirmation of the molecular ion of an analyte of interest. 

Interpretation of mass spectra depends on the type of mass spectrometer and ionisation technique used. Hard ionisation methods such as El produce molecular ion fragmentation, which can be used to identify diagnostic fragmentation patterns and functional groups. Softer ionisation techniques such as ESI and MALDI provide pseudomolecular ion formation, and rules in accordance with spectral information can be used to identify corresponding molecular structure and elemental composition. Table 13.3 lists some of the types of information that can be provided by mass spectrometry, and Table 13.4 gives dehnitions of molecular masses that are highly relevant in mass spectrometry. 

A wide variety of other MS techniques are used to detect explosives. Two notable techniques are Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and CE-MS. FT-ICR-MS is used to probe pseudomolecular ion formation of RDX, PETN, and TNT using several ionization sources including EDI, El, electron capture ionization (EC), and chemical ionization (Cl). Analyses are performed both in the positive and negative ionization mode, and identities are assigned to the major pseudomolecular ion peaks seen in the spectra from each explosive [198]. TTie composition of several explosive compounds from postblast residue is assessed with FT-ICR-MS by identifying the explosive and inactive ingredients in a smokeless powder, TNT,... 

One of the main advantages of these soft ionisation techniques is that they lead to the formation of multiply charged, pseudomolecular ions (z can be greater than 30). Hence the mass range of the spectrometer can be extended to over 105 Da (to include proteins, polysaccharides and other polymers) (Fig. 16.20). These ionisation devices are often coupled to the mass spectrometer through a heated capillary transmitting the ions. 

Ion of the molecular species ion resulting from the ionization of a molecule by the addition or removal of a proton or an hydride, or by the formation of an adduct with another ion, such as Na+, Cl-, acetate, and so on. They allow the molecular mass to be deduced. In the past the terms pseudomolecular ion or quasimolecular ion have been used but should be avoided. 

Pseudomolecular Ions. In contrast to the traditional MS, the highest mass peaks in ESI/APCI spectra are not always the molecular ion of interest. Instead, pseudomolecular ions, or noncovalent complex ions, are commonly observed. The pseudomolecular ions are generally formed by the analyte-adduct interaction in the solution system that is preserved as a result of the soft ionization of the ESI/APCI process. These ions are also formed by analyte-adduct gas-phase collisions in the spray chamber [49]. The exact mechanisms of how the analyte adducts are formed in ESI/APCI still remain unresolved at this point. More often than not, the adduct ion formation is a major cause for the low detection limit for ESEAPCI MS. However, these associative processes have also created interest in the study of drug-protein/ drug-oligonucleotide gas-phase complexes that benefit from the ability of ESI/APCI MS analysis. 

Of these reactions, the formation of [M-pH] is the most common. Species formed in these reactions by addition or subtraction of atoms are sometimes referred to as pseudomolecular ions as they do not have the same elemental composition as the charged molecule but the term appears to be falling out of favor. 

First, if a multiple mass analyzer instrument is used, the pseudomolecular ion can be colHded with a stream of gas to generate collisionally induced dissociation (CID) and the formation of structurally significant fragment ions. MS/MS with selected reaction monitoring in a triple quadrupole is the method of choice for quantitation. Structure determination or confirmation is done by means of any of the available MS/MS fragmentation techniques triple quadrupole, ion trap, quadrupole-TOF, or FT-ICR. 

The methane PICI spectrum of TATP obtained with a linear quadrupole mass analyzer has been reported to contain ions of m/z 103,117,133, and 223, corresponding to the pseudomolecular ion [TATP -i- H]" [35]. A full-scan methane PICI spectrum of TATP has also been reported to contain ions of m/z 43(100%), 59,74,75,91, and 223(<5%), as shown in Figure 16.4B [30].The LOD for methane PICI analysis of TATP with linear qudaru-pole and ion trap mass analyzers were reported in the same work as 1 and 2 ng, respectively. Quantitation was based on extracted ions m/z 43, 59, 75, and 91 in both cases. When the m/z 91 ion was isolated in an ion trap mass analyzer and fragmented under CID, ions of m/z 43 and 74 were observed [30]. The third-order tandem mass spectrometry (MS ) experiment involving isolation of m/z 91 with subsequent CID and isolation of the m/z 74 product ion leads to the formation of a m/z 43 ion upon further CID. These results provide a scheme for analysis of TATP by selected reaction monitoring (SRM). 

The most important ionization process with the soft ionization techniques in positive-ion mode is protonation, i.e., formation of [M+H]+. Next to protonation, ion-attachment or cationization proeesses with sample-related Na - or K -ions may contribute to the ionization of polar analytes in these ionization techniques. [M+Na] and [M+K] have been reported for virtually all soft ionization techniques discussed in Sect. 7.2. The ESl-MS mass spectrum of the iridoid glycoside globttlarinmay serve as an example of this behavior, showing [M+H]+, [M+NH ], [M+Na], and M+K]+ (see Fig. 7.7, see also Sect. 7.5.2). It should be emphasized that the terms protonated molecule, sodiated molecule, and potassiated molecule should be applied rather than the frequently, but erroneous term molecular iorr. The term moleeular ion is reserved for radieal ions Also the terms qua-simolecular ion or pseudomolecular ion should not be used [92]. 

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