Pseudomolecular ion

Figure 5.41 The total-ion-current (TIC) trace and reconstructed ion chromatograms from the predicted pseudomolecular ions of Indinavir m/z 614) and its mono- (m/z 630) and dihydroxy metabolites (m/z 646), generated from full-scan LC-MS analysis of an incubation of Indinavir with rat liver S9. Reprinted by permission of Elsevier Science from Identification of in vitro metabolites of Indinavir by Intelligent Automated LC-MS/MS (INTAMS) utilizing triple-quadrupole tandem mass spectrometry , by Yu, X., Cui, D. and Davis, M. R., Journal of the American Society for Mass Spectrometry, Vol. 10, pp. 175-183, Copyright 1999 by the American Society for Mass Spectrometry.

Both positive and negative ions are produced during the sputtering process, and either can be recorded by an appropriate choice of instrumental parameters. Positive ions are the result of protonation, [M + H]", or cationiz-ation, [M +cation], whereas negative ions are preponderantly [M-H], but can also be formed by the addition of an anion, that is, [M+anion]". The type of pseudomolecular ion produced is governed by the chemical nature of the sample and by the composition of the matrix from which it is ionized. 

Consequently, the only pseudomolecular ions produced are [M+Na]" species. These are usually abundant, permitting the observation of molecular ions up to at least 6000. 

All f.a.b. spectra are characterized by (a) abundant, pseudomolecular ions for both the sample and the matrix, and (b) a relatively high level of chemical noise, resulting in a signal at every mass number up to the... 

Pseudomolecular ions do not appear as single, clean signals in f.a.b. spectra. Instead, clusters of signals are always present, partly because of the presence of molecules containing the C isotope, the natural abundance of which is 1.1%, and partly because oxidations and reductions can occur in the matrix during the f.a.b. experiment. For example, underivatized... 

F.a.b.-m.s. is a powerful technique for examining mixtures of carbohydrates. Many examples of such analyses are given in Sections V and VI. Unless the components have very different chemical structures, all will give molecular ions. However, the relative abundance of the ions will not necessarily reflect the relative concentrations of the components. Furthermore, if more than one class of carbohydrate is present, different pseudomolecular ions may be produced for each class. An example of such a phenomenon is given in Fig. 4. 

Proteins and peptides are most often seen in the mass spectra as pseudomolecular ions, that is, molecules with attached charge-carrying protons (in the negative-ion mode, proteins and peptides lose protons and thus acquire a negative net charge). This additional proton has to be taken into consideration in order to predict correctly the m/z value at which the peptide of interest will be seen in a mass spectrum. For example, a peptide whose molecular weight (MW) (or molar mass) is equal to 2000 Da, when singly ionized, will be detected at 2001 m/z (for simplification, we assume the mass of proton as equal to 1) ... 

The pseudomolecular ion MH + (which is not a radical ion) is usually more stable than the M + ion produced by electron ionisation. Hence, fewer fragment ions are observed in chemical ionisation and so it is called a soft ionisation technique. 

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. 

Mass spectrometry, obtained through the use of soft techniques (FAB, ESP MS) gives information about molecular weight (pseudomolecular ions [M+H]+ or [M-H] ) and some very limited sequence and branching confirmation. [124-127]. 

Fig. 16. Fragmentation in FAB MS-MS of the oligosaccharide (22) from LPS of Vibrio salmonicida,34 FAB MS of the oligosaccharide (57) from OPS of Yersinia ruckerii Ol,36 and ESI MS-MS of the trisaccharide-serine (11) from pilin of Pseudomonas aeruginosa 1244.15 (For abbreviations, see Figs. 3, 5, and 7. Pseudomolecular ions [M + H]+ at m/z 1867.6 and 771.5 were used as precursor ions in analysis of 22 and 11, respectively.)...

Particle induced desorption methods are commonly used to ionize low-volatility compounds. Cesium ion desorption (or cesium ion secondary ion mass spectrometry, SIMS) uses a primary beam of cesium ions to desorb and ionize a non-volatile sample. This technique has been used with FTMS to produce pseudomolecular ions of vitamin B12, ((812)2 + Cs - 2CN +, at m/z 2792 (24) and molecular ions of beta-cyclodextrin (m/z 1135) (92, 93). Detection limits of 10"13 mol for the peptide gramicidin S has been demonstrated using Cs+ SIMS with FTMS (25), and additional structural information was obtained using MS/MS processes. 

The primary advantage of laser desorption is that abundant molecular or pseudomolecular ions are produced for many different classes of compounds. Positive pseudomolecular ions are most often formed by attachment of a cation, typically a proton, potassium, or sodium ion, to the parent molecule. Negative pseudomolecular ions can also be formed by laser desorption, usually by loss of a proton, or by electron attachment to molecules with a positive electron affinity. Often little fragmentation occurs (making this... 

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