Ionization methods, mass multiply charged ion

In contrast to most other ionization methods, the majority of ions produced by electrospray are multiply charged. This is of great significance as the mass spectrometer measures the m/z (mass-to-charge) ratio of an ion and the mass range of an instrument may therefore be effectively extended by a factor equivalent to the number of charges residing on the analyte molecule, i.e. an ion of m/z 1000 with... 

Electrospray ionization, in contrast to the majority of other ionization methods employed in mass spectrometry, produces predominantly multiply charged ions of the intact solute molecule. This effectively extends the mass range of the mass spectrometer and allows the study of molecules with molecular weights well outside its normal range. 

With external ion sources it became feasible to interface any ionization method to the QIT mass analyzer. [171] However, commercial QITs are chiefly offered for two fields of applications i) GC-MS systems with El and Cl, because they are either inexpensive or capable of MS/MS to improve selectivity of the analysis (Chap. 12) and ii) instruments equipped with atmospheric pressure ionization (API) methods (Chap. 11) offering higher mass range, and some 5-fold unit resolution to resolve isotopic patterns of multiply charged ions (Fig. 4.47). [149,162,172,173]... 

A new ionization method called desorption electrospray ionization (DESI) was described by Cooks and his co-workers in 2004 [86]. This direct probe exposure method based on ESI can be used on samples under ambient conditions with no preparation. The principle is illustrated in Figure 1.36. An ionized stream of solvent that is produced by an ESI source is sprayed on the surface of the analysed sample. The exact mechanism is not yet established, but it seems that the charged droplets and ions of solvent desorb and extract some sample material and bounce to the inlet capillary of an atmospheric pressure interface of a mass spectrometer. The fact is that samples of peptides or proteins produce multiply charged ions, strongly suggesting dissolution of the analyte in the charged droplet. Furthermore, the solution that is sprayed can be selected to optimize the signal or selectively to ionize particular compounds. 

Whatever the ionization method used, the mass spectra of oligosaccharides analysed by ESI, MALDI or FAB display intense ions of the molecular species resulting from protonation (M -(- H)+ or cationization by an alkali metal ion (M + alkali metal)"1" in the positive ion mode or from deprotonation (M — II) in the negative ion mode. In ESI, multiply charged ions also are produced. 

The soft technique of electrospray ionization (ESI) mass spectrometry has widespread applications in chemical and biochemical analysis of high-molecular mass compounds (M, < 200 000). In contrast to the methods described above, both singly and multiply charged ions are observed, making the ESI technique valuable for analysis of ionic compounds. 

Solvent-free separation using IMS-MS when combined with solvent-free sample preparation and appropriate ionization methods provides TSA that are independent of analyte solubility and is applicable to complex mixtures. A new ionization method, LSI, which operates at AP with laser ablation of samples prepared in a MALDI matrix, has been effectively interfaced with a SYNAPT G2 IMS-MS instrument. Early results show separation of protein mixtures and that protein ion structures from LSI and ESI are similar. Further, a new matrix allows LSI multiply charged ion formation to be extended to solvent-free matrix preparations. Thus TSA by LSI-IMS-MS is a new approach to tissue imaging at high spatial resolution on high-end mass spectrometers such as the SYNAPT G2 and Orbitrap instruments. LSI has... 

In comparison with other ionization sources, ESI represents an even softer ionization technique and causes no fragmentation of analyte ions. A major benefit of the generation of multiply charged ions from polypeptides is that they can be readily analyzed with a less sophisticated instrument with limited mass range, such as a quadrupole. A serious problem is its poor tolerance to matrix, such as mobile-phase buffers, when ESI-MS is used for metallomics and metalloproteomics studies. Since the MS response significantly depends on solvent and sample composition, ion signal intensities of a given analyte do not necessarily correlate with its concentration in samples. Therefore, the internal standards are essential for the quantitative analysis. Another solution is the use of ICP-MS as complement.The detailed methods will be described later. 

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