Electrospray ionization multiply charged ions produced

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. 

For an ion, the cross section for electron capture (EC) roughly increases with the square of the ionic charge. [137] This makes multiply charged ions as produced by electrospray ionization (ESI, Chap. 11) the ideal targets for this process, e.g. ... 

Figure 5 Examples of Data Generated on an Electrospray Ionization Mass Spectrometer, (a) Proteins Typically Produce Positive, Multiply Charged Ions and (b) Oligonucleotides Generate Negative, Multiply Charged Ions. Inset are the Computer-Generated Molecular Weight Spectra...

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. 

For compounds less than 1000 Da, electrospray produces mainly protonated molecular ions MH" for basic compounds and deprotonated molecular ions (M-H) for acidic compounds. Some adduct ions containing ammonium (MNHa)", sodium (MNa)" " or (M-H+Na), potassium (MK)" or formate (M-H+HC02) may be observed as well, and sometimes can be exploited to detect compounds inefficiently ionized by proton transfer [25]. Higher molecular mass compounds produce multiply charged ions of the type (MH-nH)" or (M-nH)" under electrospray conditions, for example... 

V. Katta, S. K. Chowdhury, and B. T. Chait, Use of a single-quadrupole mass spectrometer for collision-induced dissociation studies of multiply charged peptide ions produced by electrospray ionization, Anal. Chem., 63 (1991) 174-178. 

Today, the two most common LC/MS interfaces are atmospheric pressure ionization interfaces, electrospray (ESI) and ion spray (ISI). Electrospray (Fig. 15.8) and its subtype, nanospray, are recommended for use with proteins and highly polar or ionized compounds. They are very soft ionization, concentration-dependent techniques that result in very little fragmentation and often produce multiply charged molecular ions. 

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