Multiply-charged molecular ions

Figure 16.20—Multiply charged molecular ions. An electrospray spectrum of horse cytochrome c, a protein of molecular weight 12360 Da is shown. Between two consecutive peaks in the molecular ion cluster, the charge state varies by one unit. The second spectrum corresponds to a high-resolution spectrum in the 772-774 m/z range. In this isotopic cluster, all ions carry the same number of charges. It is possible from either of these spectra to calculate the approximate molecular weight and the number of charges carried by the ions (spectra reprinted with permission from F. W. McLafferty et al.. Anal. Chem., 1995, 67, 3802-5. Copright 1995 American Chemical Society).

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. 

The ESI mass spectra of biological macromolecules normally correspond to a statistical distribution of consecutive peaks characteristic of multiply charged molecular ions obtained through protonation (M + zH)z+, or deprotonation (M - zll) , with minor if any contributions of ions produced by dissociations or fragmentations. However, as the measured apparent mass is actually m/z, to know m one needs to determine the number of charges z. 

Multiply Charged Molecular Ions and the Determination OF Charge State... 

The mass spectra of 2-aryl-5-phenyloxazoles 198 have also been studied,as have linked bis-oxazoles of types 199 and 200, which fragment through ring cleavage and with facile formation of multiply charged molecular ions. ° A study of cleavage patterns and phenyl migration in 2,4,5-triphenyloxazole 201 has appeared (Fig. 2.20). ° ... 

The presence of multiply charged molecular ions in the MALDI-TOF spectrum can improve the accuracy of mass determination by averaging the values measured for different charge states. The gain, however, can be very little, if any, because of the significant spread in the molar mass data derived from the multiply charged ions. ... 

Figure 2 Mass spectrum of horse heart myoglobin obtained by ESI-MS. Sample introduction was performed via a syringe pump at 6 nL/min at a concentration of 2 pmol/nL (CH3CN/H2O 1 1). The two most abundant multiply charged molecular ions are shown and the measured mass is given as compared with the calculated molecular weight. The standard deviation of 100ppm (1.8 Da) results from the calculation of the average mass based on all of the multiply charged molecular ions (see text for details of the calculation).

FIGURE 4.16 Plasma desorption mass spectrum of papain, (a) structure and identity of sequence fragments, (b) mass spectrum showing the multiply charged molecular ions, and (c) expansion of the spectrum in the range m/z 1000-2520. (Reprinted with permission from reference 34). 

Trimpin, S. Inutan, E. D. Herath, T. N. McEwen, C. N., Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Method for Selectively Producing Either Singly or Multiply Charged Molecular Ions. Anal. Chem. 2010, 82,11—15. 

Atypical ESI spectrum for a protein (lysosyme) is shown in Fig. 11. The multiply charged molecular ion pattern is clearly recognizable. Note that although this ESI spectrum corresponds to only one protein, there is a mixture of ions in the spectrum each of which has a different mass-to-charge ratio (reminder In mass spectrometry, the m z ratio is measured). To calculate the molecular mass (or MW) of the protein, the charge states of the individual ions should also be determined. Thus, we have two unknowns, the MW and the charge state (n). To determine... 

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