Multiply-charged precursor ions

It is widely accepted that current ETD generally favors multiply charged precursor ions and is not effective for ions exceeding m/z 1400 or Multiply charged glycopeptides at miz above this range when acted on... 

Figure 9 Mass spectra of multiply charged fragment ions of the peptide renin substrate tetradecapeptide. The precursor was the (M+4H+)4+ ion at /r /z 440. The insets show the resolution of the isotopic peaks for the +1 to +4 ions. Reproduced with permission of The American Chemical Society from Thomson BA, Douglas DJ, Corr JJ, Hager JW, Jolliffe CL (1995) Analytical Chemistry S7 1696-1704.

It may happen that an ion formed in the analyzer loses a water molecule, formed by the OH- anion from the carboxy terminus and the H+ ion from the amino terminus. Such ions, deprived of water, are denoted with the ° symbol, for example, a°. If an ion loses the -NH2 group, it is described with, for example a. Formation of multiply charged fragments is also possible, especially if the precursor ion was originally endowed with a multiple charge. If the charge is different from +1, it is shown in the top right-hand side of the symbol, for example, b22+. 

In tandem MS mode, because the product ions are recorded with the same TOF mass analyzers as in full scan mode, the same high resolution and mass accuracy is obtained. Isolation of the precursor ion can be performed either at unit mass resolution or at 2-3 m/z units for multiply charged ions. Accurate mass measurements of the elemental composition of product ions greatly facilitate spectra interpretation and the main applications are peptide analysis and metabolite identification using electrospray iomzation [68]. In TOF mass analyzers accurate mass determination can be affected by various parameters such as (i) ion intensities, (ii) room temperature or (iii) detector dead time. Interestingly, the mass spectrum can be recalibrated post-acquisition using the mass of a known ion (lock mass). The lock mass can be a cluster ion in full scan mode or the residual precursor ion in the product ion mode. For LC-MS analysis a dual spray (LockSpray) source has been described, which allows the continuous introduction of a reference analyte into the mass spectrometer for improved accurate mass measurements [69]. The versatile precursor ion scan, another specific feature of the triple quadrupole, is maintained in the QqTOF instrument. However, in pre-... 

For each mass value, a signal quantity at the detector is recorded as counts per second, but may also be in another form. Since there are generally many mass values acquired, there is a ion intensity for each mass. This can be displayed as a bar chart of m/z ratio versus signal intensity. This chart is known as a mass spectrum. Spectra come in many variants. Full scan mass spectra show a range of masses with individual values at regular intervals of 1 or even 0.1 Da. SRM measures only the precursor ion mass and its product ion. Some mass spectra are processed data that convert m/z to mass. This is often seen in protein analysis where the multiply charged mass spectra are converted to a spectrum of just mass, simplifying the spectra interpretation. 

Another dissociation lechnique that has been applied to large multiply charged ions is eketron-cupwe ilixsndalion (KC D), in which precursor ions capture a low-energy electron to produce an intermediate that rapidly dissociates. In some cases, a background gas is added to aid in the dLssociation process,... 

---