Resolution high-mass ions

The essential independence of mean ion velocities on the molecular weight of the analyte leads to an approximate linear increase of the mean initial kinetic energies of the analyte ions with mass. High-mass ions therefore carry tens of elec-tronvolts of translational energy before ion acceleration. [33,41,50] The initial velocity of the ions is superimposed onto that obtained from ion acceleration, thereby causing considerable losses in resolution with continuous extraction TOP analyzers, in particular when operated in the linear mode. 

Ions formed in an external ion source and injected into the ICR cell are subject to effects which are analogous to the case of particle desorption ionization in the ICR cell. However, experimental results indicate limits in the resolution of high mass ions due to the finite duration of the time-domain image current signal. This observation is consistent... 

Recent reviews [264,265] show the possibilities of these different techniques of soft ionization, with various analyzers to separate the high-mass ions in particular, with a time-of-flight, it has been possible to transmit ions of m/z 12651 ( 10) [266]. Recently, resolution of TOF instruments has been improved [267], enabling the determination of metastable transitions. The FAB [268], SIMS [269] and laser desorption [270] techniques are consistent with MS/MS methodology. 

TOP analyzers, like quadrupoles, scan the mass spectrum rapidly. Resolutions of 500 can be obtained. These analyzers are popular for high mass ion detection since they have no real upper mass Limit. 

An algorithm that capitalizes on the correlation between the positions and velocities of desorbed ions has been developed to increase the mass resolution of linear TOP instruments. ° It is applicable to low- and high-mass ions and may also improve the performance of reflectron mass analyzers. 

Low mass Q1 resolution High mass Q1 resolution Ion energy 1 Entrance Collision energy Exit... 

Low mass Q2 resolution High mass Q2 resolution Ion energy 2 Multiplier (V)... 

To detect the correct product ions, selection of the mass window for a precursor ion is critical in the product-ion analysis mode. To this end, either a small mass window centered on the ion peaks should be selected or a mass spectrometer with high mass accuracy/resolution should be employed to eliminate any complications due to the overflow from the adjacent peaks. With the help of high mass accuracy/resolution possessed by a mass spectrometer, a wider mass window could be employed [45]. For the majority of the unit mass resolution instruments, one can scan only the miz of the molecule ions present in a full scan mass spectrum. 

In the high mass accuracy/resolution mass spectrometry-based shotgun lipidomics approach, the overlaps between SM and M-f1 C isotopologue of PC species, and between PC subclasses can be resolved [35]. Moreover, the linkages of alkyl vs. alkenyl at the sn-1 position of glycerol and the fatty acyl chains of diacyl species are also identified by product-ion analysis [35,36]. Clearly, this approach provides much more structural information than that of the tandem MS approach as aforementioned. 

To achieve identification of the quantified species, an approach with data-dependent product-ion analysis would be useful. However, an increased duty cycle of the instrument employed is required as the number of the analyzing lipids is increased. Alternatively, a high mass accuracy/resolution mass spectrometer would help to resolve the isobaric molecular ions from different lipid classes although isomeric species resulting from the regiospecificity and/or the double-bond location still cannot be resolved. 

Example I The low-resolution (LR) positive-ion ESI mass spectrum and the charge-deconvoluted molecular weight spectrum inset) of bovine serum albumin (BSA) as obtained from a quadrupole ion trap instrument are compared below (Fig. 12.24). In case of such high-mass ions the resolving power of a quadrupole ion trap is insufficient to separate different cationization products of equal charge state. Nonetheless, charge deconvolution reveals that ion series A belongs to the noncovalent BSA dimer, while series B results from the monomer [26]. 

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