Multiply charged analytes

A characteristic feature of ESMS is the detection of multiply charged analytes. Macromolecules, such as proteins have multiple sites where protonation or deprotonation (the two most common charge inducing mechanisms in electrospray—other routes to charge induction include, ionization through adduct formation, through gas-phase reactions, and through electrochemical oxidation or reduction) occur. These are desorbed effectively in ESMS and... 

Electrostatic binding [11] may provide another very useful approach to preconcentration analysis. Enhancement of the redox ion concentration in the ion-exchange polymer volume should permit very sensitive analysis when combined with an appropriate electroanalyti-cal method [12,13]. However, the sensitivity of the ion-exchange equilibrium to the sample solution electrolyte composition and concentration and the necessity of having a multiply charged analyte ion may limit the usefulness of the electrostatic binding approach. 

Acquire mass spectra over the m/z range 200-1000. This allows observation of multiply charged analytes and multiple conjugates. 

ThG SGparation Process. Once the anal3d is in the gas phase as an ion, analytes must be separated on the basis of mass to charge (m/z). A variety of separation techniques are available. The most common form of ms separation is the quadrupole mass filter (3,4). Because this method is limited to m/z less than 3000 or so, it is not generally used with MALDI. However, a quadrupole is commonly used with esi where multiply charged analytes allow one to examine high masses for m/z less than 3000. 

Figure 5.3. Sketch of the major processes proposed in cluster models of MALDI ionization. A, analyte m, matrix R, generic counterion. Preformed ions, separated in the preparation solution, are contained in clusters ablated from the initial solid material. Some clusters contain a net excess of positive charge, others net negative (not shown). If analyte is already charged, here by protonation, cluster evaporation may free the ion. In other clusters, charge may need to migrate from its initial location (e.g., on matrix) to the more favorable location on analyte (secondary reaction). For multiply charged analytes, hard and soft desolvation processes may lead to different free ions. Neutralization by electrons or counterions takes place to some degree but is not complete.

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