Lucky survivor model

One of the strengths of the lucky-survivor model is that it can be equally well appUed to account for the formation of negative ions (if deprotonable groups are present) and positive ions from basic analytes such as peptides and proteins (in solution precharged by protonation Eqs 1.9 and 1.10), as well as for positive (if protonable groups are present) and negative ions from acidic analytes (in solution precharged by deprotonation Eqs 1.11 and 1.12) such as nucleic acids ... 

Fig. 11.7. Pathways of formation of singly charged ions from sample-matrix preparations according to the lucky survivor model, (a) Positive ion formation, (b) negative ion formation M analyte molecule, T matrix molecule. Cat small cation. Me metal, B base. Adapted from Ref. [42] with permission. John Wiley Sons., Ltd., 2000.

In contrast, no such difference in basicity between the negatively charged ions of matrix molecules and peptides exists, because both are typically carboxylate anions with very close proton affinities. Peptide anion generation competes with simple dissociation of the neutral analyte-matrix anion adduct and limits the yield of analyte anions (see Eq. 1.8 for the CPCD model and Eq. 1.10 for the lucky survivor ). 

Both models - protonation of neutral analytes in the gas phase according to the CPCD model as well as the revised lucky survivor - are discussed in detail in Refs [3, 24, 65]. 

The ionization typically proceeds in two steps. In the first step (primary ion formation), the matrix absorbs the laser energy. Together with intact macromolecules, the formed matrix ions desorb into the gas phase. This process is very fast and happens in a few nanoseconds. A dense plume is formed in which the second step, the charge transfer from the matrix ions to the maaomolecules, occurs. This is mostly done by a gas phase cation (H, Na, K ) transfer. A quantitative two-step rate equation model of the ionization process was developed by Knochenmuss. This approach was extended by introducing a quantitative molecular dynamics model. According to Karas et al.. ..single charged ions are the lucky survivors.... These ions are accelerated in an electric field of several kilovolts and introduced into the mass analyzer. 

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