Post-source fragmentation, peptide ions

We have implemented scanning methodologies using MALDI-TOF mass spectrometry to partially purified venom from C. striatus and C. ermineus. We have carried out specific derivatizations in order to deduce composition and sequence information. Together with an intact mass these measurements are used to determine whether an ionized species observed in the MALDI mass spectrum corresponds with the intact protonated molecule of a previously characterized conotoxin. The information obtained from derivatizations is also important when the ionized species does not correspond with the intact mass of peptides of known sequence. In that case, post source decay of the native and derivatized species may help assign the fragment ions. 

A certain amonnt of structural information, snch as sequencing of peptides, can be obtained from single-analyzer MALDl-TOF systems used in the reflectron mode becanse some ions leaving the ion source are metastable (Section 3.3.1.2) and fragment as they move down the flight tube the process is called post-source decay (PSD). The technique is limited because it is not possible to preselect ions for fragmentation and becanse stepwise changes in the parameters of the reflectron are needed to obtain prodnct ion spectra. 

In-Source Decay (ISD). In the next chapter we will describe a method for amino acid sequencing of peptides on reflectron TOF instruments known as post-source decay (PSD). This method exploits the considerable metastable fragmentation that occurs in the drift region after the ions are extracted from the ion source. The resulting product ions spend a shorter time in the reflectron, and therefore arrive at the detector sooner than their precursors. 

However, compared to ESI, MALDI has always been limited by the fact that desorbed ions become vibrationaUy excited on desorption (i.e., they are hot ). This metastable activation is often sufficient to cause them to fragment, often on a > 1 -ps timescale. In 1994, Spengler et al., first took advantage of this metastable activation to sequence peptides and coined the two terms post-source decay and in-source decay . The differentiation between fragmentation post-source and in-source is instrument-dependent because the two terms simply refers to metastable fragmentation that occurs after the ion leaves the MALDI source and enters the mass analyzer or before the ion extraction voltage pulse is applied. 

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