Metastable fragmentation MALDI

MALDI Post-Source Decay The fast-metastable fragmentations have a decaytime constant on the order of the acceleration event and are considered a nuisance for the MALDI-TOF-MS analysis of large oligonucleotides. In contrast, the... 

From the very beginning, time-of-flight mass spectrometers have had a reputation as low-resolution instruments. The plasma desorption and matrix-assisted laser desorption-ionization instruments that are currently popular today produce mass spectra with resolutions from 300 to 800 that often appear to be dependent upon the nature or quantity of the sample or (in the case of MALDI) the laser power. Extraordinary mass resolutions of up to 1 part in 25,000 have been demonstrated using reflectrons, but have been by no means routine. The problem is that the time-of-flight axis reflects many properties of an ion in addition to its mass, including uncertainties in the time of ion formation, its initial location in the extraction field, its initial kinetic energy (before acceleration), and metastable fragmentation. 

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. 

Karas, M. Bahr, U. Strupat, K. Hillenkamp, E Tsarbopoulos, a. Pramanik, B. N. Matrix dependence of metastable fragmentation of glycoproteins in MALDI TOF mass spectrometry. Anal. Chem. 1995, 67, 675-679. 

Note In particular the MALDI-TOF community has coined some sort of an own terminology, e.g., in-source decay (ISD) for all fragmentations occurring within the ion source, post-source decay (PSD) instead of metastable ion dissociation and fragment analysis and structural TOF (FAST) for the specific mode of operation of a ReTOF to detect metastable ions. 

Figure 4. MALDI sequencing of a chymotryptic peptide fragment.The overlapping mass ranges for metastable ions are due to steps in the reflector voltage.

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