Drift region fragmentation

Reflectron time-of-flight mass spectrometers can be used to generate fragment spectra. Before the introduction of delayed extraction, it was observed that the ions had a tendency to disassemble into fragments while they were traveling to the reflectron (Spengler et al., 1992). This behavior is called postsource decay (PSD). The ions collide with matrix ions in the gas phase when they are accelerated with several kilovolts. Their decay occurs in the first field-free drift region. 

The factor in curly brackets is the normalised probability that A is produced between t and t -I-dt from A,.,., with energy . The second exponential gives the probability that A does not decay in the interval tj — f. The relevant time f, of this experiment is the time the cluster passes through plate PI in fig. 2. Note that it is not the time to pass through P2, as all ions which decay between PI and P2 are not focused onto one mass peak, but contribute to a broad background. The probability F that A fragments into A , -E A in the field-free drift region (and that Af, can be focused on one peak by the TOP spectrometer) is... 

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. 

Mass Dispersion of Product Ions in a Single-Stage Reflectron. If metastable fragmentation occurs in the first field-free drift region (Zq) of a single-stage reflectron instrument ... 

Product ions (wj,) have the same velocity as their precursor ions in the first drift region (Zq) where fragmentation occurs. Because the reflectron returns ions to the second drift region (Z ) with the same kinetic energy, product ions will also have this same velocity in the second drift region. Thus, the time that product ions spend in the linear regions of the mass spectrometer will be the same as their precursors ... 

Embryos accrete most of their mass from a feeding zone centered on their orbit, with a full width 10 Hill radii - roughly 0.01 AU for a 10-3M body at 1 AU. Because embryos mostly accrete local material, objects in different regions of the disk are likely to have different compositions. However, the inward drift of planetesimals and collision fragments means that some radial mixing occurs at this stage. 

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