Important enhancements to MALDI-TOF experiments are provided by additional instrumentation. Examples for improving instrumentation are the use of delayed extraction methods and reflectron devices. The principle of delayed extraction is to have a small time window between the ionization pulse and the extraction pulse. During this time window, ions are allowed to disperse according to their initial velocities, and neutral (non-ionized) molecules that are generated during desorption are pumped away. Therefore ion-molecule collisions are minimized, which otherwise would contribute to a spread of translational energies. The application of delayed extraction1-4-1 leads to significant increases in mass resolution (e.g., at 3000 Da from about 5000 without DE to over 10,000 with DE) and accuracy (from about 100 ppm to about 10 ppm). Another important perform ance enhancement comes with the application of reflectron devices. A reflectron essentially is an electrostatic ion mirror. It consists of a series of electrical lenses with increasing repelling potential. The reflectron corrects for initial energy and positional dispersions of isomass ions. Faster-moving ions spend less time in the drift region but enter to a greater depth into the reflecting field. The extra time spent in the field compensates for their shorter flight time in the drift regions, with all isomass ions arriving simultaneously at the detector. The performance enhancement compared to nonreflectron instruments (called linear TOF instruments) is significant, especially for molecules with masses greater than 10 kDa. An excellent overview on the underlying principles of MALDI-TOF and the effect of reflectron devices is provided by Guilhaus.
An important technique used to conduct structural studies with MALDI-TOF MS is the so-called postsource decay (PSD). The MALDI process results in different classes of desorbed particles, ions, neutral particles, and metastable particles that undergo decay during their drift toward the detector. The decay is initiated by low-energy collisions of neutral matrix molecules with analyte ions. To analyze the PSD products, precursor ions are preselected (based on mass) via an ion gating device. Products and precursors are resolved through the use of single-staged or two-staged reflectrons. Some applications for the PSD approach are described in ''Applications in Proteomics.''
Was this article helpful?
The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.