MALDI velocity

The molecular ions produced in the MALDI process have relatively high initial velocities, which can cause reduction in mass resolving power and transmission, primarily for TOF analyzers with axial ion extraction (see Section 2.2.1). Hence, the MALDI-MS mass resolving power depends strongly on laser fluence and is highest when the laser fluence is close to the threshold level. 

The mechanisms of ion formation in MALDI are a subject of continuing research. [30-34] The major concerns are the relationship between ion yield and laser flu-ence, [28,35] the temporal evolution of the desorption process and its implications upon ion formation, [36] the initial velocity of the desorbing ions, [29,37,38] and the question whether preformed ions or ions generated in the gas phase provide the major source of the ionic species detected in MALDI. [39,40]... 

Berkenkamp, S. Menzel, C. Hillenkamp, F. Dreisewerd, K. Measurements of Mean Initial Velocities of Analyte and Matrix Ions in Infrared MALDI-MS. J. Am. Soc. Mass Spectrom. 2002, 75, 209-220. 

Juhasz, P. Vestal, M.L. Martin, S.A. On the Initial Velocity of Ions Generated by MALDI and Its Effect on the Calibration of Delayed Extraction-TOF Mass Spectra. 

Karas, M. Bahr, U. Fournier, L Gltick-mann, M. Pfenninger, A. The Initial Ion Velocity As a Marker for Different Desorption-Ionization Mechanisms in MALDI. Int. J. Mass Spectrom. 2003, 226, 239-248. 

Gluckmann, M. Karas, M. The Initial Ion Velocity and Its Dependence on Matrix, Analyte and Preparation Method in Ultraviolet MALDI. J. Mass Spectrom. 1999, 34,461 All. 

Principle of an LTOF instrument tuned to analyse positive ions produced by MALDI. After their formation during a laser pulse, ions are subject to the applied electric field. Ions are continuously accelerated and drift in a free-field region. They travel through this region with a velocity that depends on their m/z ratios. Ions are thus dispersed in time. 

Capillary isoelectric focusing can be applied as a micropreparative tool for protein analysis by matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF-MS) [69,70]. The exact timing of the collector steps in the interface is based on determining the velocity of each individual zone measured between two detection points close to the end of the capillary. During the collection a sheath flow fraction collector is used to maintain the permanent electric current. 

Time lag focusing presumes that (a) ionization occurs within the source and (b) the ionization mechanism imparts upon the ions a distribution of transient energies and velocity vectors. An ion source such as that used in MALDI can greatly benefit from time lag focusing, and this technology is becoming standard on commercial MALDI/TOF instruments. 

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