Velocity distribution, MALDI ions

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. 

The technology of MALDI-MS has been considerably improved in recent years. One factor that has led to much better resolution is the addition of a reflectron. This is an ion mirror with the electrode voltage arranged in such a way that it largely corrects for the initial velocity distribution of a packet of particular ion species. The correction increases the mass resolution, which is especially useful in case a mixture of analytes is assayed. 

The important instrumental development of delayed (ion) extraction (DE) or time-lag focusing (TLF) [279] has had a great impact on MALDI development [280,281]. The major limitation to the resolution provided by MALDI-MS is the initial velocity distribution of the ions. Ions with the same mass/charge ratio but different energy distributions yield a broad peak with a decrease in resolution. Correction for this by the use of pulsed extraction for time-lag focusing [282] has greatly improved the quality of the mass spectra from low masses (<100 Da) up to high masses. The term time-lag... 

Today, the modern version of time-lag focusing, or delayed extraction,14 16 is used in almost all MALDI-TOF mass spectrometers. It is in fact a special case of the Wiley and McLaren method, in which the initial spatial distribution (usually a very thin sample and matrix mixture dried on a stainless steel plate) is assumed to be zero. Therefore, after the delay time the velocity and spatial distributions are correlated, i.e. ions of highest velocity have moved the greatest distance. This space velocity correlated focusing has been described by Colby and Riley,17 and provides considerably better focusing than can be obtained from two independent and uncorrelated initial distributions in space and velocity/energy that comprise the general Wiley-McLaren case. 

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