Space-velocity correlated focusing

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. 

Figure 13.8 Schematic of (a) space focusing on a dual-stage source and (b) space-velocity correlated focusing from a dual-stage source after delayed extraction.

Colby, S.M. and Reilly, J.P. (1996) Space-velocity correlation focusing. Anal. Chem., 68,1419-1428. 

King, T.B., Colby, S.M., and Reilly, J.P. (1995) High- resolution MALDI-TOF mass-spectra of 3 proteins obtained using space-velocity correlation focusing. Int. J. Mass Spectrom. Ion Processes, 145, L1-L7. 

Figure 10.7. (a) Positions of ions following delayed extraction showing normal mass dependence of space-velocity-correlated focusing, (b) Positions of ions after correction using mass-correlated acceleration. 

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