Wiley-McLaren condition

The TOE under Wiley-McLaren conditions and the Doppler techniques are similar in that the initial velocity produces a linear response in the measured quantity (T — T and Vobs o> respectively). Likewise the energy analysis of product ions produces a signal... 

When reaction products are formed in a finite region, rather than a point source, it is best to extract the ions with a combination of acceleration and drift regions which are consistent with the Wiley-McLaren space focusing condition (Wiley and McLaren, 1955). Specific ratios of acceleration and drift distances for given electric fields are imposed so that the ion TOP is independent of the precise initial position of ionization. This is easily explained as follows. Suppose that two ions (with initially zero kinetic energy) are formed at positions Uj + 8 and tij — 8 as measured from the second acceleration, or drift region. Clearly the ion closer to the drift region will reach this... 

Figure 5.25 Ion TOP distributions for isotropic angular distribution of dissociation products. The mass of the fragment ion is 30 amu, the electric fields in the first two acceleration regions are 10 and 100 V/cm and the acceleration distances are 2 and 1 cm, respectively. The drift distances are indicated in the figure. The d = 55 cm corresponds to the Wiley-McLaren space focusing condition. For each drift distance, the ion TOF was calculated for three product translational energies of 0, 0.5, and 1.0 eV.

The initial velocity spread is converted to a spatial spread by allowing a delay between ionization and acceleration. Ions separate to create a correlation between position and velocity. Spatial focusing can be applied to correct this. This method was also developed by Wiley and McLaren and the conditions (length of delay) must be adjusted for each miz. Time-lag focusing does not work well when a significant spatial and velocity spread are present simultaneously. As will be discussed later, time-lag focusing has recently been adapted to greatly increase the resolution in MALDI TOF-MS. 

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