Space-focus plane

There is a point along the flight tube, called the space focus plane , where the spatial distribution is minimized for ions of any given m/z. If the space focus plane is not at the detector then, after travelling through the space focus plane, ions with the same m/z will have different flight times to the detector (Fig. 21B). 

The space focus plane is essentially due to two opposing factors affecting the spatially distributed ions during extraction and how this spatial distribution affects the ions arrival time. Firstly, ions in the source that are more distant from the detector have further to travel to the detector, and secondly, ions formed more distant from the detector experience a larger extraction potential and therefore reach a higher velocity in the drift tube. 

Steering Field Primary Space Focus Plane Field-free Region... 

For efficient removal, these plates should ideally be placed at the point at which ions attain their tightest temporal and spatial focus prior to detection. Although deflection of this type would be challenging with a swept-beam geometry, the space focus plane of the two-stage acceleration design offers an excellent opportunity to deflect the maximum number of ions over the shortest distance in the shortest time. This placement also ensures the most resolution of mass deflection. For an ICP-TOF-MS, the efficiency of removal for this parallel-plate scheme was promising [24]. 

We noted above that the space focus plane for a single extraction source is just twice the mean distance s in the source, or d 2s. For a two-stage extraction... 

When ions are formed by gas-phase methods, tailing of the mass-spectral peak is likely to be more symmetric. There is, however, a focal point, known as the space-focus plane, in the drift region at which faster ions formed toward the rear of the source catch up with slower ions formed near the front of the source. This point is independent of mass (although ions of different mass arrive there at different times) and is located at a distance 2s, where s is the full depth of the source region for desorption methods, or the distance from the extraction grid to the center of the ionization zone for gas-phase ionization methods. 

As we shall see below and in the next chapter, there are two ways to correct for the loss of resolution due to the spatial distribution of ion formation. First, it is possible to devise a two-stage extraction system which pushes the space-focus plane to the entrance of the detector. The second approach considers the space-focus plane to be a virtual source with ions differing only in kinetic energy. These are then focused using a reflectron. 

The Space-Focus Plane. The early time-of-flight mass spectrometers utilized electron impact (El) to ionize volatile molecules in the gas phase. Ionization in the gas phase results in considerable uncertainties in the initial position of the ions in the extraction field that are less problematic in the desorption (PDMS and MALDI) instruments in use today. Sdll, it is useful to consider this problem (and the means by which it is corrected), since spatial distributions do exist when ions are desorbed from irregular or insulating surfaces. In addition, it is likely that compact EI-TOF instruments will continue to be developed (and commercialized) for environmental monitoring or as gas-chromatographic detectors. 

One can demonstrate that ions formed at Sg and Sg + Asg reach d at the same tirtie by showing that the difference in time spent in the source region is compensated for by the difference in time spent to reach the space-focus plane, i.e., that At, - -At. The difference in time spent in the source region is ... 

Similarly, the difference in time spent to reach the space-focus plane is ... 

Dual-stage Extraction and Space Focusing. As we noted above, the location of the space-focus plane is independent of mass, while ions of different mass are focused at the space-focus plane at different times. However, mass dispersion at this short distance from the source is usually not sufficient to allow the location of a detector at this point. Thus, one solution is to move the location of the space-focus plane to... 

If the second extraction field ( ) is considerably larger than the first (Eq), then an ion will spend a very short time in that region compared with the time spent in the source and drift regions. With this assumption one can obtain a rough estimate of the location of the space-focus plane that can be useful in determining the overall dimensions of the instrument. 

Thus, A% = when 4 = and the space-focus plane can be found... 

In particular, when = r, the space-focus plane is found (very approximately) ... 

I pnc recalls the equation that was used to determine the space-focus plane for a duai- istage extraction system ... 

Comparison of Reflectrons Used for Post-Source Decay. Figure 8.27 shows the voltage profiles for time-of-flight mass spectrometers equipped with several different kinds of reflectrons. In all cases, ions begin their flight from an ion source held at the accelerating voltage V, and the fiscal point for the reflectron analyzers is the space-focus plane at a distance equal to 2X the width of the source. Ly and Zj... 

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