Space focus

The currently perhaps most popular approach to cell-based partitioning in low-dimensional chemical spaces focuses on the so-called the BCUT metric... 

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]. 

BD Gardner, JF Holland. Nonlinear ion acceleration for improved space focusing in time-of-flight mass spectrometry. J Am Soc Mass Spectrom 10 1067—1073,1999. 

E j Single stage, first order space focusing... 

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... 

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.

A Journey through Chemical Space - Focused Libraries and Scaffold Hopping... 

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