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A New Tool for Small Crystals - FOX2D CU 25_25P Symmetrical Focussing Optics

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News

A New Tool for Small Crystals - FOX2D CU 25_25P Symmetrical Focussing Optics

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As an extension of the existing FOX 2D product range this new optics enables small samples analysis in various applications such as protein crystallography.

The analysis of small crystals on home lab machines remains an important challenge and has pushed Xenocs to develop an optics designed and adapted to boost the performance of common x-ray generators.

An important feature of such a system is a maximum flux density in the focal spot together with a low background level.

According to Xenocs, the higher source size acceptance of the mirror, the higher efficiency of the single-reflection concept, and the 1:1 imaging of the source, enables this new optics to yield a significant increase in flux density when compared to double reflection optics.

A full dataset using the FOX2D CU 25_25P mirrors was collected at MRC Cambridge by Dr Andrew Leslie from a small cryo-cooled crystal of IP3-3 kinase, supplied by Dr Beatriz Gonzalez-Perez.

These mirrors were then exchanged for traditional double reflection multilayer mirrors (side-by-side geometry) and a second dataset was collected from the same crystal under identical experimental conditions.

The generator was an MSC/Rigaku RuH3R running at 50kV, 100mA (300 µm focus) and the data were collected on a Mar345 image plate detector.

The crystal was a thin plate with approximate dimensions 200x75x50 µm. The crystal belongs to space group C222 with cell dimensions a=72Å, b=97Å, c=191Å. The merging statistics for the two datasets are listed below.

Side-by-side
Xenocs

FOX2D CU 25_25P
Exposure time per frame
4 min
4 min
Rmerge (22.7.-2.43A)
8.8%
6.4%
Rmerge (2.57-2.43A)
44.1%
26.2%
/ (22.7.-2.43A)
12.1
15
/ (2.57-2.43A)
2.5
4.1
Mean multiplicity
3.3
3.3
In both cases the standard deviation estimates from MOSFLM were adjusted in SCALA to reflect the true discrepancies between symmetry related reflections.

The results for the highest resolution shell show an improvement in signal to noise by a factor of 1.64. This is consistent with the average scale factor of 2.43 between the two datasets, which provides a good estimate of the relative X-ray flux hitting the crystal.

If the data quality is determined solely by counting statistics, an increase in flux by a factor of 2.43 would give a factor of sqrt(2.43) = 1.56 in the signal to noise. Use of the Xenocs mirrors provides a clear improvement in data quality for this relatively small crystal.

To get more information please have a look at the complete application note.
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