Studies of nanostructures such as suspensions and nanoparticle powders aim to determine their characteristic lengths, such as interparticle distance or diameter. In SAXS investigation, these parameters are determined by fitting mathematical models to the collected data. Hence accurate data is needed to ensure meaningful interpretations can be made.
The capability of the
Xeuss 2.0 SAXS/WAXS system to reveal the nanostructure of a sample is related to its ability to achieve low detected wave vector (q min) values, without compromising angular resolution.
Demonstration of q min Values for Silica Powder Characterization
SAXS measurements using a range of slit apertures have been performed on a SiO
2 powder (Ø 150nm) to demonstrate the capability of the Xeuss 2.0 SAXS/WAXS system to achieve a low q min value combined with a high angular resolution.
The capability to obtain very low q values is demonstrated by obtaining scattering curves using very high resolution (VHR) and high resolution (HR) settings. This is seen in Figure 2. The beam is very clean at the beamstop edge, thanks to the Scatterless 2.0 technology. By shifting the beamstop as shown in the 2D pattern, ultimate q
min values down to 0.025nm -1 are achieved.
Figure 1. 2D SAXS pattern of SiO 2 powder with beamstop offcentered.
Figure 2. Scattering curves from SiO 2 powder. Influence of the collimation setting on the data quality. Exposure time = 600s.
Switching between different slit settings is done by a single instruction in the acquisition software, with no need for any further intervention. Control of the beam stop position is possible with the same ease of use.
In this case, it was possible to observe the particle signature up to around six oscillations, and related local minima are emphasized using the VHR setting. One can thus choose between higher peak definitions with the VHR setting to improve data analysis, while samples screening can be performed using the HR setting.
At a sample-to-detector distance equal to 2.5m, over 14 experimental data points (no over sampling) between two local maxima of the scattering curve are displayed, as shown on Figure 3. The associated pixel resolution is equal to Δq = 0.003 nm
-1. Hence the system can study particles up to 250 nm and beyond.
Figure 3. Inset in low q_region of the scattering curve for VHR setting
Comparison of Scatterless Slit 2.0 with Standard Germanium Pinhole
Figure 4. Measurement set-up
At BESSY II, tests were done at the PTB four-crystal monochromator beamline, in Berlin, Germany, using 8keV radiation. The tested apertures were aligned to the primary beam to enable a maximum photon flux. Sequential comparative measurements were done between the commercially available scatterfree germanium pinhole and Scatterless slits 2.0.
The beam size was systematically varied with the S1 upstream slits as shown in Figure 4 for evaluating the impact of photon flux on cleaning capabilities. In order to obtain a good comparison the size of Scatterless slits 2.0 was set to 0.9mm to obtain the same downstream photon flux as with the commercially available Ge-pinhole of 1mm.
Figure 5 shows the comparative measurements (Data courtesy of Dr Michael Krumrey at PTB Berlin, with a four-crystal monochromator Beamline and 8 keV radiation). These two-dimensional patterns show that the Scatterless slits 2.0 performance is more than that of commercially available Ge-pinhole, regardless of the beam opening value.
Figure 6 compares the single dimensional scattering curves obtained from 2D patterns. In the measured q_range [0.04-0.4] nm
-1, scattering from the Scatterless slits 2.0 remains lesser than that of the scatterfree Ge-pinhole. At low q value (0.04 nm -1) there is a ratio up to 10 favoring the Scatterless 2.0 slits.
Figure 5. 2D scattering patterns obtained with various upstream slits aperture (S1). 60 s exposure time.
Figure 6. Scattering curves obtained with S1 = 0.8 mm. 60 s exposure time.
Demonstration of High Signal-to-Noise Ratio
There is a need for a high signal-to-noise ratio (I/σ) at every q value in order to investigate weak scattering systems such as diluted proteins or surfactant solutions. Combining Scatterless slits 2.0 technology with a low-noise camera in new generation
Xeuss 2.0 SAXS/WAXS system ensures high data quality collection and accurate analysis.
As shown in Figure 7, “empty camera”, 10min measurement shows the ultra-low background level of the Xeuss 2.0. Figure 8 shows the following data in absolute intensity scale (mm
Xeuss 2.0 typical beam profile as determined on the detector
A typical scattering curve of a protein solution (10 mg/ml subF) not subtracted from buffer
Level of intensity of water
Level of intensity of a typical polymer
Figure 7. Zoom on the beamstop region at lowest q m No sample data. Exposure time 10 min.
Figure 8. Rebuilt 1D scattering curves from Xeuss camera beam profile and typical samples
The beam profile measurement shows the ability of the Xeuss 2.0 to define a clean beam. A clean beam implies that there is a very low level of parasitic scattering propagated in the q-space meaning that the background signal remains low even at small q-values.
The noise level is observed to be lower than the water scattering intensity allowing reliable water measurement. The diluted protein’s scattering curve shows a higher scattering than the noise signal, especially at high q values, demonstrating the ability to perform faithful buffer subtraction on the collected data.
Moreover, considering the incident beam of the Xeuss 2.0 SAXS/WAXS system, the signal-to-noise ratio is higher than 4x10
9. This high level of signal-to-noise ratio capacity results from the integration of the Low Noise technologies developed by Xenocs in the Xeuss 2.0 SAXS/WAXS system, such as the Scatterless slits 2.0. Using a Dectris detector helps in taking full advantage of these features.
In applications such as polymer materials and colloids, high quality SAXS investigation of much larger nanostructures with characteristic lengths of 500nm is of interest. It requires a q
min value down to 0.01nm -1 or below. High quality data analysis can only be performed with an associated nominal resolution equal to Δq = 0.001 nm -1. This ensures data with a sufficient number of data points and enables accurate model fitting procedure. The USAXS version of the Xeuss 2.0 SAXS/WAXS system provides such features.
Application fields like characterization of diluted systems, ie BioSAXS, require high signal-to-noise ratio (I/σ) at every q values, to ensure improved buffer subtraction for accurate data analysis. Similarly, investigation of large nanostructures with weak scattering power involves high resolution collimation which affects useful flux on sample. For such challenging applications, high I/σ at low q
min is very essential, and increasing such factor offers a direct improvement in data quality.
Using low scattering slits contributes to improving the signal-to-noise ratio. The Scatterless slits 2.0 are fully integrated in the new generation
Xeuss 2.0 SAXS/WAXS system. Further to improving performance, the Scatterless 2.0 slits enable one to change the SAXS/WAXS system resolution by automatic recall of slits settings through a single software push button action without further intervention.
The Xeuss 2.0 SAXS/WAXS system also displays a high signal-to-noise ratio that enables SAXS measurements of proteins. Measurements performed on protein sub-F demonstrates that reliable data is obtained compared to synchrotron results and enables the protein structure resolution.
This information has been sourced, reviewed and adapted from materials provided by Xenocs.
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