Vol.12 - Winter Issue 2011
High performance SAXS / WAXS instrument with GeniX low divergence
A GeniX (50W, 50 µm) equipped with a FOX 3D 12_INF optic was installed in fall 2010 in Tel-Aviv University in the experimental biophysics laboratory of Dr Roy Beck-Barkai at the School of Physics and Astronomy, faculty of exact sciences on a small and wide angle x-ray scattering (SAXS / WAXS) laboratory beamline.
Dr Roy Beck-Barkai principal research interests are in the field of nanostrutured self assembled polymers and biopolymers, biophysics of supramolecular complexes, statistical mechanics and hydrodynamics of polymers, protein-protein and lipid-protein interactions. Among other techniques, SAXS and WAXS is the cornerstone of characterization of such systems, providing a direct insight of molecular organization and interactions, from the atomic level up to 100 nanometers or more. Using a SAXS/WAXS setup allow Dr. Beck-Barkai's team to directly probe the macromolecular forces and interactions with extended characteristics length-scales at hydrated condition.
In this SAXS / WAXS beamline (figure 1), the GeniX low divergence system is coupled to two sets of scatterless slits collimation and two detectors are used to cover a wide range of measuring methods: a large Image Plate detector – MAR345 (Mar Research) is used for WAXS measurements and a high performance Pilatus 300K detector (Dectris) for SAXS measurements.
Figure 1: View of the motorized laboratory setup at Dr. Roy Beck-Barkai laboratory, Nanocenter and Nanotechnology building, Tel Aviv University, Israel. Complete long SAXS beamline is captured as seen from detector side. Top: from left to right, GeniX beam delivery system, including the FOX3D 12-INF optic, the scatterless slit #1, vacuum tube. Bottom: from left to right: the scatterless slit #2, the sample area, the WAXS detector MAR 345 Image Plate, the evacuated beam path, the motorized beam stop chamber in vacuum and reinforced exit window, the Pilatus 300K detector on a X-Z motorized platform.
Combination of scatterless collimation and high brilliance microsource
The quality of the GeniX low divergence arises from the combination of high brilliance microfocus source and single reflection aspheric multilayer optics to provide a highly parallel beam with high photon density. Despite using low power, its performance in SAXS is several times higher than standard sealed tube systems or traditional high power rotating anode generators due to very low divergence of the resulting beam.
The GeniX is coupled to a collimator made of two scatterless slits and a telescopic tube adapted to change the beam properties in a rapid and stable manner. The use of scatterless slits technology for beam collimation is a unique advantage in terms of brilliance preservation as a simplified collimation with only two slits is possible for high resolution SAXS . The low scattering signal from the slits is also a benefit in terms of ease of alignment as no clean up adjustment is needed after changing beam size. The highly parallel and stable beam provided by the GeniX is also a key feature when reconfiguring the collimator length and slits aperture for different flux/resolution settings.
Depending on collimator length and slit size, typical flux range is 2.5x107- 5x107 photons/sec with 0.8mmx0.8mm beam size.
Figure 2: Typical Silver Behenate SAXS pattern recorded with a Pilatus 300K detector placed at 2.5m from sample after 5 minutes exposure. a/ 2D scattering pattern. b/ Fully azimuthal average of 2D pattern. Q_min better than 4.4x10-2 nm-1 (0.0044 Å-1) enables probing length scales up to 150nm. Delta Q_detector resolution approx. 2.8x10-3 nm-1. Data courtesy of Dr. Roy Beck-Barkai, Tel-Aviv University, Israel.
Modular design for a large range of applications
The high quality of microfocus source is well demonstrated with the q_min = 0.044nm-1 (0.0044Å-1) obtained with a long sample-to-detector distance of 2452mm as observed on figure 2.
The SAXS / WAXS system is quite modular and use of shorter diffracted beampath offers intermediate resolution (sample to detector distance of 1m) with a high flux configuration using the high sensitivity 300K Pilatus detector. Such configuration is illustrated in Figure 3 with a 30 minutes measurement in a low scattering contrast sample (a lipid multi-lamellar vesicle sample).
Figure 3: Intermediate SAXS (~1m sample to detector distance) pattern of a DPPC 30mg/ml sample recorded for 30min with a Pilatus 300K detector. a/ 2D scattering pattern. b/ Fully azimuthal average of 2D pattern of the sample and of corresponding buffer subtracted data. Data courtesy of Dr. Roy Beck-Barkai, Tel-Aviv University, Israel.
The second important asset lies in the combined usage of two detectors
To simplify WAXS studies and avoiding displacing the SAXS detector, a second large area image plate detector (MAR345) is positioned close to the sample (27cm). This is greatly facilitated with the true parallel beam from GeniX low divergence and scatterless collimator, with low parasitic level: overlap with SAXS data is straightforward. Beam being stable in space, swap of detector is made in a snap, dedicated WAXS beamstop being already aligned from previous WAXS session and with only extremely limited mechanical maneuvers. For instance, a large detector as such as the Mar345, sligthly offset and positioned at approximately 27cm offers q_range [0.04, 3.00]Å-1 or 2-theta [0.5, 43.0]°, as seen on figure 4 for WAXS data on Ag Behenate sample.
Figure 4: a/ Typical Silver Behenate WAXS pattern collected with 2nd detector( MAR345 from Marresearch) placed at approx. 0.27m from sample. Exposure time is 10min. Total air path between the sample and the MAR345 detector is in air. The point in center of 2D image is a detector artifact. b/ 1D azimuthal averaging of data. 1st order of Ag Behenate at characteristic repeat distance of 5.87nm is clearly detected. Data courtesy of Dr. Roy Beck-Barkai, Tel-Aviv University, Israel.
Moreover, the setup also provides combined usage of the two detectors aligned together to study WAXS and SAXS simultaneously. The setup can be aligned together in the long distance SAXS position (~2.5m) combined with WAXS (27cm) or the intermediate SAXS position (~1.2m) combined with WAXS (27cm).This increases the q range of the WAXS and gives the advantage of simultaneous measurements which reduces exposure time. The combo setup for the intermediate SAXS setup offers q range up to 0.4Å-1 for the SAXS and 1.1-4Å-1 for WAXS (figure 5).
Figure 5. SAXS / WAXS combo measurement. 1D azimuthal averaging for the 2D combo SAXS and WAXS images for DLPE (30mg/ml) in HEPES buffer solution. SAXS q range up to 0.35Å-1 and for the WAXS 1-4Å-1.Data courtesy of Dr. Roy Beck-Barkai, Tel-Aviv University, Israel.
Rona Shaharabani and Guy Jacoby are greatly acknowledged for their deep implication in the experimental work.
i Youli Li et al , J. Applied Crystallography (2008) 41, 1134 - 1139