Rapid high quality data collection for macromolecular crystallographic applications has become a priority for home laboratory environments. To meet that need, two competitive detector technologies have emerged: charge coupled device (CCD) based area X-ray detectors and multiple image plate (IP) based large area X-ray detectors. Shown is the Rigaku HighFlux HomeLab, our most popular complete home X-ray diffraction system, equipped with both detector types to enable optimal data collection for a wide range of protein crystal types.
X-ray area detectors comprise just one of the core components of a home laboratory protein crystallography system. Other critical constituents of an integrated solution include: X-ray generators, X-ray optics, cryocooling systems, data acquisition and structure solution software, robotics for crystal mounting, alignment and storage, as well as service and support.
Analysis of instrumentation used in deposited PDB structures confirms Rigaku dominant share of the home laboratory macromolecular crystallography marketplace, with an 89% X-ray generator share in 2005. Since 1996, home laboratory productivity share for Rigaku detectors has been steadily increasing. Rigaku detector share increased from 38% in 1996 to 66% in 2005, the latest year for complete data.
Two competitive detector technologies have emerged: charge coupled device (CCD) based area X-ray detectors and multiple image plate (IP) based large area X-ray detectors. While CCD detectors are often considered first for rapid data acquisition, new high-throughput imaging plate detectors now provide an alternative while preserving the unique characteristics that make IPs often desirable for weakly diffracting applications and where maximum experimental versatility is needed.
Designing, developing and internally manufacturing advanced area X-ray detectors for protein crystallography is a core endeavor at Rigaku. New products are introduced to the market on a regular basis. Recent innovations range from a triple plate IP detector for high throughput crystallography and a dual plate curved IP for high resolution macromolecular work to 3rd generation large area single CCD and 2x2 CCD detectors for applications where maximum absolute speed, coupled to exceptional performance, is essential.
Large apertures, high spatial resolution, lack of image distortion, and broad dynamic range (nearly six orders of magnitude) all provide appeal for the macromolecular crystallographer. Very low and constant background noise make IP X-ray detectors exceptionally desirable where long exposures are required. Rigaku is the only supplier of multi-plate IP detector systems, with the industry standard R-AXIS IV++, the high throughput R-AXIS HTC, and the unique high resolution curved-plate R-AXIS HR.
An imaging plate is a film-like radiation image sensor comprised of specifically designed phosphors that trap and store X-ray radiation energy. It utilizes the "photostimulated luminescence" (PSL) phenomenon. Energy from X-ray photons impinging on such a IP are stored and stable until scanned with a laser beam during the read operation. PSL light, released upon laser excitation during the readout operation, is collected through a condensing lens onto two photomultiplier tubes and subsequently digitized to afford a two dimensional (2D) electronic image.
Because of the ongoing importance of imaging plate technology (IP) for crystallographic and X-ray diffraction applications, Rigaku is the IP industry leader in both engineering research and product innovation. For example, improvements in the PSL phosphor have improved sensitivity and reduced the points spread function (PSF). Combined with higher power lasers, better optics and improved readout systems, read noise was reduced from 2-3 copper X-ray photon equivalents to less than 0.5 while the readout improved 3900%.
With a large aperture, high spatial resolution, lack of image distortion, and broad dynamic range (nearly six orders of magnitude), the R-AXIS IV++ provides a broad appeal for macromolecular crystallographic applications. For difficult applications where long exposures are required, the very low and constant background noise level provided by imaging plate technology make it a superior choice for data acquisition.
Early imaging plates suffered from relatively low throughput, however the R-AXIS IV++, with its unique 2-plate design for concurrent expose and read operations, is well suited to experimental regimes that are typically employed in a home lab environment. A flexible stainless steel belt is used as both a support and transport system. In the expose position, the steel belt and IP are held against a flat platen by vacuum, provided by an external pump, to ensure an undistorted imaging area.
R-AXIS IV++ shown with an Inverted-φ goniometer and a helium beam path as part of the Phasing HomeLab. The external shroud, shown extending from the front of the detector, is purged with He(g) to dramatically reduce X-ray intensity attenuation by air when using a chromium (Cr) wavelength X-ray source. Chromium radiation (2.29 Å) doubles the available anomalous signal from elements such as S, Ca and Se, as compared to the signal available with copper radiation (1.54 Å), and is thus valuable to investigators who wish to increase the throughput of de novo structure solution in their home laboratory.
The R-AXIS HTC represents the state of the art in imaging plate (IP) detector technology, with its unique three-plate system allowing for simultaneous expose, erase and readout operations. The duty cycle is thus limited only by the speed of the IP transport from one position to the next, making the R-AXIS HTC highly efficient for screening samples or for experiments with brief exposure times. It combines the high-throughput capability of a CCD with the large aperture size and dynamic range of an IP.
Relative to CCDs, imaging plate (IP) detectors have traditionally suffered from relatively low throughput. Rigaku's R-AXIS HTC, with its novel 3-plate design for concurrent expose, read, and erase operations, overcomes this limitation. While one plate behind the aperture is collecting data, a second plate is in the readout station being processed. And, at the same time, a third plate is positioned in an erase station, preparing for the next exposure.
A curved imaging plate detector designed for high resolution macromolecular crystallography, the Rigaku R-AXIS HR features a dual plate design for faster data collection. The cylindrical design not only allows for higher resolution data to be collected but also improves the quality of that data since reflections hit normal to the detector surface and are thus not seen as elongated reflections at the edges of the image (which is the case for all flat detectors).
Rigaku internally designs and manufacturers a line of third generation CCD detectors that are optimized for protein crystallography. All systems employ the most advanced Kodak® CCD chip with indium tin oxide (ITO) technology for superior sensitivity. Both CCD detectors use a large format sensor that is optimized for high dynamic range and superior spatial resolution. The Saturn 944+ is a single chip design with advanced ITO technology and 18-bit readout while the Saturn A200 is a high dynamic range large area 2x2 array detector.
The Rigaku Saturn 944+ single chip CCD area X-ray detector is comprised of four basic components housed inside a vacuum canister fitted with a thin polymer entrance window. Spots of diffracted X-ray beams from a sample crystal impinge on a phosphor on the front of a fiber optic taper. Light from the phosphor (in the visible spectral range) travels through the taper and is imaged onto a large format Kodak® Blue Plus™ KAF-4320E CCD sensor assembly thermally stabilized by a CryoTiger® closed-cycle Joule Thompson cryogenic cooler.
Incident photons pass through the overlaying transparent indium-tin-oxide (ITO) and/or polysilicon gate structures to subsequently generate electron-hole pairs in the CCD during the integration period. The photo-generated electrons are then collected in the potential well of each pixel. The amount of charge accumulated in each pixel varies linearly as a function of the incident illumination level and the integration period.
After photons composing the image have been collected, the CCD undergoes readout by shifting rows of image information in a parallel fashion, one row at a time, to the serial shift register (shown as a linear set of light blue elements at the bottom of the array). The serial register then sequentially shifts each row of image information to an output amplifier as a serial data stream. The entire process is repeated until all rows of image data are transferred to the output amplifier and off the chip to an analog-to-digital converter.
The Saturn 944+, a 3rd generation CCD area X-ray detector designed and manufactured by Rigaku, now offers a further four-fold increase in readout speed. Superior dynamic range and high sensitivity is achieved through the use of transparent indium-tin-oxide (ITO) CCD sensor technology from Kodak®. Combining a large format CCD with an optimal fiber optic demagnification ratio, the Rigaku Saturn 944+ is the definitive single chip CCD detector specifically optimized for the high throughput and spatial resolution needs of protein crystallography.
The Saturn A200 is a large aperture CCD detector designed for use on the beamline or for home labs that desire a larger aperture CCD detector. This 3rd generation large aperture detector has been designed and manufactured by Rigaku to provide faster readout times and better sensitivity than earlier models. It is the first 2 x 2 tiled large area CCD detector designed to have the sensitivity and speed required for accurate measurement of weaker reflections both at the synchrotron and in the home lab.
While both detector types provide excellent performance, CCDs are optimal for data collection of well diffracting crystals as well as rapid screening of crystals prior to a synchrotron trip. Image plate X-ray detectors, on the other hand, afford maximum experimental flexibility - allowing about anything that can be done with a CCD (albeit not as fast) but also the collection of very long exposures on poorly diffracting crystals.
Many labs like to have one of each type to maximize their capabilities and find having both an IP and CCD detector is central to their success.
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