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Please check out our X-Ray Diffractometer (XRD Instruments) sections for more information or to find manufacturers that sell these products
X-ray diffraction—analyzing the diffraction patterns that result when a beam of X-rays scatters off of atoms in a crystal structure—is a powerful tool for studying molecular structure. Analyzing the specific angles of the diffracted X-rays, as well as their intensities, allows scientists to ascertain the size and shape of the molecules in the crystal, the atoms within those molecules, and their spatial arrangements and bonds. X-ray diffraction is valuable in many fields, having been used to determine molecular structures for the design of therapeutic drugs, and to understand the bonds and folds in three-dimensional protein structures. X-ray diffraction is also important for the structural analysis of many types of molecules, thin film analysis, examining crystal phase and structure, and sample stress and strain.
X-ray diffraction is not a new technique, having begun over 100 years ago, yet it continues to evolve and deliver more refined information from samples prepared with less fuss and bother than ever before. Unlike the earliest crystallographers, scientists today can purchase an entire X-ray diffraction system, or perhaps use one in a research core facility. The systems, while smoothing the technique’s work flow, are not themselves simple—they are composed of many parts, each with a vital role to play. Some of these parts can vary, depending on the type of X-ray diffraction needed. An overview of X-ray diffraction systems technology can help with understanding why some components are important for particular applications.
Goniometers
To understand the importance of an X-ray diffraction system’s parts, it is useful to consider them in the context of their roles in a linear pathway through an experiment. In a typical X-ray diffraction experiment, a prepared crystallized sample is scooped up with a pin or plastic loop, and then positioned on the head of a goniometer. A goniometer is like a high-tech sample holder; it serves to hold the crystal in the beam of X-rays, but it also rotates the sample to precise degrees. This allows the crystal to be struck by X-rays in the many different orientations needed to collect enough diffraction data for quality analysis. One of the most common types of X-ray diffraction goniometers used today is known as a kappa goniometer, which characteristically consists of four circles, and provides three angles of rotation for the crystal sample. One of these angles allows the sample holder component of the goniometer to swing out, which makes it easier for the operator to mount the crystal sample.
X-ray generators
With the sample loaded into the goniometer, next we turn to the source of X-rays: the X-ray tube or generator. Because it is a vacuum tube, the X-ray tube contains a cathode that emits electrons into the vacuum, and an anode that collects them. The resultant flow of current is accelerated by a high-voltage power source connected across the cathode and anode. When the anode material—usually tungsten, molybdenum, or copper—is bombarded with electrons, a small fraction of the energy is released as X-rays, while the rest is lost as heat. Most common X-ray diffraction systems use X-ray tubes as generators.
Another type of source is synchrotron radiation, produced by accelerating electrons by magnets through a closed path, which yields X-rays with intensities that are orders of magnitude higher than conventional X-ray tube generators (the electrons travel at nearly the speed of light). However, most researchers do not have access to the large synchrotron radiation facilities required to generate these types of X-rays.
Focusing the X-ray beam
After the beam of X-rays leaves the X-ray tube, it is filtered to a single wavelength, and focused or collimated into a beam of X-rays of a single direction. This is accomplished using filters, focusing mirrors, a monochromator, and/or a collimator. A collimator can be as simple as a long metal tube, or more complex, such as in polycapillary collimating optics that are used in micro X-ray diffraction (μXRD), which is just like standard X-ray diffraction, but specialized for very small samples.
For example, in contrast to standard X-ray diffraction (with a spatial resolution ranging in diameter from hundreds of microm- eters to several millimeters), polycapillary focusing optics in μXRD focus the jumble of X-rays emitted from the X-ray generator to a small spot on the crystal (tens of micrometers in diameter), allowing analysis of smaller features. Having an X-ray beam of greater intensity concentrated down to a smaller focal spot delivers better spatial resolution than standard X-ray diffraction normally provides, and better diffraction analysis of especially small specimens. Another notable feature of μXRD is its much smaller power requirements, compared with its standard cousin.
A detector for sensing the diffracted X-rays
As the X-rays hit faces of the crystal, they scatter in distinct patterns, which are sensed by a detector in order to be recorded for subsequent analysis. There are several types of detectors used in X-ray diffraction systems. Earlier models used multiwire detectors, which are less common now. More widespread today are imaging plates, charge-coupled device (CCD) sensors, and the newer pixel array detectors.
Imaging plates provide a luminescent readout of diffraction patterns using scintillation material, but the data must be recorded and stored relatively soon, as the signal begins to decay within days. Despite this, image plates are commonly used because of their many other strengths: good linear response, high quantum efficiency, wide dynamic range, high spatial resolution, and low price tag. A potential drawback of imaging plates might be a slower response if using them with a synchrotron X-ray generator.
Another type of sensor is CCD detectors, which convert the diffraction pattern into a digital image. They also have a good linear response, as well as low noise and high sensitivity. One possible drawback is that their response can be saturated by very intense X-ray diffraction, which might entail repetitions with a range of exposures to collect the data needed.
A third and newer type of detector is the pixel array detector, especially useful for data collected using synchrotron X-ray sources. Essentially an integrated circuit coupled to a diode detector, pixel array detectors have an advantage over CCD detectors in that they are faster in cases where high-speed imaging is desired. They have a higher signal-to-noise ratio compared to imaging plates, which rely on detecting secondary photons produced by the scintillation material within the plates. Lastly, pixel array detectors boast better spatial resolution than imaging plates due to the differences in detector materials—the pixel array detector collects charge directly, while the imaging plates rely on photons first produced by X-rays striking the scintillation material.
Recent developments in X-ray diffraction techniques
As a technique, X-ray diffraction continues to evolve because its parts are regularly improved by scientists who try to achieve better results, or to accomplish something never before attempted. One example of such improvements is the relatively new, dual-wavelength X-ray diffraction systems (as opposed to the original single-wavelength systems). These systems have co-mounted sources of high-intensity X-rays, for example, from both molybdenum- and copper-based generators. Some of these systems have also been developed with higher-throughput X-ray diffraction in mind. Greater X-ray intensity allows the use of smaller or less pure crystal samples, as well as shorter exposure times, which speed throughput.
The well-documented evolution of X-ray diffraction systems over the past century provides a solid foundation on which scientists continue to improve. Watch for more recent advancements in this amazing technology to enhance the speed and quality of your own X-ray diffraction research.
For more information on X-ray diffraction systems, please visit www.labcompare.com.
Caitlin Smith is a freelance science writer who has a Ph.D. in Neuroscience from Yale University and postdoctoral work in Electrophysiology and Synaptic Plasticity; e-mail: [email protected].
Please check out our X-Ray Diffractometer (XRD Instruments) sections for more information or to find manufacturers that sell these products