What is XRPD - X-Ray Powder Diffraction?

At DANNALAB, we use XRPD to identify the properties of pharmaceuticals. Sometimes we are confronted with the question: what is XRPD analysis? The answer "X-Ray Powder Diffraction analysis" is not very illuminating. We will answer this question here.

Terminology

XRPD or X-Ray Powder Diffraction is a method for measuring the X-Rays scattered by a polycrystalline sample as a function of the scattering angle. Analysis of this distribution gives a lot of information about the microstructure and properties of the sample.

In practice, the term XRPD is often substituted by XRD - "X-Ray Diffraction". Without the "P" the acronym XRD is a much broader term describing all fields of X-Ray diffraction, such as monocrystal diffraction, fiber X-Ray diffraction, the aforementioned X-Ray powder diffraction, X-Ray diffraction on epitaxial layers so on. These different techniques are distinguishable by differences in geometry, instrumentation, mathematical treatment of the data and the types of samples analysed.

An XRPD sample is a "polycrystalline" sample consisting of many small randomly oriented crystallites (like on the above banner). This makes it different from a sample used, for example, in monocrystal X-Ray diffraction. Generally polycrystalline samples exist in different forms: solid form (metals, ceramics), as a loose powder, in the form of a film or in the form of a liquid suspension. If you are looking for information related to the analysis of samples with a polycrystalline nature, it is advisable to use the term XRPD (for example, during a web search).

Note: If the sample is in a solid form, it has a surface. There are specific methods for the characterization of polycrystalline materials like this. These are texture analyses and residual stress analyses. Both methods use the intensity of X-Ray diffraction to obtain information about the anisotropy of crystallite orientation and the microstructure relative to the surface. These methods will not be mentioned below, though keep in mind this difference of what is called X-Ray Powder Diffraction traditionally.

Major questions answerable with XRPD

XRPD gives the information about the microstructure of a material of interest. This information is linked to the physical properties of said material. Below is a list of common questions that can be answered with the use of XRPD:

Questions not answerable by XRPD

Methods and applications

• Crystallography and qualitative phase analysis: The indexing and the determination of a crystallographic unit cell. The 6 parameters of the crystallographic unit cell; a, b, c, alpha, beta and gamma, may be determined from the analysis of the XRPD peak positions. There are several databases available with peak positions and full patterns that can be used to identify different substances.
• Quantitative phase analysis: These are methods to determine the concentration of different crystalline phases (or an amorphous phase) in a mixture. These methods are often based on building calibration a curve and performing linear regression, or on full-pattern analysis (figure 1).

Figure 1. XRPD patterns with different amounts of amorphous phase. This can be used to quantify the amount of an amorphous phase in an unknown sample.
• Atomic structure refinement and ab-initio structure determination: "Rietveld analysis" allows for the refinenement of the actual atomic positions starting from a known atomic model. The refinement is based on criteria for the best fit between experimental and simulated patterns (figure 2). There are novel methods (for example "Charge flipping") that also enable "ab-initio" structure determination.

Figure 2. Example of Rietveld analysis of D-Mannitol. The measured pattern (blue) is overlayed with the atomic model-based pattern (red). The atomic structure model is refined to achieve the best fit between the two of them.
• Determination of crystallite size and micro-deformations: "Line Profile Analysis". Nowadays this is often combined with "Rietveld analysis".
• Microdiffraction: the above methods conducted with a very small X-Ray beam to obtain information about a specific spot on a sample.

Instrumentation and geometry

XRPD experiments are conducted using an X-Ray powder diffractometer (figure 3).
A laboratory-based X-Ray powder diffractometer consists of the following component:

• X-Ray source (X-Ray tube).
• X-Ray generator delivering a high voltage to the X-Ray source.
• Holder to carry the sample to be investigated.
• Detector to measure X-Rays scattered by the sample.
• Incident beam- and diffracted beam- conditioners. These may consist of apertures, parallel-plate apertures, crystal-monochromators, multilayer mirrors and capillary optics.
• Goniometer providing precise angular positioning of the X-Ray source and detector, relative to the sample.

The XRPD pattern is obtained by recording the intensity of X-Rays as a function of the diffraction angle.

Geometries of X-Ray Powder Diffraction experiments

There are several possible geometries for X-Ray Powder Diffraction experiments. The most commonly used geometries in X-Ray powder diffraction are "line-focus geometries", which are listed below:

• "Bragg-Brentano focusing scheme in reflection mode" (figure 4.1). A divergent incident beam in the equatorial plane illuminates a flat sample in reflection mode. Scattered radiation is registered by a relatively small detector with pixels arranged one-dimensionally. The sample surface is oriented along the focusing circle and should be ideally flat. The incident beam may be conditioned by a primary monochromator or an X-Ray mirror.
• "Bragg-Brentano focusing scheme in transmission mode" (figure 4.3). A divergent incident beam is turned into a convergent beam by a monochromator or an X-Ray mirror. The convergent beam that is focused onto the detector illuminates a flat sample in transmission mode. The scattered radiation is registered by a relatively small detector with pixels arranged one-dimensionally. The sample should ideally be flat, but this is not as critical as in the prior case.
The advantages of Bragg-Brentano focusing schemes are the high resolution and the compatibility with small linear detectors.
• "Parallel beam geometry" (figure 4.2, 4.4). A parallel incident beam in the equatorial plane created by a graded parabolic X-Ray mirror. The incident beam illuminates the sample in reflection (4.2) or in trasmission mode (4.4). The scattered radiation is intercepted by an optical setup accepting beams coming only from a given direction before entering the single detector. This is achieved with the use of a parallel plate collimator, X-Ray mirror or a secondary monochromator. The sample type, surface orientation or the flatness has no effect on the resolution.
• "Debye-Scherrer geometry" (figure 4.5). A parallel narrow incident beam in the equatorial plane created by a collimator or an X-Ray mirror. The incident beam illuminates the sample in the form of a narror cylinder (usually a capillary that rotates). The scattered radiation is registered by a large one-dimentional position sensitive detector in the form of an arc.

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• Some diffractometers use different geometry (that are similar to the sychrotron beamlines): a narrow pen-like incident beam in combination with a large 2D position-sensitive detector(figure 4.6). This geometry requires intensive movements of the sample to bring a sufficent number of crystallites into the reflection position.

How are XRPD samples prepared?

Ideal X-Ray diffraction experiments should provide clean low-background patterns with a high resolution. A range of special experimental techniques exist to achieve this, with each requiring different methods of sample preparation. This is the non-trivial part of the process because most polycrystalline substances may be altered during the preparation. Texture (the preferred orientation of crystallites), amorphisation, defects, and even phase transformations may be introduced during the preparation of the sample. Nevertheless, methods do exist to minimise the influence on the actual sample during preparation.

For the characterisation of polycrystalline materials by XRPD the following types of samples are most commonly prepared:

• Powder pressed to a flat surface for measurements in reflection mode(figure 4.1). Drawbacks include possible texture effects, influence from the milling process and exposure of the sample to the atmosphere.
• Powder prepared as a flat layer between transparent foils for measurement in transmission mode (figure 4.3). The texture effects and the influence from the atmosphere are less compared to the example above.
• Powder or slurry prepared in a thin capillary for measurement in transmission (figure 4.5). Benefits of this method is that the sample is protected from the environment, the texture is nearly absent, and a quantity below 1mg is sufficient. Though the milling of aggregates may be required.
• Powder deposited as a thin layer on a surface (something which can also be achieved through drying)(figure 4.1). This results in reduced texture, with only a small quantity being sufficient. A drawback is that the sample is not protected from the environment.
• Powder deposited as a loose layer with a low density (20%-40% from bulk)(figure 4.2). This exhibits a reduced texture, though a drawback is that the sample is unprotected from the environment.
• In special cases the solid sample may be measured as is or by cutting a small part of the sample. If the surface is flat one should use Bragg-Brentano geometries (figure 4.1 or 4.3), or otherwise parallel beam geometries (figure 4.2 or 4.4).

What does the data look like?

When reading a scientific paper or report you may come across an XRPD pattern. It is important to understand what kind of information can be extracted from such a pattern. The analysis of XRPD patterns is normally done with the help of analytical software. A typical XRPD pattern is shown in Figure 5. Along the abscissa (horizontal axis) the so-called 2Theta value is shown: the angle between the incident and the diffracted X-Ray beams. The ordinate (vertical axis) shows the intensity of the scattered X-Rays registered by the detector.

The intensities of the XRPD peaks are related to the actual atomic arrangments inside the crystallographic unit cell. The XRPD peak positions are used to determine the crystallographic unit cell parameters a, b, c, alpha, beta and gamma. Using Rietveld analysis, the actual positions of atoms in the unit cell can be recovered. Crystal structure determination by XRPD is an alternative to the more common method of structure determination by monocrystal diffraction. The latter requires a monocrystal sample and different instrumentation.

Information is also encoded in the widths of the peaks themselves. Peak broadening increases with higher levels of (so-called 2nd order) microdeformations inside the crystallites and with smaller crystallite dimensions. The resulting peak shape is a convolution of "physical" broadening, spectral width and the broadening from instrumental aberrations.

The peaks in the diffraction pattern may have a specific structure due to the presence of double wavelength components, the so called Ka1 and Ka2 peaks. This happens when the measurements are performed without a "Ka1" monochromator. Peaks may also exhibit asymmetry due to instrumental aberrations. These instrumental aberrations appear due to different trajectories in the diffraction system. The diffraction angle of each trajectory is therefore slightly different from the "ideal" angle of detector, which leads to a visible deformation of the diffraction peaks.

Where can I find XRPD?

As a rule to thumb - if you are a student or a researcher, the best place to look is at your university. This should be the most affordable option, though caveats may include delivery time and the interpretation of the data. The measurements will be conducted by either a student, an appointed specialist or you will be allowed to do it yourself. If you have a budget, you may consider asking a commercial CRO.

Sometimes XRPD studies are performed at the synchrotron beamlines: large-scale facilities allowing enourmous power of the X-Ray beam. You need to book your session far upfront, travel and submit your sample following specific procedures. The measurement time itself is short, but everything else should be prepared very carefully to get the results in your appointed time window. At every university there is a group of beamline users who could help you with starting. In our view the interface to use XRPD at the beamlines is perfect for academic customers but not exactly flexible for the industrial users. Organisational overheads of going to the beamline might be particularly high.

If you are coming from industry and are looking for XRPD services, it is strongly advised to look for a CRO that is specifically dedicated to your field. You speak the same language, an NDA can be signed, delivery times are respected and most often results can be delivered in a format that you will be able to use. For regulated industries like pharma the CRO should have an appropriate accreditation as otherwise the data might be of no use.

How much does it cost?

The costs of commercial XRPD analysis may vary significantly. For Northern Europe the price tag would start from hundreds of euros per study and may rise by one or two orders of magnitude. This all depends on the scope, supplier, required quality framework and where the results are going to be used. Any sort of quantification studies are much more expensive that qualitative identification jobs. GMP studies devoted to new method development and validation are usually among the most expensive.

As everywhere established long-term cooperation could result in a cost reduction.

Collection of test cases

Samples used in XRPD are not always nicely prepared powder samples. Sometimes XRPD is useful for the analysis of real-life polycrystalline materials. Special instrumentation and some skills are usually required for such studies.
A few examples from our collection are presented below:

Powders

Films

Ceramics

Metals

• Phase analysis of debris from fireworks factory explosion in Enschede (2000)
• Testing of bulletproof material
• Identification of counterfeit drugs (link)
• Study of deposits within enrichment reactor - ppt
• Wrong active substance in pharmaceutical product
• X-Ray scattering on mesoporous silicate film
• Superconductive film created by pulsed laser deposition
• Transition in superconducting film structure
• Enrichment of weartrack of ZrO ceramic
• XRPD on ceramic ball bearing
• Hydroxyapatite biocoating quality testing
• Self-assembled monolayers
• Quality testing of boron-carbide ceramic
• Rutile-anataze quantification
• Materials transfer in drilling bit
• Stress measurements in polycrystalline thin film
• Texture measurements in TiN films
• Texture in rolled copper
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• Study of sample in a drop of water - just link to dannalab test cases
• Collapse of peptide structure - just link to dannalab test cases
• Some other examples related to the characterization of pharmaceuticals are available at our partner DANNALAB B.V. website here.