The path from molecule to medicine is paved with complexity, and developers face a persistent challenge: how to fully characterize solid forms, especially polymorphs, salts and co-crystals, early and accurately. These forms can dramatically influence a drug’s stability, solubility, bioavailability and manufacturability. Yet traditional analytical techniques often fall short in resolving subtle structural differences or detecting low-level impurities in complex mixtures.
This is where X-ray powder diffraction (XRPD), enhanced by advanced geometric and computational analysis, offers a transformative edge.
Rather than replacing existing techniques like spectroscopy or thermal analysis, XRPD complements and significantly improves the analytical toolkit. It provides a direct window into the crystalline structure, enabling developers to not only identify phases but also quantify them with precision — even in multi-component systems. When paired with modern data processing and modeling, XRPD becomes a high impact tool for decision-making across development stages, from early screening to final formulation.
Transmission vs. Reflection Geometry
XRPD is a powerful analytical technique used to determine the crystallographic structure, chemical composition, and physical properties of materials. When utilizing XRPD, scientists often set up the experiment using one of two common geometries — transmission and reflection.
While both geometries have their applications, transmission geometry offers several advantages over reflection geometry, making it a preferred choice in certain scenarios. Transmission geometry excels in providing high quality data and is superior for quantitative analysis. The choice between transmission and reflection is guided by the nature of the sample. Generally, transmission is better for low absorbing samples (e.g. organic compounds) whilst reflection is better for dense, opaque materials (e.g. metals or inorganic powders).
One significant advantage of transmission geometry is the reduction of sample preparation errors. Analysis using reflection geometry is prone to preferred orientation effects and sample displacement errors. Transmission geometry can also use smaller sample volumes and offers the added benefit of increased safety to the analyst as materials are fully contained.
Preferred Orientation
In reflection geometry, the sample surface is irradiated, and the diffracted X-rays are collected from the same surface. This can lead to preferred orientation, where certain crystallographic planes are overrepresented in the diffraction pattern, i.e. the crystallites in the powder are oriented in one direction. In real samples, preferred orientation of particles is always present as a random orientation of particles can only exist if their shape is spherical. The sample preparation techniques employed for reflection geometry, where materials are pressed into cavities and smoothed out, exacerbate preferred orientation often resulting in the absence of inflections for some crystalline components. In contrast, transmission geometry involves passing X-rays through the sample, which minimises the impact of preferred orientation and provides a more representative diffraction pattern.
Sample Displacement Error
If the sample height is different from the focal plane, this results in shifted peak positions known as sample displacement errors. With the orientation in reflection, it is difficult to pack the sample in such a way that its surface is exactly level with the sample holder surface. Additionally, diffraction of low angle X-rays can be blocked by the sample in reflection geometry.
Transmission analysis is more conducive to a random orientation of crystals as a very small quantity of sample (as low as 10 mg) is placed between two layers of transparent media. This mode of sample preparation is also safer (as mentioned above) as the sample is contained between films rather than the exposed surface of a reflection sample.
Improved Data Quality for Thin Films and Small Samples
Transmission geometry is particularly advantageous for analyzing thin films and small samples. In reflection geometry, the penetration depth of X-rays is limited, which can result in weak diffraction signals due to the limited penetration depth. Transmission geometry allows X-rays to pass through the entire sample, enhancing the diffraction signal and improving data quality. This is especially beneficial for studying thin films, nanoparticles and other small-scale materials. The focusing mirror employed with transmission geometry also gives better data quality up to 40º2θ compared to the reflection geometry set up.
Better Suitability for Anisotropic Samples
Anisotropic crystalline solids, where properties such as refractive index and electrical and thermal conductivity vary according to the direction in which their internal structure is ordered, can pose challenges in reflection geometry XRPD. The orientation of the sample surface can cause peak broadening and intensity variations, leading to inaccurate results. Transmission geometry, on the other hand, allows X-rays to pass through the sample in multiple directions, providing a more comprehensive analysis of anisotropic materials. This is particularly useful for studying materials with complex crystal structures and directional properties.
Case Study 1
Extension of a method to include an additional polymorph
A method to ensure absence of crystalline Form 1 and Form 2 in tablets, with a limit of detection (LOD) of 1% w/w, where the API was an amorphous form, was validated by Almac Sciences in transmission and reflection geometries.
The result of the method validations were compared. An overlay of the diffractograms acquired in each geometry (Figure 4) illustrates that peak profiles are identical in both geometries. The much broader peak shapes observed in reflection geometry can lead to hidden peaks and can inhibit calculation of accurate LOD values. The diffractogram acquired in transmission utilized ~20 mg of sample material that was safely contained, protected from air and humidity, between films whereas the reflection analysis used ~300 mg of material open to the air.
Case Study 2
Identity method for API validated in transmission geometry
A method was validated by Almac Sciences for identity of an API in both transmission and reflection geometries.
The result of the method validations were compared. An overlay of the diffractograms acquired in each geometry (Figure 5) illustrates that peak profiles are identical in both geometries. The transmission method had the advantages of requiring less sample material, providing better containment for a hazardous material and producing data of better quality with sharper, better resolved peaks.
The case studies presented illustrate the versatility of XRPD in solving real-world challenges; from identifying unexpected polymorphs during early screening, to ensuring batch-to-batch consistency in commercial manufacturing. Whether used to de-risk formulation strategies, support regulatory filings, or troubleshoot stability issues, XRPD proves to be more than just an analytical tool — it is a strategic asset. Its ability to deliver precise, geometry-based insights into solid-state forms makes it indispensable across the development life cycle, offering clarity where other techniques may fall short.
Sample preparation is often simpler for transmission geometry XRPD compared to reflection geometry. In reflection geometry, the sample surface needs to be flat and smooth to ensure accurate diffraction measurements. This can require extensive sample preparation, including polishing and mounting. In transmission geometry, the sample can be in the form of a powder, thin film, or even a small piece, reducing the need for extensive preparation and making the technique more versatile.
Conclusion
While both transmission and reflection geometries have their place in XRPD, transmission geometry offers several distinct advantages. It reduces preferred orientation and sample height effects, improves data quality for thin films and small samples, is better suited for anisotropic samples, requires less sample preparation and protects the sample from air and humidity. The ease of sample preparation together with the reduction in the volume of material required bring added cost benefits to the client. These advantages make transmission geometry XRPD a valuable tool for a wide range of materials science and crystallography applications.