Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is a powerful analytical technique for the study of powdered and rough surface materials. It is widely employed for quality control, materials characterisation and catalytic studies in both research and industrial settings. However, optimal sample preparation and instrument setup are crucial to obtain the most reliable results and ensure robust data interpretation. This article outlines some best practices to assist quality control managers, researchers and other R&D professionals working with DRIFTS equipment.

Understanding DRIFTS: a unique spectroscopic tool

Unlike traditional Fourier transform infrared (FTIR) spectroscopy that relies on transmission measurements, DRIFTS captures diffusely scattered infrared radiation from powdered samples due to specular reflection (see Figure 1). This makes it particularly useful for materials that are difficult to analyse with conventional transmission-based setups, while its high sensitivity to surface interactions means it is ideal for studies of adsorption processes, catalytic reactions and chemically modified solid surfaces. Importantly, DRIFTS requires no optical contact with the sample, so gas flow through a sample can be maintained during data acquisition. This is a key consideration for real-time in situ monitoring of gas–surface interactions, for example, as encountered in studies of catalytic processes. The DRIFTS technique also offers the advantages of minimal sample preparation and non-destructive analysis.

In practical terms, if a sample is not reflective and cannot be analysed by conventional transmission FTIR spectroscopy, diffuse reflectance is a suitable alternative.

Figure 1. Schematic illustrating the distinction between diffuse and specular reflections for a powdered sample.

The Kubelka–Munk (KM) transformation in DRIFTS analysis

The Kubelka–Munk (KM) transformation is a valuable tool for quantitative analysis of DRIFTS spectra, as it provides a mathematical model of diffuse reflectance properties (based on certain assumptions).2,3 Briefly, Kubelka and Munk’s equation describes the relationship between diffuse reflectance and analyte concentration in terms of the absorption and scattering coefficients of the sample.2 Due to inherent practical limitations in the model, diffuse reflectance is always measured relative to a reference sample.3

Applying the KM transformation allows the concentration of surface species to be measured, making the interpretation of complex spectral data more straightforward. This is useful in various fields including catalysis, materials science and pharmaceuticals, where analysing molecular interactions on solid surfaces is essential. In the case of catalysis, the KM transformation helps researchers explore active sites and adsorbed species on materials such as metal oxides and zeolites. When applied to DRIFTS data, it generates clearer, more reliable insights and allows chemical reactions to be readily monitored.

Figure 2 shows a typical result you might obtain when using Harrick’s Praying Mantis Diffuse Reflection Accessory. The KM units on the y-axis indicate that the KM transformation has already been applied to the raw reflectance data, to make it more suitable for quantitative analysis. The x-axis represents wavenumber (in cm−1), with key vibrational bands of surface hydroxyl groups highlighted due to their importance in catalysis applications. These signals could potentially be monitored in real time to follow catalytic reactions.

Figure 2. Example data obtained from a DRIFTS experiment that examined three zeolite samples, after KM transformation.

Best practices for sample preparation

1. Use an appropriate non-absorbing matrix

To enhance signal quality and minimise spectral artefacts, the sample should be diluted in a non-absorbing matrix or reference material. Typical matrix materials include:

  • KBr or KCl, for mid-IR measurements.
  • Diamond powder, for applications requiring extreme robustness.

2. Optimise particle size

Finer particle sizes improve reproducibility by ensuring a uniform scattering environment. Therefore, the particle size should ideally be <40 µm and preferably in the 5–10 µm range. To achieve ideal sample characteristics:

  • Use a Wig-L-Bug grinder or mortar and pestle to achieve the desired consistency.
  • Avoid excessive grinding, which can adversely impact the sample properties and introduce data artefacts.

3. Control moisture content

Moisture can lead to unwanted spectral interference. To prevent this:

  • Oven-dry the reference material before use.
  • Store samples in a desiccator prior to measurement.

4. Ensure consistent packing density

A consistent sample packing density in the sample holder is important for reproducibility (see Figure 3). To ensure appropriate packing:

  • Lightly tap the sample cup to remove air pockets.
  • Avoid using excessive pressure, which can result in specular reflection artefacts.

5. Verify the optical alignment of your instrument

Proper optical alignment is critical for obtaining high-quality DRIFTS spectra, since misalignment can cause signal loss, spectral distortions or unwanted artefacts. To ensure reliable performance and maintain consistency in measurements:

  • Regularly check the alignment of mirrors and optical components in the DRIFTS accessory.
  • Verify that incident light is correctly focused on the sample.
  • Follow manufacturer recommendations for calibration and alignment procedures.

Figure 3. Image showing a correctly packed sample in the Microsample Cup of the Harrick Praying Mantis Diffuse Reflection Accessory.

Step-by-step sample preparation and analysis

Grinding: Begin by grinding your sample using a mortar and pestle or a Wig-L-Bug mill, to achieve a fine and uniform particle size.

Drying: Next, dry your reference material (e.g., KBr) in an oven and store it in a desiccator to keep it free of moisture.

Mixing: Dilute your sample in the reference material. Aim for 2–15% sample concentration in the reference matrix, depending on how strongly absorbing your sample is, and ensure thorough blending to achieve a uniform mixture.

Background spectrum: Load the Praying Mantis Sample Cup with your dry reference material and flatten the surface to prepare it for data acquisition. Then, record a background spectrum.

Sample spectrum: Empty the reference material from the Sample Cup and load your sample mix. Adjust the height of the sample stage and level the surface to complete the preparation process. After loading the Sample Cup into the Praying Mantis Diffuse Reflection Accessory, the sample spectrum can now be acquired (see Figure 4).

Figure 4. Loading the Sample Cup, containing a properly prepared sample, into the Praying Mantis Diffuse Reflection Accessory.

Instrument optimisation for reliable spectra

1. Choose the right optical accessory

A well-designed DRIFTS accessory enhances diffuse signal collection while minimising specular reflectance. For instance, Harrick’s Praying Mantis Diffuse Reflection Accessory uses an ellipsoidal mirror design to ensure only diffusely scattered light is collected, improving spectral quality. With the optional reaction chambers and temperature controllers, the Praying Mantis range of accessories is ideal for analysing samples at high temperatures and across a wide range of pressures.

2. Optimise background and sample measurements

  • Background spectrum: Be sure to collect a background spectrum using a well-packed non-absorbing reference matrix, before making sample measurements.
  • Sample height adjustment: Samples should be positioned at the optimal focal point of the optical system to maximise the collected signal.
  • Acquisition parameters: Use an appropriate resolution (typically 4 cm−1) and signal averaging for improved signal-to-noise ratio.

3. Detector considerations

  • For mid-IR DRIFTS, a liquid nitrogen cooled mercury–cadmium–telluride (MCT) detector provides superior sensitivity.
  • For broader applications, a standard deuterated l-alanine-doped triglycine sulphate (DTGS) detector may be sufficient.

How to avoid common spectral artefacts

1. Minimise specular reflection

Specular reflection can distort DRIFTS spectra, especially for highly absorbing materials. To mitigate this:

  • Use an off-axis collection setup or an accessory that deflects specular components away from the detector.
  • Ensure uniform sample distribution to reduce surface gloss.

2. Avoid reststrahlen bands

Highly absorbing materials can also produce distorted bands known as reststrahlen bands (a term that means “residual rays” in German), particularly in the mid-IR range.  To suppress these signals:

  • Increase the dilution ratio of the sample in the reference material, if reststrahlen bands are observed.
  • Use spectral subtraction techniques to isolate the true sample signal.

3. Factor in thermal effects in high-temperature studies

For in situ DRIFTS studies that involve temperature variation:

  • Compensate for expansion and contraction effects by ensuring precise control of sample height (i.e., to keep the sample at the optimal focal point).
  • At high temperatures, use appropriate shielding to prevent sample self-emission.
  • Always match background and sample spectra at identical temperatures.

Enhancing DRIFTS applications in research and industry

1. Catalysis and surface chemistry

In the petrochemical industry, DRIFTS is widely used to study heterogeneous catalysts for refining processes, which often require high temperatures and pressures. It enables real-time monitoring of catalyst surfaces under these conditions, allowing researchers to optimise chemical processes such as fluid catalytic cracking (FCC) and hydrocracking.

The pharmaceutical industry uses DRIFTS to monitor solid-state reactions and characterise the interaction of active pharmaceutical ingredients (APIs) with excipients, which is a key factor in drug stability and can help improve formulation strategies.

2. Quality control and material characterisation

  • In polymer science, DRIFTS is used to evaluate surface treatments and modifications, such as plasma treatments for enhanced adhesion.
  • In agriculture, it is applied in fertiliser and soil analysis to facilitate optimal nutrient content.
  • In ceramics, DRIFTS is employed to assess material composition and structural integrity during production. Notably, ceramics include a broad range of inorganic, non-metallic materials encompassing clay-based products (e.g., tiles, porcelain) and advanced ceramics that find use in aerospace applications, biomedical implants and electronics.

3. In situ and operando studies

The non-destructive nature of DRIFTS makes it ideal for real-time analysis of chemical processes, particularly in high-pressure and high-temperature environments. A good example in heterogenous catalysis is the study of adsorption–desorption mechanisms in catalytic converters, which are crucial to help control vehicle emissions.

In conclusion, a well-optimised DRIFTS setup is essential to obtain reliable and reproducible spectra. An awareness of proper sample preparation, regular instrument calibration and strategies to avoid potential artefacts will all ensure high-quality results. By following these best practices, researchers and other R&D professionals, as well as quality control managers can fully leverage DRIFTS as a powerful technique for detailed materials analysis and process monitoring.

This article is based on a TechTalk delivered by Jim Delaney, Applications Scientist at Specac, during Pittcon 2025. If you have any questions, please get in touch with him directly: Jim Delaney

References

  1. M. B. Mitchell, in Structure–Property Relations in Polymers, ed. M. W. Urban and C. D. Craver, American Chemical Society, Washington, DC, 1993, ch. 13, pp. 351–375.
  2. J. P. Blitz, in Modern Techniques in Applied Molecular Spectroscopy, ed. F. M. Mirabella, John Wiley & Sons, Inc., New York, 1998, ch. 5, pp. 185–219.
  3. J. Sirita, S. Phanichphant and F. C. Meunier, Anal. Chem., 2007, 79, 3912–3918.

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