Operating around the world and represented by employees in the UK, USA and China, Specac are experts in the manufacture of FTIR accessories for spectrometers and FTIR sample preparation equipment. Ever since we launched our Golden Gate single-reflection diamond ATR accessory in the 1990s, we have been at the forefront of this technology – so if you’ve ever wanted to learn more about ATR-FTIR spectroscopy, you’ve come to the right place!

What is ATR-FTIR spectroscopy?

ATR-FTIR (which stands for ‘Attenuated Total Reflectance Fourier-Transform Infrared’) spectroscopy is a powerful analytical technique, used to investigate the molecular composition of materials. It combines the principles of infrared spectroscopy with the ATR sampling method to gain valuable information about the chemical bonds, as well as functional groups present in a sample.

ATR-FTIR spectroscopy operates by measuring the absorption of infrared light by a sample placed in contact with a high-refractive-index crystal. The infrared light interacts with the sample at the crystal-sample interface, causing a portion of the light to be absorbed, while the remaining light is reflected back. By analysing the resulting infrared spectrum, valuable insights into the molecular structure, composition, and chemical properties of the sample can be obtained.

ATR-FTIR spectroscopy is widely used in various scientific fields – including chemistry, materials science, pharmaceuticals, environmental analysis, and forensic science – due to its non-destructive nature, high sensitivity, and broad applicability.

How does ATR-FTIR spectroscopy work?

The infrared beam is reflected off the under surface of a refracting crystal, which then interacts with a sample of lower refractive index placed onto the upper side of the same surface. There is a tiny interaction between the sample material and the beam of light, due to the phenomenon known as an ‘evanescent wave’, which is shown in the diagram below.

Figure 1: Graphical representation of a single bounce ATR

In order to ensure that the beam of light is reflected rather than transmitted through the surface of the ATR crystal the angle of incidence must be above the critical angle, which varies depending on the refractive index of the crystal material. In traditional ATR-FTIR spectroscopy, a high-refractive-index crystal – such as diamond, germanium, or zinc selenide – is used as the ATR element.

Read more: The Basics of ATR-FTIR spectroscopy

What is penetration depth in ATR-FTIR spectroscopy?

The depth of penetration in ATR-FTIR spectroscopy is typically on the order of a few micrometres, making ATR particularly suitable for the analysis of thin films, coatings, and surface layers of materials. By interrogating only a shallow region of the sample, ATR-FTIR spectroscopy provides valuable information about the molecular composition and chemical bonds present near the surface.

The advantage of this shallow sampling depth is that it allows for the analysis of samples without extensive sample preparation, such as slicing, grinding, or dissolution. It is also beneficial when working with heterogeneous samples, as ATR can provide localised information about different regions or layers.

However, limited penetration depth is also a consideration in ATR-FTIR spectroscopy. If the region of interest is located deeper within the sample, the ATR measurement may not capture the desired molecular information accurately. In such cases, alternative sampling techniques, such as transmission spectroscopy – or diffuse reflectance – may be more suitable.

Additionally, penetration depth affects the intensity of the infrared signals. The deeper the penetration, the weaker the signal becomes due to absorption and scattering processes within the sample. Therefore, when comparing spectra or quantifying the concentration of specific components, it’s important to consider the penetration depth and ensure that the samples and measurement conditions are consistent.

Download our Technical Note TN21-02 ATR Penetration Depth

What are the different types of ATR-FTIR spectroscopy?

There are several variations and configurations of ATR-FTIR spectroscopy, which all offer specific advantages. These are some of the most common types:

• Single Reflection ATR. This is the most basic configuration where the incident infrared light undergoes a single reflection at the crystal-sample interface. It provides a simple and straightforward ATR measurement, suitable for routine analysis and samples with relatively high refractive indices.

• Multiple Reflection Horizontal ATR. In this configuration, the infrared light undergoes multiple internal reflections within the crystal before exiting. This increases the effective path length and enhances sensitivity, particularly for samples with low concentrations or weak absorptions. Multiple reflection ATR accessories, such as the trough or waveguide ATR, are used to achieve this extended path length.
• Silicon Wafer ATR. For this configuration of ATR spectroscopy, a silicon wafer is used as the ATR element, instead of using traditional high-refractive-index crystals. Aside from being cost-effective, silicon has the advantage of a high refractive index.

How does ATR-FTIR differ from other IR spectroscopy techniques?

ATR-FTIR spectroscopy is a specific configuration of FTIR spectroscopy that employs the ATR sampling method, enabling convenient analysis of solid, liquid, and semi-solid samples without extensive sample preparation. But there are other methods of analysing samples, such as directly by transmission or using other kinds of “external” reflectance such as diffuse or specular reflectance.

The main technique that ATR-FTIR is often compared to is the transmission technique. This is where the IR beam is passed through the sample directly, which is simpler in concept but often more difficult in practice. To make a transmission measurement, the sample needs to be prepared as a thin film, placed between very closely spaced windows in a transmission cell, or diluted in a non-absorbing matrix such as KBr, so that enough signal can pass through it.

The ATR technique is generally much easier, since it only requires good contact with the ATR element to produce a good measurement. However, there are some spectral differences. The strength of the ATR effect is dependent on wavelength, whereas the transmission measurement is not. This can be seen in the relative intensities of these two spectra:

How do you choose the right crystal to use in ATR-FTIR spectroscopy?

Choosing the right crystal for ATR-FTIR spectroscopy depends on several factors, specific to your sample and its analysis requirements.

Here are some considerations that you’ll need to make…

• Refractive index. The refractive index of the crystal should be higher than that of the sample to ensure that the light is reflected in the crystal. Matching the refractive indices helps to minimise reflection losses and enhance the sensitivity of the measurement.
• Chemical compatibility. The crystal material should be chemically compatible with both the sample and the environment in which the analysis is conducted. Some samples or experimental conditions may be corrosive or reactive, requiring chemically resistant crystal materials such as diamond, germanium, or zinc selenide. Consideration should also be given to potential interactions or contamination between the sample and the crystal.
• Wavelength range. Different crystal materials have varying transmission ranges for infrared light. The crystal should be transparent to the desired wavelength range of the analysis. For example, diamond is suitable for a broad range from mid-infrared to far-infrared, while germanium has good transmission in the mid-infrared region.
• Sampling requirements. The physical properties of the crystal can impact the sampling technique and ease of use. For example, diamond crystals are robust and durable, making them suitable for harsh conditions or repetitive measurements. On the other hand, germanium or zinc selenide crystals may require more delicate handling due to their relative softness.
• Different crystal materials have varying costs, and the selection should align with the budget constraints of the analysis.

Our technical note TN21-03 ATR Crystal Choice and Quest Puck Guide has more information.

 

What are the applications of ATR-FTIR spectroscopy?

The characteristics of ATR spectroscopy make it the most commonly applied form of FTIR analysis. Below are some examples of applications where ATR spectroscopy is most useful:

• Checking and confirming the identity of substances. ATR-FTIR spectroscopy, in combination with appropriate sample preparation and data analysis, provides a reliable and non-destructive method for checking and confirming the identity of substances. It is widely employed in industries such as pharmaceuticals, chemicals, materials science, forensic analysis, and environmental monitoring, where accurate identification and quality control are essential.
• Monitoring cure times for adhesives. By offering valuable insights into the curing process of adhesives, ATR-FTIR spectroscopy helps to optimise formulations, understand curing mechanisms, assess curing completeness, and ensure product quality. It allows for real-time monitoring and analysis, facilitating the development and control of adhesive systems in various industries – including automotive, aerospace, electronics, and construction.
• Chemical classification and machine learning. By integrating ATR-FTIR spectroscopy with machine learning algorithms, chemical classification and identification tasks can be efficiently and accurately performed. This approach finds applications in various fields – such as pharmaceuticals, environmental analysis, food quality control, forensic science, and materials characterisation. It’s especially useful to enable rapid and automated chemical analysis, enhance process control, and support decision-making in complex chemical systems.
• Analysis of biomolecules, organisms, and other macromolecules with horizontal ATR. In all these applications, ATR-FTIR spectroscopy with horizontal ATR facilitates direct analysis of samples, providing rapid and valuable information about the molecular composition, structural characteristics, and chemical properties of biomolecules, organisms, and macromolecules. It offers non-destructive, label-free, and real-time analysis capabilities, contributing to diverse areas such as biochemistry, biomedical research, pharmaceuticals, biotechnology, and materials science.

 

ATR-FTIR spectroscopy in action: The spectroscopy of biological samples.

ATR-FTIR spectroscopy is widely used in the spectroscopic analysis of biological samples due to its ability to provide valuable information about their molecular composition, structural characteristics, and chemical properties.

Key applications include:

• Biomolecular structure and conformation
• Disease diagnosis and biomarkers
• Cellular and tissue analysis
• Biological fluids
• Microbial analysis
• Drug delivery and pharmaceutical analysis

… and more.

ATR-FTIR spectroscopy is a versatile and powerful technique applied in diverse areas, including biomedical research, clinical diagnostics, pharmaceuticals, biotechnology, and environmental sciences, contributing to the understanding and advancement of biological systems.

Keen to explore ATR-FITR spectroscopy in more detail?

Take a look at our FTIR theory articles.

At Specac, we stock a comprehensive range of FTIR accessories for a variety of sampling techniques, including Attenuated Total Reflectance (ATR), Specular and Diffuse Reflectance (DRIFTS), and Transmission cells. This includes sampling kits and packs for rapid setup of academic and teaching labs.

We’re a team of ATR-FTIR spectroscopy experts. So, if you have a question or need a quote, please contact us or read our FAQs.