Fourier transform infrared (FTIR) spectroscopy is a versatile method applicable to many different sample types and scientific fields. The technology’s wide range of capabilities also comes with a wide range of different techniques, components and features to choose from. When it comes to sample handling, different methods, modules and accessories may be necessary to obtain the best results for a particular sample in a specific application; the analysis of a pharmaceutical tablet will require different tools than the analysis of a gaseous substance, or of the coating on a piece of construction material, for example.
While exploring the vast array of available FTIR sample handling products, use this guide to help narrow down the best options for your laboratory, samples and applications:
Attenuated Total Reflectance (ATR): Fast and Flexible for Many Common Samples
Attenuated total reflectance (ATR) is the most common sample handling technique for FTIR spectroscopy due to its ease of use and applicability to a large number of common sample types including hard solids, powders, liquids and even challenging, thick and strongly absorbent samples such as black rubber.1 This technique utilizes a crystal with a specific geometry and refractive index that, when struck by IR radiation, produces an evanescent wave that hits the sample in contact with the crystal surface. The beam is attenuated where the sample absorbs energy and exits the other end of the crystal before reaching the detector.
This method requires little-to-no sample prep as the sample can be placed directly on the crystal, as long as a sufficient surface area is in contact with the crystal surface. The method also only requires small sample sizes, such as just a drop of liquid or just enough powder to cover the crystal surface, although larger samples can also be used. If you want to cut back on preparation time and are working with a wide range of solid, liquid, organic and inorganic samples, or working with highly absorbent materials, the ATR method is a reliable choice for the majority of applications. However, ATR can be more costly than other methods due to the cost of the crystal accessories, and the method is not used for gas analysis.
Choosing the correct crystal for your analysis is also crucial for getting the most out of your ATR-FTIR analysis. Here are a few factors you’ll need to consider:
Crystal Material
Different materials used for ATR crystals have different refractive indices, different spectral ranges and different levels of robustness in terms of hardness, water solubility and susceptibility to different pH conditions.2 A higher refractive index will result in a lower depth of penetration (dp), so the properties of your sample will need to be considered to ensure the dp is not too weak or too overwhelming for results to be interpreted effectively.
Diamond is one of the most common crystals used due to its robustness and a refractive index that is suitable for most samples. Germanium, while not as robust, has a high refractive index, allowing for a minimal dp for highly absorbent samples like black rubber. Zinc selenide is another option that will offer the same dp as diamond at a lower cost, but is much softer and thus may be damaged by hard solid samples.
Angle of Incidence
ATR crystals can come with different bevel angles such as 30°, 45° or 60°, and this will also affect the dp. A wider angle will produce a shallower dp for highly absorbent samples, will a narrower angle can increase the dp for less absorbent or low concentration samples.
Number of Reflections
ATR crystals may be single-reflection or multiple-reflection (also known as single bounce or multi-bounce) with the number of reflections being positively correlated with the intensity of the resulting signal due to multiple contacts with the sample. A longer crystal with a larger number of reflections could help resolve weak, indistinguishable absorption bands for samples of low concentration.3
Products to Consider:
- Nicolet Summit FTIR Spectrometer from Thermo Fisher Scientific
- Golden Gate Versatile ATR Accessory from Specac Inc.
Transmission: Low-cost, Established Method for Solids, Liquid and Gas
Transmission FTIR spectroscopy, in which the sample is placed directly in the path of the IR beam, is a simple, traditional method through which many spectral libraries have been built. The method, while no longer the most popular compared to ATR, is still used in many laboratories and provides high-quality qualitative and quantitative results at a lower cost than most other methods.4 However, this method does require more extensive sample prep, as solids must be ground and pressed into pellets with potassium bromide (KBr) or mixed into mulls, and mulls, liquids and gases must be placed in windowed cells that can require some assembly. The method can also suffer from lower reproducibility if sample preparation is not carried out in an effective and consistent manner.
Transmission may be the best method for your lab if you need to save costs, and, with proper sample prep, offers excellent sensitivity and high quality spectra - it also has the advantage of an abundance of established reference libraries. For mulls, liquid and gas samples, there are some factors you’ll need to consider when selecting your transmission cells:
Window Material
The windows in liquid and gas transmission cells through which the IR beam passes can come in many different materials, and similarly to ATR crystals, your material selection will depend on factors like refractive index, spectral range and robustness depending on your application and sample type. Cost may also be a factor in your decision.
KBr and sodium chloride (NaCl) windows are commonly used due to their wide wavenumber range and low cost, but are soluble in aqueous liquids and thus not suitable for all samples.5 Caesium iodide (CsI) is also soluble but offers an extended transmission range down to 140 cm-1. ZnSe windows are insoluble in water and applicable to most mid-IR applications but are also more costly. Thallium bromoiodide (KRS-5) also has lower solubility and is commonly used in gas cells for samples that contain water vapor.6
Path Length and Spacers
Gas transmission cells come in short or long paths depending on the sample concentration and detection limit needed. Short cells in the cm range are available for general use when very low detection limits are not necessary, but when ppm or ppb detection limits are required, cells with path lengths of several meters can be used for much higher sensitivity and resolution. Long-path cells utilize mirrors to achieve multiple reflections that extend the path of the beam within the cell. Regular aluminum mirrors may be used for non-corrosive samples while gold-plated mirrors are needed when there is risk of corrosion.6
In liquid cells, spacers are used to achieve different path lengths in the 0.01 mm to 10 cm range. Longer path lengths (1-10 cm) are typically used for measuring oils, while a path length of around 0.1 mm is usually suitable for most common samples. Liquid cells may come with built-in spacers and a fixed path length, or can be variable through manual assembly with different sized spacers.5
Products to Consider:
- ALPHA II FTIR Spectrometer from Bruker Optics
- Omni-Cell Traditional Liquid Transmission Cell from Specac Inc.
Reflectance-Absorbance (IRRAS): Ideal for Thin Films and Coatings
IR reflectance-absorbance spectroscopy (IRRAS) is a method most commonly used for analyzing a thin layer of sample on a flat reflective surface such as metal. In this method, the IR beam passes through the sample at a specific angle, reflects off the substrate and then passes through the sample again before reaching the detector. This technique is non-destructive and can detect extremely thin layers and monolayers depending on the angle of the beam.7 While there is little-to-no sample preparation needed, additional accessories will be needed to attain the grazing angle and sensitivity needed to analyze extremely thin sample layers:
Grazing Angle Accessories
IRRAS accessories contain windows and mirrors and are designed to direct the IR beam at a specific angle onto the sample; grazing angle accessories provide an incident angle as close to 90° as possible to ensure a sufficient path through a very thin layer down to a monolayer, usually around 80°. These accessories may come with a fixed angle or a variable angle that can be adjusted by the experimenter. These accessories often come equipped with a polarizer and may also include a photo-elastic modulator (PEM) for extremely sensitive polarization modulation IRRAS (PM-IRRAS).
Polarization Modulation
When analyzing an extremely thin film or coating, absorbances from water and gases in the atmosphere can significantly interfere with the spectroscopic measurement due to the increased sensitivity needed to analyze these thin layers. PM-IRRAS (sometimes called PEM-IRRAS) allows for suppression of these interferences through rapid switching between s-polarization and p-polarization. The p-polarized light is absorbed by the sample while s-polarized light only undergoes atmospheric absorption, thus the difference can be subtracted and the sample spectra can be distinguished.8 Keep in mind that PM-IRRAS involves a relatively complicated setup compared to other methods and requires dual channel data acquisition in order to record both sets of spectra simultaneously, so ensure your FTIR system is amenable to this method before using this type of accessory.
Products to Consider:
- FT/IR-6000 Series FTIR Spectrometers from JASCO
- Free-Standing Polarizers from Specac Inc.
Diffuse Reflectance (DRIFTS): Minimal Sample Prep for Fine Powders
While not as seen as commonly as other methods in this guide, diffuse reflectance for infrared Fourier transform spectroscopy (DRIFTS) is still used in many labs that commonly deal with samples that are easily ground into fine powders, such as in the pharmaceutical industry where it can be used to analyze ground tablets.9 The advantage of this method for these sample types is that there is no sample prep needed other than grinding with a mortar and pestle and mixing with KBr - the powder is simply transferred into a sample cup without being pressed or mulled. This method can also be used for in situ experiments involving high temperatures and pressure.
In the DRIFTS technique, the IR radiation is directed at the sample/KBr mixture and interacts with the fine particles, scattering throughout the sample as it reflects off the particle surfaces. The diffused energy is then directed to the detector by parabolic output mirrors and the collective signals of the scattered energy are recorded. This method is frequently used for near-IR (NIR) applications, sometimes without the need for dispersion in KBr.
This method can also be used for rigid polymers and paints with the use of silicone carbide (SiC) paper or an SiC disc to scrape a small amount of sample from a large or irregular surface. One disadvantage of this method is that, like transmission, consistency in sample prep (particle size and proportioning/homogeneity with KBr) is needed for reproducibility.
DRIFTS Accessories
Sample compartments for DRIFTS will contain a mirror or mirrors to direct the scattered light and cups to hold samples and references. Sample cups can come in different sizes and mirrors can come in different shapes, materials and configurations; for example, mirrors may be ellipsoid or parabolic in shape, may be aluminum or gold-plated and one or two mirrors may be used. DRIFTS modules may also come with a grinder implement made from SiC or diamond included.
Another accessory option for DRIFTS is environmental chambers for temperature and pressure experiments. Environmental chambers specifically designed for FTIR will have windows for the IR beam to pass through; material type, temperature control range, pressure control range and safety features are specifications to consider when selecting an environmental chamber for DRIFTS.
Products to Consider:
- IRTracer-100 FTIR Spectrophotometer from Shimadzu
- Selector Diffuse Reflectance Accessory from Specac Inc.
References
- "FTIR Sample Techniques: Attenuated Total Reflection (ATR)," Thermo Fisher Scientific. https://www.thermofisher.com/us/en/home/industrial/spectroscopy-elemental-isotope-analysis/spectroscopy-elemental-isotope-analysis-learning-center/molecular-spectroscopy-information/ftir-information/ftir-sample-handling-techniques/ftir-sample-handling-techniques-attenuated-total-reflection-atr.html
- "Crystal Selection for ATR," Pike Technologies. https://www.piketech.com/atr-crystal-selection/
- "ATR: Single vs. Multiple Reflection," Pike Technologies. https://youtu.be/SKHNVNdlWKQ
- "FTIR Sample Techniques - Transmission," Thermo Fisher Scientific. https://www.thermofisher.com/us/en/home/industrial/spectroscopy-elemental-isotope-analysis/spectroscopy-elemental-isotope-analysis-learning-center/molecular-spectroscopy-information/ftir-information/ftir-sample-handling-techniques/ftir-sample-handling-techniques-transmission.html
- "Measurement Methods for Liquid Samples," Shimadzu. https://www.shimadzu.com/an/service-support/technical-support/analysis-basics/ftirtalk/talk9.html
- "Measurement Methods for Gas Samples," Shimadzu. https://www.shimadzu.com/an/service-support/technical-support/analysis-basics/ftirtalk/talk13.html
- "FTIR Sample Techniques - True Specular Reflectance/Reflection-Absorption." Thermo Fisher Scientific. https://www.thermofisher.com/us/en/home/industrial/spectroscopy-elemental-isotope-analysis/spectroscopy-elemental-isotope-analysis-learning-center/molecular-spectroscopy-information/ftir-information/ftir-sample-handling-techniques/ftir-sample-handling-techniques-true-specular-reflectance-reflection-absorption.html
- "PM-IRRAS," Bruker. https://www.bruker.com/en/products-and-solutions/infrared-and-raman/ft-ir-research-spectrometers/pm-irras.html
- "FTIR Sample Techniques - Diffuse Reflectance (DRIFTS)," Thermo Fisher Scientific. https://www.thermofisher.com/us/en/home/industrial/spectroscopy-elemental-isotope-analysis/spectroscopy-elemental-isotope-analysis-learning-center/molecular-spectroscopy-information/ftir-information/ftir-sample-handling-techniques/ftir-sample-handling-techniques-diffuse-reflectance-drifts.html