FTIR Sampling Techniques
Choosing the right sampling technique is critical for obtaining high-quality FTIR spectra. Each method has its strengths — the best choice depends on your sample type, preparation time, and the accessories available for your FTIR spectrometer.
Comparison Overview
Transmission
Best for: Gases, thin films, KBr pellets
Prep: Moderate–High
Limitations: Sample must transmit IR
ATR (Attenuated Total Reflectance)
Best for: Solids, liquids, pastes, powders
Prep: Minimal
Limitations: Surface technique (~0.5–5 μm depth)
DRIFTS (Diffuse Reflectance)
Best for: Powders, rough surfaces
Prep: Minimal (dilute in KBr)
Limitations: Quantification is difficult
Specular Reflectance
Best for: Coatings on reflective substrates
Prep: None
Limitations: Requires flat, reflective substrate
Transmission
How it works
The simplest FTIR sampling mode: the infrared beam passes directly through the sample. The detector measures the intensity of transmitted light, and the spectrum shows which wavelengths were absorbed.
When to use it
Ideal for gas-phase analysis (using gas cells with long pathlengths), thin polymer films, and solid samples pressed into KBr pellets. It provides the most straightforward Beer-Lambert relationship between concentration and absorbance.
Practical tips
For KBr pellets, grind the sample finely (< 2 μm particle size) and mix at about 0.2–1% concentration (roughly 1 mg sample per 100–200 mg KBr). Press at ~10 tonnes under vacuum to produce a clear disk. Keep KBr dry — it is hygroscopic and will introduce water peaks if exposed to humidity.
ATR (Attenuated Total Reflectance)
How it works
The sample is pressed against a high-refractive-index crystal (diamond, ZnSe, or Ge). The IR beam enters the crystal and undergoes total internal reflection at the sample interface. An evanescent wave extends a short distance (typically about 0.5–5 μm, depending on wavelength and crystal) into the sample, where it is partially absorbed.
When to use it
ATR is now one of the most widely used FTIR sampling techniques. It requires virtually no sample preparation — simply place the sample on the crystal and apply pressure. Excellent for liquids, pastes, powders, films, and solid surfaces.
Practical tips
Ensure good contact between the sample and crystal — poor contact produces weak, noisy spectra. Clean the crystal with isopropanol between samples. Diamond ATR crystals are the most durable but most expensive. ZnSe is a good general-purpose alternative for near-neutral samples (about pH 5–9), though it is attacked by acids and strong bases.
DRIFTS (Diffuse Reflectance)
How it works
Infrared light is focused onto the surface of a powdered sample. The light penetrates into the powder, undergoes multiple scattering events and partial absorption, and emerges in all directions. Collection optics gather this diffusely reflected light and focus it on the detector.
When to use it
DRIFTS is excellent for analyzing powders without pressing pellets. It is commonly used for catalysis studies, soil analysis, and pharmaceutical raw material identification. Samples are typically diluted in ground KBr at 2–5% concentration.
Practical tips
Particle size has a strong effect on DRIFTS spectra — grind samples finely and consistently. Use the Kubelka-Munk transform rather than transmittance or absorbance for more linear concentration relationships. Always use the same KBr background preparation as the sample.
Specular Reflectance
How it works
The IR beam hits the sample surface at a fixed angle and the specularly (mirror-like) reflected light is collected. For thin coatings on metal substrates, the beam passes through the coating, reflects off the metal, and passes through the coating again — doubling the effective pathlength.
When to use it
Use specular reflectance for analyzing coatings, films, and surface treatments on metals and other reflective substrates. Grazing angle reflectance (high angle of incidence) increases sensitivity for very thin films down to monolayer thickness.
Practical tips
The angle of incidence affects both sensitivity and the apparent peak positions. For very thin films (< 100 nm), use grazing angle (70–85°) to maximize signal. The substrate must be clean and highly reflective — any surface roughness will reduce the specular component and add noise.
Want to learn more about ATR?
ATR-FTIR is a rich topic on its own. Beyond the basics covered here, there are important considerations around crystal material selection (diamond, ZnSe, germanium), depth of penetration calculations, and sample-specific preparation techniques for different material types.
Frequently Asked Questions
What is ATR in FTIR spectroscopy?
ATR (Attenuated Total Reflectance) is a sampling technique where the sample is pressed against a high-refractive-index crystal (diamond, ZnSe, or germanium). An evanescent wave penetrates about 0.5–5 μm into the sample surface, depending on wavelength and crystal, measuring its infrared absorption. ATR requires minimal sample preparation and works with solids, liquids, pastes, and powders.
Do you need to prepare samples for FTIR?
It depends on the technique. ATR requires virtually no preparation — just place the sample on the crystal. Transmission mode may require pressing a KBr pellet (grinding sample with KBr and pressing under vacuum) or preparing a thin film. DRIFTS requires diluting powdered samples in ground KBr at 2–5% concentration.
What is the difference between ATR and transmission FTIR?
In transmission, the IR beam passes directly through the sample, requiring it to be thin enough to transmit light. In ATR, the beam reflects internally within a crystal, with an evanescent wave probing only the sample surface (about 0.5–5 μm depth). ATR is faster and easier with minimal prep, but only measures the surface; transmission measures the bulk.
References
- Griffiths, P. R.; de Haseth, J. A. Fourier Transform Infrared Spectrometry, 2nd ed. Wiley, 2007.
- Smith, B. C. Fundamentals of Fourier Transform Infrared Spectroscopy, 2nd ed. CRC Press, 2011.