IMPORTANT MEDICAL DISCLAIMER: The information on this page was generated by an Artificial Intelligence model and has not been verified by a human medical professional. It is for informational purposes only and does not constitute medical or dental advice. This content is not a substitute for professional consultation, diagnosis, or treatment from a qualified doctor, dentist, or other health provider. Never disregard or delay seeking professional medical advice because of something you have read here. Relying on this information is solely at your own risk.
Infrared (IR) spectroscopy remains one of the most vital tools in the analytical chemist’s arsenal for identifying functional groups and verifying molecular structures. However, the quality of an IR spectrum is almost entirely dependent on how the sample is presented to the spectrometer. A poorly prepared sample results in “bottomed out” peaks, scattering noise, or atmospheric interference that can render data useless.
This guide provides a technical walkthrough of the three primary sampling techniques used in Modern Fourier Transform Infrared (FTIR) labs: Attenuated Total Reflectance (ATR), Potassium Bromide (KBr) pellets, and Nujol mulls. To better understand how these results are processed, you may also want to refer to our practical guide to FTIR spectroscopy and functional group interpretation.
Table of Contents
- 1. Attenuated Total Reflectance (ATR)
- 2. The KBr Pellet Method
- 3. Nujol Mull (Liquid Film)
- Troubleshooting Common Preparation Issues
- Summary of Key Takeaways
- Sources
1. Attenuated Total Reflectance (ATR)
ATR has largely replaced traditional transmission methods in modern laboratories due to its speed and minimal cleanup. Instead of the IR beam passing through the sample, it reflects off the internal surface of a high-refractive-index crystal (typically diamond, Zinc Selenide, or Germanium) in contact with the sample [1].
When to Choose ATR
Best for: Rapid screening, liquids, pastes, polymers, and hard solids.
Avoid for: Samples with very low concentrations or materials that might scratch the crystal (if using ZnSe).
Step-by-Step Procedure
- Clean the Crystal: Use a lint-free wipe with isopropanol or acetone. Perform a “Background” scan of the clean, open crystal to subtract atmospheric CO₂ and water vapor.
Apply the Sample:
Liquids: Place one drop directly over the crystal area.
Solids: Place a small amount of powder or a piece of the material on the center of the crystal.
- Apply Pressure: For solids, use the built-in pressure clamp. According to technical specs from JASCO Inc., the IR beam only penetrates the sample by about 0.5 to 5 micrometers. Without high-pressure contact, the signal will be weak or non-existent.
- Acquire and Correct: Because ATR penetration depth is wavelength-dependent, the resulting peaks at lower wavenumbers often appear stronger than in transmission spectra. Turn on “ATR Correction” in your software to normalize the intensities [2].
ATR is significantly faster and requires minimal cleanup because it eliminates the need for complex sample preparation like making KBr pellets or Nujol mulls. It allows for the direct analysis of liquids, pastes, and solids by simply placing the sample in contact with a high-refractive-index crystal.
The IR beam only penetrates the sample surface by 0.5 to 5 micrometers, so intimate contact between the sample and the crystal is required. Without applying sufficient pressure using a clamp, the IR signal will be too weak to produce a usable spectrum.
Avoid ATR for samples with very low concentrations that require the higher sensitivity of transmission methods, or when using materials that could scratch sensitive crystals like Zinc Selenide (ZnSe).
2. The KBr Pellet Method
The KBr pellet technique is the traditional “gold standard” for transmission IR. Potassium Bromide is used because it is transparent to IR radiation in the mid-IR range (4000–400 cm⁻¹) and acts as a carrier medium [2].
When to Choose KBr Pellets
Best for: High-resolution powders, inorganic salts, and samples where quantitative accuracy is required.
Avoid for: Hygroscopic (water-attracting) samples or those that react with halides.
Step-by-Step Procedure
- Preparation (The 1% Rule): Aim for a concentration of approximately 1% sample in KBr. Typically, this is 1–2 mg of sample mixed with 100–200 mg of optical-grade KBr powder [3].
- Grinding: Use an agate mortar and pestle to grind the mixture until it has the consistency of fine flour. If the particles are too large, you will experience “Christiansen Effect” scattering, causing distorted peak shapes.
- Pressing: Transfer the powder to a pellet die. Apply between 8,000 and 10,000 psi using a hydraulic press for 1–2 minutes. A vacuum attachment is recommended during pressing to remove air, which prevents the pellet from being cloudy or fragile.
- Analysis: The resulting pellet should be nearly transparent. Place it in a universal holder for transmission analysis.
Tip: If your KBr pellet turns white or opaque after removal, it likely contains moisture. Bake your KBr powder at 110°C overnight before use.
KBr is used because it is transparent to infrared radiation in the mid-IR range (4000–400 cm⁻¹), allowing the beam to pass through the sample without the carrier medium interfering with the spectra.
The 1% rule suggests mixing approximately 1–2 mg of your sample with 100–200 mg of optical-grade KBr powder. This specific concentration prevents the sample from being too opaque, which would otherwise ‘bottom out’ the peaks in the resulting spectrum.
A cloudy pellet usually indicates the presence of moisture or trapped air. To fix this, bake your KBr powder at 110°C overnight to remove humidity and ensure you use a vacuum attachment during the pressing process to evacuate air.
3. Nujol Mull (Liquid Film)
A “mull” is a suspension of a solid in a heavy mineral oil (Nujol). This is a legacy technique still used when ATR is unavailable or when a sample is sensitive to the high pressures of KBr pressing.
When to Choose Nujol
Best for: Quick transmission checks of solids when a hydraulic press is unavailable.
Avoid for: Samples where you need to see C-H stretching regions (2850–3000 cm⁻¹), as Nujol is a hydrocarbon and will completely mask these peaks [4].
Step-by-Step Procedure
- Grind the Solid: Grind ~5 mg of sample in a mortar until very fine.
- Add the Oil: Add 1–2 drops of Nujol oil and continue grinding until a smooth, toothpaste-like paste forms.
- Sandwich: Using two Salt Plates (NaCl or KBr), place a small amount of the paste on one plate and press the second plate on top, rotating slightly to create a thin, even film [4].
- Clean Up: Salt plates are extremely sensitive to water. Clean them only with anhydrous solvents like chloroform or methylene chloride. For a deeper look at the underlying physics of how light interacts with these materials, see our physics guide to spectroscopy.
Nujol is a hydrocarbon mineral oil, meaning it has intense absorption peaks in the C-H stretching region (2850–3000 cm⁻¹). This will completely mask any C-H functional groups present in your actual sample.
Because salt plates (NaCl or KBr) are extremely water-soluble, they must never be cleaned with water or aqueous solutions. Instead, use anhydrous solvents like chloroform or methylene chloride and store them in a desiccator to prevent fogging from atmospheric moisture.
Troubleshooting Common Preparation Issues
| Symptom | Probable Cause | Solution |
|---|---|---|
| Flat-topped peaks | Sample is too thick/concentrated | Use less sample (KBr) or a thinner film (Mull) |
| Noisy baseline | Poor contact (ATR) or scattering (KBr) | Increase clamp pressure or grind sample finer |
| Huge broad peak at 3400 cm⁻¹ | Water contamination | Dry your KBr; avoid breathing on the sample |
| Shifted Baselines | Refractive index mismatch | Ensure sample is finely ground to reduce light scattering |
Distorted peaks, often called the Christiansen Effect, occur when the sample particles are not ground finely enough. Ensuring the sample-KBr mixture is ground to the consistency of fine flour in an agate mortar can mitigate this issue.
A background scan accounts for current environmental factors like atmospheric CO₂ and water vapor. Subtracting this background ensures that the final spectrum only represents the chemical structure of your sample rather than the air inside the spectrometer.
Summary of Key Takeaways
| Method | Sample Physics | Primary Advantage |
|---|---|---|
| ATR | Surface Reflection | No preparation; non-destructive |
| KBr Pellet | Transmission | High resolution; quantitative |
| Nujol Mull | Transmission | Alternative for pressure-sensitive solids |
Decision Matrix: Which Method to Use?
Use ATR for 90% of routine lab work. It is non-destructive, requires no salt plates, and is the fastest method for liquids and solids.
Use KBr Pellets when you need high-quality transmission spectra for publication or when looking for very weak signals that ATR might miss.
Use Nujol only if you are disinterested in the C-H region and need a quick transmission alternative for a solid powder.
Action Plan for Beginners
- Identify your Regions of Interest: If you need to see the 3000 cm⁻¹ region, skip Nujol.
- Check Sample Solubility: If your solid is soluble in a volatile solvent, you can “cast” a thin film directly onto a salt plate instead of making a pellet.
- Perform a Background Check: Always run a background scan before every sample to account for changes in lab humidity and CO₂.
- Clean Promptly: Salt plates and ATR crystals can be etched by acidic or aqueous samples. Clean them immediately after your scan is complete.
IR spectroscopy is as much an art as it is a science. By mastering these three preparation techniques, you ensure that your data is a true reflection of the molecular structure rather than an artifact of poor lab technique.
ATR is recommended for roughly 90% of routine work because it is non-destructive, requires no special salt plates, and is the fastest method for both liquids and solids.
Use ATR for speed and convenience with most solids and liquids. Choose KBr pellets when you need high-resolution transmission data for publication, and use Nujol only as a quick transmission alternative if you don’t need to see the C-H stretching region.