FTIR sample preparation for ATR, KBr pellets, films, liquids, and powders
The practical role of sample preparation in FTIR
FTIR sample preparation is the step that makes a real material suitable for measurement without changing the chemistry the spectrum is intended to show. The right route depends on sample form, concentration, optical behavior, and whether the result should represent the surface or the bulk. ATR is often the fastest option for routine identification because many solids and liquids can be measured directly. Transmission methods, including KBr pellets, thin films, liquid cells, and gas cells, take more preparation but give more control over path length and bulk absorption. Powders may also be prepared by diffuse reflectance or as mulls when pressing a pellet is not suitable. For broader workflow context, see the sample preparation section.
Public technical guidance from instrument manufacturers and pharmacopeial methods is consistent on one point: poor sample contact, excessive thickness, moisture, and scattering can distort an otherwise capable FTIR measurement. In practice, sample preparation is not a cosmetic step before the scan. It determines whether the instrument records a clean absorbance pattern or a mixture of chemistry, geometry, and handling artifacts.
A sample-type decision guide
The simplest way to choose an FTIR preparation method is to start with the physical form of the sample, then decide how much path length control is needed. The table below summarizes common laboratory choices rather than mandatory rules. Regulated methods, validated quality-control procedures, and instrument-specific instructions should take priority when they apply.
| Sample type | Common preparation route | Why it is used | Main limitation |
|---|---|---|---|
| Flat solids and films | ATR or transmission film | ATR is quick; transmission is useful when the film is already thin and uniform | ATR is surface-weighted; transmission can saturate if the film is too thick |
| Powders | ATR, KBr pellet, DRIFTS, or mull | Multiple options allow a balance between speed, bulk response, and dilution | Particle size, scattering, and moisture can strongly affect spectra |
| Hard or irregular solids | ATR with sufficient pressure, or microtomed thin section | ATR avoids cutting in many routine checks | Poor contact gives weak or non-reproducible bands |
| Liquids | ATR drop, demountable liquid cell, sealed cell, or fixed path cell | Liquids can often be measured directly; cells provide path length control | Strong absorbers and solvents may require very short paths |
| Aqueous samples | ATR, short path liquid cell, or method-specific accessory | ATR reduces the practical burden of measuring water-containing samples | Water absorption can mask analyte bands |
| Gases and vapors | Gas cell, often with a defined optical path length | Provides a controlled gas-phase measurement | Cell materials, pressure, concentration, and safety controls matter |
For routine unknown identification, ATR usually offers the best first attempt because it is fast and uses little material. For low-concentration components, validated quantitative work, or comparison with reference spectra collected in transmission, a transmission-style preparation may be more appropriate. For coarse powders and heterogeneous solids, DRIFTS can increase sampling volume, but particle size and dilution still need control.
Preparing solids and powders
ATR for solids, powders, and coatings
ATR-FTIR works by pressing the sample against an infrared-transparent crystal. Instead of passing through the full sample thickness, the infrared beam interacts with a shallow region near the sample-crystal interface. Technical guides commonly describe this interaction depth as only a few micrometers, depending on wavelength, crystal material, refractive index, and angle of incidence. This makes ATR especially useful for surfaces, coatings, polymer films, tablets, and many routine identity checks.
The preparation is simple, but it still has to be controlled. A flat, clean, representative area should touch the crystal. Powders need enough pressure to create intimate contact rather than a loose pile. Hard samples should be pressed only within the accessory limits, especially when softer crystal materials such as ZnSe are used. Diamond ATR crystals are widely used because they are robust, but they still require clean handling and compatible solvents.
ATR has two important interpretation limits. First, it is more surface-sensitive than bulk transmission. A coated, oxidized, contaminated, or weathered surface may produce a spectrum that differs from the underlying material. Second, band intensities in ATR are not identical to transmission spectra because penetration depth changes across the infrared range. Many FTIR software packages include ATR correction for library searching, but analysts should still compare spectra measured by the same mode whenever possible.
KBr pellets for transmission spectra
The KBr pellet method remains a classic preparation for powdered solids in transmission FTIR. The sample is finely ground with dry infrared-grade potassium bromide and pressed into a transparent or translucent disc. Bruker, Shimadzu, Agilent, and pharmacopeial sources all describe variations of this general approach. The appeal is that transmission can provide a bulk spectrum with a more direct relationship between absorbance, concentration, and path length than surface ATR measurements.
Typical manufacturer examples use a very small amount of sample relative to KBr. Shimadzu technical guidance describes approximately 0.1 to 1.0 percent sample in 200 to 250 mg alkali halide for a 13 mm pellet, with fine grinding and pressing under vacuum. European Pharmacopoeia guidance has historically described a few milligrams of substance in a few hundred milligrams of dried potassium bromide or potassium chloride for a 10 to 15 mm disc. These values are useful starting points, not universal recipes. Strong absorbers require less sample; weak absorbers may require more, provided the pellet still transmits enough light.
KBr preparation usually fails for practical reasons. KBr is hygroscopic, so moisture introduces broad O-H absorption and can weaken pellets. Coarse particles scatter light and create sloping baselines. Too much sample causes total absorption, where major peaks flatten or disappear into saturated regions. Excessive grinding may also alter sensitive crystalline materials, a limitation recognized in pharmacopeial language that warns analysts to consider possible changes in crystal form.
DRIFTS and mulls when pellets are not the best option
Diffuse reflectance infrared Fourier transform spectroscopy, often shortened to DRIFTS, is another route for powders and rough solids. Thermo Fisher Scientific educational material describes DRIFTS as useful for organic and inorganic samples that can be ground into a fine powder and, when needed, mixed with an infrared-transparent matrix such as KBr. It avoids pellet pressing and can be convenient for hard polymers, tablets, and powder mixtures.
Mulls are older but still useful in selected cases. A finely ground solid is dispersed in a mulling agent, commonly a mineral oil or fluorinated oil, and spread between infrared windows. The benefit is that the sample may be handled without dissolving or pressing it. The drawback is that the mulling agent has its own absorption bands, so the analyst must know which spectral regions are obscured.
Preparing liquids, films, and gases
Liquids can often be measured directly by ATR: place a small drop on the crystal, cover the active area, collect the spectrum, and clean the crystal immediately afterward using a compatible solvent. This works well for many neat liquids and viscous materials. Volatile, flammable, toxic, or reactive liquids require the laboratory’s ventilation and safety procedures. Some FTIR instruments contain hot sources, and manufacturer manuals caution against allowing flammable vapors to accumulate around the sample compartment.
Transmission liquid cells are preferred when path length must be controlled. A demountable cell with windows and spacers can produce short paths for strong absorbers, while fixed-path or sealed cells are useful for reproducibility and volatile samples. The path length must match the absorbance strength. If the path is too long, solvent or analyte peaks saturate. If it is too short, weak bands may disappear into noise. See also: analytical methods.
Films and thin sections require close attention to thickness. Bruker educational guidance notes that transmission measurements are generally practical only when samples are sufficiently thin, and thin films are a common transmission use case. Uniformity is as important as average thickness. A wedge-shaped film can create interference fringes or variable band intensity across the measurement area. For polymers, adhesives, coatings, and packaging layers, ATR may be enough for identification, while transmission or microscopy may be needed to understand layered structure.
Gas-phase FTIR requires a gas cell rather than a solid or liquid accessory. The cell path length, window material, pressure, and concentration determine whether bands are measurable without saturation. Reactive or hazardous gases should only be handled with a validated setup, appropriate purge procedures, and compatible cell materials.
Choosing between ATR and transmission
ATR and transmission are not competing answers to the same problem; they answer slightly different questions. ATR shows what is present near the surface in optical contact with the crystal. Transmission shows what absorbs through the prepared optical path. That difference matters for coated materials, weathered plastics, powders with surface treatment, pharmaceuticals, and samples where the reference library was collected by a specific method.
| Question | ATR is usually stronger when | Transmission is usually stronger when |
|---|---|---|
| How fast must the result be? | Routine identity checks need minimal preparation | Preparation time is acceptable for better path control |
| Is the sample surface representative? | The surface is the target or is chemically uniform | The bulk composition is the target |
| Is quantitative work required? | A validated ATR method controls contact, pressure, and matrix effects | A controlled path length supports Beer-Lambert-style calibration |
| Is the reference spectrum method-specific? | Reference data were collected by ATR or corrected appropriately | Reference data were collected as KBr pellets, films, or liquid cells |
| Is the sample very hard, rough, or granular? | Good crystal contact can be achieved safely | Grinding, dilution, or sectioning gives a more reliable optical path |
An efficient laboratory workflow often begins with ATR for screening. If the ATR spectrum is weak, surface-biased, or inconsistent with the analytical question, the analyst can move to KBr pellet transmission, DRIFTS, a thin film, or a liquid cell. This staged approach saves time without assuming that the fastest method is always the most defensible method.
Quality control and troubleshooting
Good FTIR preparation includes a clean background, a representative sample, and a quick review of spectral quality before interpretation. Background spectra should be collected with the same accessory configuration used for the sample. ATR crystals, salt windows, pellet dies, and liquid cells should be cleaned between measurements to avoid carryover. For hygroscopic salts and moisture-sensitive windows, dry storage and desiccation are part of the method, not optional housekeeping.
- Weak ATR spectrum: improve sample contact, use a flatter area, increase pressure within accessory limits, or measure several spots.
- Sloping baseline in a pellet or DRIFTS spectrum: grind more finely, reduce particle-size differences, or improve dilution with the matrix.
- Saturated peaks: reduce sample thickness, lower sample concentration in KBr, or shorten the liquid cell path length.
- Broad water bands: dry KBr, check sample moisture, use fresh background spectra, and store salts in a desiccator.
- Unexpected extra bands: check for cleaning solvent residue, mulling agent absorption, plasticizers, window contamination, or previous sample carryover.
- Poor reproducibility: standardize pressure, particle size, sample mass ratio, path length, drying time, and the number of replicate spots.
For regulated or quality-critical work, preparation details should be written into the method: accessory type, crystal or window material, sample-to-matrix ratio, drying condition, pressure setting, path length, background procedure, acceptance criteria, and cleaning protocol. Without these details, two spectra may look different because the chemistry changed, or simply because the preparation changed.
Frequently asked questions
Do all FTIR samples need preparation?
No. Many samples can be measured directly by ATR with little preparation beyond cleaning the crystal and making good contact. However, transmission, DRIFTS, mulls, liquid cells, and gas cells still require deliberate preparation when the analytical question needs bulk response, dilution, path length control, or method comparability.
When should I use a KBr pellet instead of ATR?
Use a KBr pellet when a bulk transmission spectrum is needed, when the reference method or library is based on transmission, or when ATR gives a weak or surface-biased result. A KBr pellet is also useful for many powdered solids, but it requires dry KBr, fine grinding, suitable dilution, and careful pressing.
Why do KBr pellets show water interference?
KBr absorbs moisture from the air. Moisture can create broad O-H bands and reduce pellet quality. Drying the KBr, cooling and storing it in a desiccator, minimizing air exposure, and using a fresh background can reduce the problem.
Can ATR-FTIR be used for quantitative analysis?
Yes, but it must be validated for the sample type and purpose. ATR quantitative results depend on contact, pressure, matrix, penetration depth, and surface uniformity. For some applications, transmission or a fixed-path liquid method may provide better control.
What is the most common cause of a poor FTIR spectrum?
The most common causes are excessive sample thickness, poor ATR contact, moisture, scattering from coarse particles, and contamination. Before changing instrument settings, check whether the sample is too concentrated, too rough, too wet, or not in consistent contact with the sampling accessory.
