IR spectroscopy sample preparation for reliable FTIR results

October 8, 2026Fitzgerald, Clara

Why sample preparation controls the quality of an IR spectrum

IR spectroscopy sample preparation is a major source of spectral quality, not a minor laboratory task. It determines whether the spectrum mainly represents the sample, the surface, the matrix or a preparation artifact. In routine FTIR work, the first decision is the sampling mode. ATR is often the fastest option for solids, liquids and films because it requires little physical preparation. Transmission methods, including KBr pellets, salt plates, liquid cells and gas cells, remain important when the analyst needs tighter control of path length, bulk composition or comparison with legacy reference spectra. Diffuse reflectance is useful for powders and surface-rich materials, especially when pellet pressing could change the sample.

Mid-infrared measurements are commonly discussed over about 4000 to 400 cm^-1. USP General Chapter <854> describes mid-infrared spectroscopy in this region and gives 3800 to 650 cm^-1 as the default range for certain pharmacopeial compliance uses unless a monograph or validated procedure says otherwise. In practical work, the preparation route should be driven by the spectral window, sample thickness, particle size, water sensitivity and required depth of analysis.

For more method articles in the same category, see the sample preparation section.

Match the preparation method to the sample and question

The common mistake is to ask which method is easiest to run. The better question is which preparation makes the relevant chemical information visible without changing it. A polymer coating, a hydrated salt, a volatile solvent and a catalyst powder may all be suitable for FTIR, but they should not be prepared in the same way.

Sample or objective	Common preparation route	Main advantage	Main limitation to check
Hard solid, film, rubber, adhesive or tablet surface	ATR	Fast, little preparation, small sample amount	Surface-sensitive; contact pressure affects intensity
Fine dry powder for bulk transmission	KBr pellet	Strong transmission spectrum and legacy comparability	KBr is hygroscopic; grinding and pressing may alter some samples
Moisture-sensitive powder or sample unsuitable for KBr	Mineral oil mull or alternative window method	Avoids some pellet problems	Mineral oil absorbs in C-H regions
Thermoplastic or soluble polymer	Pressed, cast or evaporated thin film	Cleaner bulk spectrum when film is thin enough	Film thickness and residual solvent can distort bands
Non-volatile liquid or solution	Liquid cell with suitable windows and spacer	Controlled path length	Window material and solvent absorption must be compatible
Gas or vapor	Gas cell	Defined path length for gaseous samples	Condensation, pressure and resolution requirements
Powder surface, catalyst or sample for heating studies	Diffuse reflectance or DRIFTS	Easier powder pretreatment than pellets and surface-rich response	Particle size, dilution and packing strongly affect intensity

ATR preparation for quick screening and surface analysis

Attenuated total reflectance is often the first choice for routine identification because the sample is placed directly against a high-refractive-index crystal. The infrared beam reflects within the crystal and interacts with a shallow region of the sample at the interface. This makes ATR convenient, but it also means the spectrum can emphasize the surface more than the bulk.

For solids, the practical goal is intimate and reproducible contact. Flat, smooth materials usually give stronger and more stable spectra than rough, curved or granular pieces. Powders can be measured by ATR, but inconsistent packing against the crystal may produce weak or variable bands. Flexible polymers, rubbers and adhesives often need controlled pressure so the same sample area contacts the crystal from run to run.

Good ATR practice

Clean the crystal before the background and again between samples with a solvent that is compatible with the crystal and sample residue.
Use enough pressure for full contact, but do not over-tighten brittle crystals or crush samples in a way that changes them.
Record the crystal type and pressure setting when comparing batches or building an internal library.
Be cautious when comparing ATR spectra with transmission libraries because relative band intensities can differ.
For layered materials, remember that ATR may mainly represent the top layer rather than the full cross-section.

ATR is an excellent screening method, but it is not automatically a universal replacement for transmission. If a target component is below the surface, or if the surface is contaminated, oxidized, coated or enriched, a transmission method, microtome section, extraction, cross-section or complementary technique may be needed.

Transmission preparation for solids, liquids and gases

Transmission FTIR measures radiation that passes through the prepared sample. This usually requires more preparation than ATR, but it gives the analyst more direct control of thickness, dilution and optical path length. It remains valuable for powders, thin films, liquids, gases and regulated or legacy methods that were validated using transmission spectra.

KBr pellets for dry powdered solids

The KBr pellet technique mixes a small amount of finely ground sample with dry infrared-grade potassium bromide, then presses the mixture into a transparent or translucent disc. Alkali halides such as KBr are widely used because they are largely transparent across much of the mid-infrared region. The method works best for dry, stable powders that can be ground without decomposition, polymorphic change or reaction.

Key preparation variables include the sample-to-KBr ratio, dryness, particle size, mixing uniformity and pressing conditions. Too much sample can cause saturated bands and noisy baselines. Too little sample may hide weak diagnostic peaks. Moisture in KBr can introduce broad water features and cloud the pellet. Uneven grinding leaves large particles that scatter light and distort the baseline.

Dry the KBr and keep it in a desiccator until use.
Grind the sample gently to a fine, uniform powder unless the material is pressure- or heat-sensitive.
Mix a small amount of sample thoroughly with KBr; many teaching and laboratory procedures use low percent-level sample loading rather than a concentrated mixture.
Transfer the mixture evenly into a clean die, avoid trapped air and press according to the die and press instructions.
Inspect the pellet for cloudiness, cracks, specks and uneven thickness before collecting the spectrum.

KBr pellets are powerful, but they are not always neutral. Hygroscopic samples, salts that may exchange halides, unstable hydrates and pressure-sensitive pharmaceuticals may require an alternative method or a validated procedure that controls these risks.

Nujol mulls and salt-plate preparations

A mineral oil mull disperses a finely ground solid in a small amount of mineral oil and sandwiches it between infrared-transparent windows. This can be useful when a powder is unsuitable for KBr pellet preparation. However, mineral oil has its own infrared absorption, especially in C-H stretching and bending regions, so it can mask bands near approximately 3000 to 2800 cm^-1 and 1500 to 1300 cm^-1. That limitation should be considered before using a mull for organic materials where C-H information is important.

Salt plates and demountable cells need careful handling. KBr and NaCl windows are moisture-sensitive, scratchable and fragile. A cloudy window, residual film or scratched surface can produce poor baselines and false features. Gloves, clean handling tools and desiccated storage are part of the preparation method, not optional housekeeping.

Thin films for polymers and soluble samples

Thin-film preparation is often suitable for polymers, coatings and soluble solids. Films may be pressed, cast from solution or formed by evaporating solvent on an infrared-transparent window. The film should be thin enough to avoid total absorption in strong bands but thick enough to show weak features. Residual solvent, uneven film thickness, crystallization during drying and thermal history from hot pressing can all alter the spectrum.

Liquid and gas cells

Liquid cells use two infrared-transparent windows separated by a spacer. Path length is critical: a path that is too long can saturate intense bands, while a path that is too short may reduce sensitivity. Window selection must consider water, acids, bases, solvents and sample reactivity. For example, common alkali halide windows are not suitable for aqueous samples.

Gas cells are selected by concentration, path length, pressure, temperature and window material. Low-concentration gases often require longer path lengths, while concentrated gases may need a shorter cell to avoid saturated peaks. Heating may be necessary when condensation could occur. Gas spectra can contain sharp bands, so instrument resolution and background handling matter more than in many routine solid measurements. See also: analytical methods.

Diffuse reflectance preparation for powders

Diffuse reflectance infrared Fourier transform spectroscopy, often called DRIFTS, is widely used for powders and surface-focused studies. Instead of pressing a pellet, the powder is usually placed in a sample cup, sometimes after dilution with KBr or KCl. The infrared radiation interacts with a scattering powder bed, and the collected diffuse reflection can provide information similar in band position to transmission spectra, although relative intensities may differ.

Shimadzu technical materials describe diffuse reflectance as easier in pretreatment than the KBr pellet method for powder samples and note its use in catalyst-related studies, including measurements involving substances on sample surfaces. The practical preparation concerns are particle size, dilution ratio, cup filling, surface leveling and reproducible packing. Coarse particles increase specular reflection and scattering effects. Overpacking, underpacking or an uneven cup surface can change the spectrum even when the chemistry is unchanged.

Diffuse reflectance is not simply powder ATR. It probes a powder bed differently and may require mathematical processing such as Kubelka-Munk transformation when comparing with transmission-like spectra or developing quantitative methods. For identification work, consistent preparation is often more valuable than aggressive data manipulation.

Common preparation errors and how to recognize them

Many poor IR spectra are caused by preparation rather than instrument failure. The following symptoms can help narrow the cause before the sample is rerun.

Observed problem	Likely preparation cause	Practical response
Broad water bands or cloudy KBr pellet	Moist KBr, humid handling or wet sample	Dry KBr, work faster, use desiccation or choose ATR/mull if appropriate
Flat-topped or missing strong peaks	Sample too thick or too concentrated	Reduce film thickness, shorten liquid path length or dilute the pellet/DRIFTS mixture
Sloping or noisy baseline in a pellet or mull	Large particles and scattering	Improve grinding gently and make particle size more uniform
Weak ATR bands	Poor crystal contact or uneven surface	Increase contact consistency, use a flatter area or prepare a thinner section
Unexpected C-H bands in a mull	Mineral oil contribution	Use an alternative mull medium or a different sample presentation
Different spectra from the same material	Surface heterogeneity, pressure differences or inconsistent path length	Standardize pressure, thickness, sampling point and background collection

A useful quality habit is to record the sampling accessory, crystal or window material, pressure setting, path length or spacer, dilution ratio, background method and any drying or grinding step. These details allow another analyst to reproduce the spectrum and help explain differences between instruments or laboratories.

How to choose a defensible workflow

A defensible preparation workflow starts with the analytical question and ends with documentation. For simple identification of a clean, homogeneous solid, ATR may be sufficient. For comparison with a validated transmission library, a KBr pellet or film may be the better match. For a powder catalyst, DRIFTS may preserve surface information that pellet pressing could obscure. For regulated testing, the validated method controls the choice even if another method is faster.

Use this short decision sequence before preparing the sample:

Define the target information: surface, bulk, trace contaminant, functional group, gas composition or batch comparison.
Check whether the sample is altered by grinding, pressure, heat, water, solvent or air exposure.
Select the sampling mode that answers the question with the least chemical change.
Control the variables that affect intensity: contact, thickness, concentration, path length, particle size and packing.
Run a background that matches the accessory and matrix as closely as practical.
Document the preparation in enough detail for repetition.

The most reliable IR results rarely come from the most elaborate preparation. They come from a preparation that is appropriate, repeatable and clear about its limitations.

Frequently asked questions

Is ATR always better than KBr pellet preparation?

No. ATR is usually faster and easier, but it is more surface-sensitive and can produce relative intensities that differ from transmission spectra. KBr pellets can be better for dry powdered solids when a bulk transmission spectrum or legacy comparison is needed.

Why does moisture cause problems in KBr pellets?

KBr is hygroscopic, meaning it absorbs moisture from the air. Moisture can cloud the pellet and introduce water absorption features, which may interfere with interpretation. Dry storage, fast handling and a suitable background reduce the problem.

When should a Nujol mull be avoided?

A Nujol mull should be avoided when the bands of interest overlap with mineral oil absorption, especially important C-H regions. It may still be useful for solids that are not suitable for KBr pellets, provided its spectral contribution is understood.

Why do ATR and transmission spectra of the same sample look different?

ATR samples a shallow region at the crystal-sample interface, while transmission measures radiation passing through the prepared sample. Differences in penetration depth, contact, refractive index effects, thickness and preparation can change relative band intensities.

What should be recorded in an IR sample preparation note?

Record the sampling mode, accessory, crystal or window material, sample amount or dilution, film thickness or path length when known, pressure setting, drying or grinding step, background method and any visible sample changes. These details make the spectrum more useful for comparison and troubleshooting.