IR sample preparation guide for ATR, transmission, and KBr pellets
Choosing the right IR sample preparation route
IR sample preparation controls how an infrared spectrum is produced in practice: sample form, optical geometry, thickness, concentration, contact pressure, background, and handling conditions. For routine identification, ATR is often the fastest starting point because many solids, powders, gels, and liquids can be measured directly after the crystal is cleaned. For bulk information, transmission library matching, low-intensity bands, quantitative work, or method-driven quality control, transmission approaches such as KBr pellets, thin films, liquid cells, or gas cells may be more suitable.
The practical question is not which technique is universally better. It is which preparation makes the spectrum representative, repeatable, and comparable with the reference data or method being used.
For related laboratory handling topics, visit the sample preparation section.
Start with the analytical objective, not the accessory
Many preparation problems start when a laboratory chooses the quickest accessory before defining what the spectrum must prove. Infrared spectroscopy may be used to identify functional groups, compare unknowns with reference spectra, monitor reactions, support incoming material checks, or quantify a component under a validated method. Each purpose places different demands on sample preparation.
If the goal is rapid screening, a clean ATR crystal and consistent pressure may be sufficient. If the goal is to compare an unknown with an older transmission library, a KBr pellet or film may give a more comparable spectrum than ATR. If the goal is quantitative trending, uncontrolled sample thickness, uneven particle size, or changing contact pressure can create more variation than the chemical difference being measured.
Standards and technical references reinforce this point. ASTM E1252 describes general techniques for obtaining infrared spectra for qualitative analysis across liquid, solid, and vapor-phase samples. ASTM E573 focuses on internal reflection spectroscopy. ISO 19087:2018, for respirable crystalline silica by FTIR, shows how sample collection, treatment, and measurement geometry can become part of the method itself. In regulated or method-driven work, the preparation route is not just a convenience choice; it is part of the analytical definition.
How major IR sample preparation methods compare
The table below summarizes common preparation routes and the trade-offs that usually matter in an FTIR laboratory.
| Preparation method | Typical sample fit | Main advantage | Main limitation |
|---|---|---|---|
| ATR | Solids, powders, films, gels, many liquids | Fast measurement with minimal preparation | Spectrum is dominated by the shallow contact region and depends on crystal contact |
| KBr pellet | Dry powders and finely ground solids | Transmission spectrum suitable for many reference comparisons | Requires dry matrix, grinding, dilution, and pressing |
| Thin film | Polymers, coatings, evaporated solutions | Useful for bulk polymer or coating spectra when thickness is controlled | Overly thick films saturate strong bands |
| Liquid film or liquid cell | Neat liquids and solutions | Defined path length is possible with suitable spacers or cells | Window material, solvent bands, bubbles, and path length must be controlled |
| DRIFTS | Powders, catalysts, rough or scattering solids | Less dependence on direct ATR contact | Particle size, packing, dilution, and reflectance behavior affect spectra |
| Gas cell | Vapors and gases | Controlled optical path for gas-phase analysis | Requires appropriate cell path length, sealing, and background handling |
ATR is attractive because it removes many preparation steps. It is not, however, a universal replacement for transmission. ATR spectra can differ in relative band intensities from transmission spectra, especially when strong absorption bands, surface coatings, fillers, or layered materials are involved. When a result is compared with a database or certificate spectrum, the measurement mode used to generate the reference matters.
ATR preparation for quick but repeatable spectra
ATR preparation appears simple: clean the crystal, place the sample, apply pressure if needed, collect a background and sample spectrum, then clean the crystal again. The hidden variable is contact. A hard plastic chip, a soft rubber, a fine powder, and a viscous liquid can all contact the crystal differently. Poor contact can weaken bands, distort relative intensities, or make replicate spectra less consistent than expected.
For solids, use a flat area where possible and apply the same pressure setting for comparable samples. For powders, cover the crystal evenly and avoid large particles that prevent intimate contact. For liquids, use enough material to cover the active area without overflow, and confirm that the liquid does not attack the crystal, seal, or accessory materials. Diamond ATR is robust for many routine samples, while germanium and ZnSe accessories have different chemical resistance and penetration behavior.
ATR is especially useful when the sample is valuable, difficult to grind, or likely to change during pellet pressing. It is also practical for surface questions, such as whether a film, contaminant, coating, or residue is present. Its limitation is tied to the same feature: ATR emphasizes the surface or near-surface region rather than a long bulk path through the material.
KBr pellets and transmission preparation
The KBr pellet method remains important because it produces a transmission spectrum from a diluted solid dispersed in an infrared-transparent salt matrix. Public manufacturer guidance commonly recommends finely grinding a small amount of sample with dry, infrared-grade KBr, then pressing the mixture into a transparent or translucent pellet. Shimadzu technical guidance gives typical examples such as about 0.1 to 1.0 percent sample in KBr for 13 mm pellets, with dry KBr stored in a desiccator and handled to reduce moisture uptake. Exact quantities should follow the laboratory method, pellet die size, and expected absorbance.
The critical variables are dryness, particle size, dilution, mixing uniformity, pellet clarity, and background choice. KBr is hygroscopic, so moisture can add broad O-H related absorption and raise the baseline. Coarse particles scatter light and create sloping baselines. Too much sample can saturate strong bands; too little sample can leave weak bands buried in noise. In some workflows, a KBr blank pellet may correct scattering and moisture effects more closely than an empty holder background.
Transmission methods also include pressed or cast polymer films, liquid films between salt windows, solutions in liquid cells, and gas cells. For liquids and solutions, path length is the central preparation variable. Strongly absorbing samples may need a very short path length, while dilute solutions may need a longer path or a concentration step. Salt windows such as NaCl or KBr are not compatible with water and some corrosive materials, so the cell material must match the sample chemistry. See also: buying guides.
Match preparation to sample type
Powders and crystalline solids
Powders can be measured by ATR, KBr pellet, mull, or DRIFTS. ATR is efficient when the powder packs well against the crystal. KBr pellets are useful when a transmission spectrum is needed, but the sample must tolerate grinding and pressure. DRIFTS can be useful for high-surface-area powders, catalysts, and materials that are difficult to press or contact consistently on ATR.
Polymers, films, and coatings
Polymer films may be measured directly by transmission if they are thin enough, or by ATR if surface information is acceptable. Thick films often produce saturated transmission bands, while layered films can produce ATR spectra dominated by the top layer. If the objective is bulk polymer identification, a controlled thin section or pressed film may be more representative than a quick surface reading.
Liquids, gels, and pastes
Neat liquids are often straightforward by ATR, but volatile liquids, water-rich samples, and aggressive solvents require careful accessory selection. Liquid transmission cells are appropriate when path length control is needed. Avoid trapped bubbles, uneven films, and carryover from previous samples, because each can create false peaks or unstable baselines.
Unknown contaminants and trace residues
Small contaminants require a preparation route that preserves the material and minimizes background interference. Micro-ATR can be effective for particles large enough to contact the crystal. If the contaminant is embedded in a surface, scraping, solvent extraction, or micro-compression may be needed, but each step can change what is being measured. In this situation, document every handling step because interpretation depends on preparation history.
Common preparation errors and how to reduce them
| Problem | Likely preparation cause | Practical correction |
|---|---|---|
| Broad water bands or unstable baseline | Moist KBr, humid handling, wet sample, or poor purge | Dry compatible materials, use a desiccator, collect a fresh background, and minimize exposure time |
| Saturated absorption bands | Sample too thick, too concentrated, or path length too long | Dilute the pellet mixture, make a thinner film, or reduce liquid cell path length |
| Weak or missing bands in ATR | Poor crystal contact or measurement of only a surface layer | Improve contact, grind powder finer if appropriate, increase pressure consistency, or use transmission |
| Sloping baseline in transmission | Scattering from coarse particles or cloudy pellets | Improve grinding, mixing, pellet clarity, and blank correction |
| Unexpected extra peaks | Carryover, window contamination, mulling oil, solvent residue, or sample reaction | Clean accessories, run blanks, use fresh preparation materials, and confirm with a second geometry |
| Mismatch against a reference library | ATR spectrum compared with transmission reference, or different sample state | Use the same measurement mode where possible or apply appropriate correction and caution |
Some materials can also react or change during preparation. Shimadzu application discussions of hydrochloride samples, for example, show why KBr pellets may not be ideal for every salt form because ion exchange or pressure-related changes can distort the spectrum. If chemistry, hydration state, polymorphism, or pressure sensitivity is a concern, confirm the result with ATR, KCl pellets, or another validated approach rather than relying on one preparation route.
A practical decision workflow for the lab
- Define the question: identification, comparison, quantitation, contaminant screening, or method compliance.
- Check the reference source: library spectrum, certificate, standard method, or internal SOP.
- Match the geometry: avoid comparing ATR and transmission spectra as if they were identical unless the method allows it.
- Control the physical variables: thickness, dilution, particle size, pressure, contact, path length, background, and cleaning.
- Record the preparation details: sample mass or approximate ratio, matrix, window material, ATR crystal, pressure setting, drying step, and replicate strategy.
- Use a second preparation only when it adds information, such as confirming surface contamination versus bulk composition.
This workflow helps prevent a common mistake: treating sample preparation as a minor step after instrument setup. In IR spectroscopy, the preparation is part of the measurement. A clean, repeatable, well-documented preparation often improves interpretation more than collecting additional scans on a poorly prepared sample.
Frequently asked questions
Is ATR enough for most IR sample preparation?
ATR is enough for many routine identification tasks, especially when the sample contacts the crystal well and the reference data are also ATR-based. It may be insufficient when the question requires bulk transmission information, controlled path length, weak bands, or comparison with transmission spectra.
How much sample should be used in a KBr pellet?
There is no single amount for every material because absorption strength differs by compound. Manufacturer guidance often uses low sample percentages in KBr and then adjusts to avoid saturated or noisy spectra. The practical target is a pellet that is uniform and transparent enough to give strong but not saturated bands.
Why does a KBr pellet show water-related absorption?
KBr readily takes up moisture from the air, and many samples are also hygroscopic. Drying compatible KBr, storing it in a desiccator, preparing pellets quickly, and using appropriate blanks can reduce water interference, although water-rich samples may require a different method.
Can ATR spectra be compared with transmission library spectra?
They can be compared cautiously, but relative intensities and band shapes may differ because ATR and transmission sample the material differently. For critical identification, use a reference collected in the same geometry or confirm with a second method.
When should DRIFTS be considered?
DRIFTS is useful for powders, rough solids, catalysts, and materials that do not make reliable ATR contact or do not form good pellets. It still requires control of particle size, packing, dilution, and background, so it should be treated as a defined preparation method rather than a shortcut.
