Infrared Spectroscopy: FTIR and ATR-FTIR

For a simple bond model, vibration frequency depends on force constant and reduced mass. Real molecules have normal modes involving coordinated atomic motion. Absorption occurs when an allowed transition matches the incident photon energy. The mode, rather than an isolated bond label alone, determines infrared activity.

A vibration must change the molecular dipole moment to be active in the usual electric-dipole approximation. A permanent dipole is not required: a molecule can lack one at equilibrium and still have infrared-active modes. Conversely, a particular symmetric mode can be inactive even when the molecule has other active modes.

The ordinary stretching vibrations of homonuclear diatomics such as N2 and O2 are inactive under this rule. That does not make ambient air free of infrared interference. Water vapour and carbon dioxide produce atmospheric features, and differences between background and specimen conditions can leave residual bands.

Wavenumber is reciprocal wavelength, commonly in cm⁻¹. Higher wavenumber means greater photon energy. At 1,000 cm⁻¹ the wavelength is 0.001 cm, or 10 µm. Conventional infrared plots often show wavenumber decreasing from left to right.

A common working mid-IR range is about 4,000–400 cm⁻¹, subject to instrument and sampling limits. Near-IR measurements often involve overtones and combination bands, while far-IR measurements can access lower-frequency motions. These regions have different applications; mid-IR is not the only region relevant to forensic or analytical questions.

Group-frequency interpretation uses bands often associated with functional groups. Their positions and intensities depend on chemical environment, coupling, hydrogen bonding and physical state. A band should be read with the rest of the spectrum, rather than assigned as an exact structure from one lookup value.

Broad O–H stretching often appears around 3,200–3,600 cm⁻¹ for hydrogen-bonded alcohols and related environments. Carboxylic-acid O–H absorption can extend much more broadly and lower, often into roughly 2,500–3,300 cm⁻¹. Water can contribute strongly. N–H and terminal alkyne C–H features can occur nearby, so shape and accompanying bands matter.

Carbonyl stretching frequently occurs in a broad region around 1,650–1,850 cm⁻¹, depending on the functional group and environment. Amides, conjugation and strained structures illustrate why a narrow 1,680–1,750 cm⁻¹ rule misses valid carbonyls. Approximate ranges guide hypotheses; supported reference comparisons test them.

Aliphatic C–H stretching commonly lies below about 3,000 cm⁻¹, with many aromatic or alkenyl C–H stretches above it. Nitrile absorption is often near 2,200–2,300 cm⁻¹, whereas alkyne stretching can be weak and depends on symmetry. These are illustrative assignments, not a complete identification table.

The region below roughly 1,500 cm⁻¹ contains many coupled and skeletal vibrations and is often called the fingerprint region. Related compounds can differ substantially here. However, a similar pattern is not an absolute guarantee of unique identity, particularly for mixtures, incomplete spectra or closely related materials.

Compare candidate references across diagnostically useful regions and investigate unexplained bands or missing features. A mismatch at a carbonyl band cannot automatically be dismissed as contamination while accepting a library hit. It can indicate a wrong candidate, mixture, chemical change, physical-state difference or measurement issue.

In a common Michelson configuration, a beamsplitter divides light between fixed and moving mirror arms. The recombined intensity varies with optical path difference. The detector records an interferogram containing contributions from the spectral range admitted by the source, optics and detector.

A Fourier transform converts the interferogram into spectral information. A background measurement accounts for the instrument path and relevant environmental response. Appropriate ratioing and processing produce a transmission or absorbance spectrum; the transform alone does not automatically remove all background or specimen artefacts.

The multiplex or Fellgett advantage concerns simultaneous acquisition of spectral information and is especially relevant when detector noise dominates. Its benefit is not universal under every noise regime. The throughput or Jacquinot advantage reflects optical throughput compared with slit-limited dispersive arrangements.

A reference laser commonly supports optical-path tracking and wavenumber accuracy. Spectral resolution depends on available optical path difference and processing. Co-adding scans can improve signal-to-noise under suitable conditions, but more scans do not repair saturation, an unrepresentative specimen or a contaminated crystal.

Choose spectral range, resolution and acquisition time for the question. A rapid routine scan and a high-resolution gas measurement need not share settings. There is no universal thirty-second acquisition, fixed sixteen-scan recipe or guaranteed improvement factor for every FTIR instrument.

Transmission measurements pass light through a specimen or preparation. A suitable thickness or concentration avoids excessive absorption. Solids can be diluted in a compatible infrared-transparent medium such as dry KBr and pressed into a pellet, but moisture, scattering, preparation-induced changes and possible chemical interaction need consideration.

A pellet procedure must suit the material and equipment. A single mass, die load or pressing time is not a universal forensic recipe. The preparation can alter the specimen, and a clear-looking pellet does not itself establish chemical integrity or representative sampling.

A mull suspends finely divided material in a liquid such as mineral oil. The mulling medium contributes its own bands and can obscure useful regions. Changing medium can expose some regions while obscuring others. Liquid films, solution cells and gas cells have their own path-length, window-compatibility and background requirements.

ATR directs light through a high-index crystal at an interface supporting total internal reflection. An evanescent field extends into the adjacent specimen and interacts with absorbing material. The field decays with distance; it does not probe a sharply bounded layer of fixed thickness.

Penetration increases with wavelength and also depends on angle and the refractive indices of crystal and specimen. A commonly quoted micrometre-scale depth is only illustrative. Under particular conditions, the depth can exceed 2 µm at low wavenumber, so 0.5–2 µm is not a universal range.

Good contact matters for solid ATR measurements. Surface coatings or contamination may dominate a response that is unrepresentative of the bulk. Pressure can mark or alter fragile material, and recovery is not automatically complete. Assess preservation and cleaning needs for the evidence rather than call every ATR measurement non-destructive.

Crystal selection depends on spectral range, chemical compatibility, mechanical requirements and desired sampling depth. Diamond, germanium and zinc selenide have different properties. ATR and transmission band intensities can differ; compare compatible references and evaluate any ATR correction rather than assume the spectra are identical.

Powders and tablets can give useful infrared patterns, particularly when a component is sufficiently abundant and accessible. Mixtures, coatings, fillers and low-concentration constituents complicate interpretation. Failure to see a drug band does not establish that no drug is present below the method's capability.

In seized-drug work, infrared spectroscopy can provide structural information within an appropriate analytical scheme. SWGDRUG recommendations address technique combinations and their limitations for that field. They do not establish a universal legal rule that all FTIR findings are merely presumptive or that every case must use one particular follow-up instrument.

Paint examinations can compare layer sequence, binders and other components. Microspectroscopy helps target individual layers while limiting contributions from adjacent material. Agreement supports a comparison within its limits; an IR match alone does not uniquely identify a vehicle or establish when paint was transferred.

Fibre and polymer examinations can help distinguish material classes and assess comparisons. Polyester, polyamide, acrylic and cellulosic materials have characteristic patterns, but dyes, finishes and blends can contribute. Polymer classification is different from individual-source identification.

Questioned-document examinations can compare selected binders, coatings or toner components. Sampling location, surface condition and other optical or microscopic findings matter. A shared polymer does not uniquely connect a page to a particular printer.

Explosive or reactive material requires an appropriate safety assessment before handling or pressure-based sampling. A library candidate should be evaluated within a suitable explosives analytical scheme, including substrate and mixture limitations. A rapid FTIR observation is not a blanket instruction to press every unknown powder onto an ATR crystal.

Quantitative infrared methods can use calibrated band measurements or multivariate models. Their performance depends on representativeness, preparation, calibration and validation. FTIR is not inherently restricted to identification, nor is every infrared quantitative method less precise than every UV-Vis method.

A spectral search ranks similarities according to the software's algorithm, preprocessing and reference collection. Scores are not standardised across products. A value of 0.95 in one system cannot be treated as a universal confirmation threshold in another.

Review the candidate spectrum alongside the specimen spectrum and examine distinguishing regions. Ensure appropriate physical state, acquisition range and sampling mode. NIST WebBook gas-phase spectra, for example, are useful reference information but are not interchangeable with every solid ATR spectrum.

Mixture spectra can contain contributions from several components, approximately additive under suitable absorption conditions. Interactions, scattering, ATR effects and processing can complicate this relationship. A top library hit can describe a major component while missing another constituent; it need not identify the whole specimen.

Infrared microscopy combines spatial targeting with spectral measurement. Resolution and sampled volume depend on wavelength, optics, aperture, detector and mode. There is no universal 10 µm resolution for every measurement, and an aperture size is not itself a demonstrated ability to isolate a layer.

Collect suitable backgrounds, check crystal cleanliness and address atmospheric differences. Performance checks can evaluate relevant wavenumber accuracy, resolution and response using suitable references. An acceptable check supports the measured characteristics; it does not guarantee every library assignment.

Report the examined material, analytical basis and justified conclusion with relevant limitations. Distinguish chemical identification, class comparison and source inference. Accreditation and legal admissibility depend on their applicable scope and jurisdiction; neither follows automatically from a library score.

Related reading: Raman Spectroscopy and Comparison with IR.

Related reading: Scanning Electron Microscopy (SEM) with EDS.

Related reading: ISO/IEC 17025 and Quality Management in Forensic Laboratories.