By SpectralBench Editorial Team14 min read
Spectroscopy Glossary
A comprehensive reference of spectroscopy terms covering FTIR, Raman, UV-Vis, NIR, and chemometrics. Each definition is written for practitioners — clear enough for a student encountering the term for the first time, precise enough for a working scientist. Search for any term or browse alphabetically. Definitions link to relevant SpectralBench tools and learn articles for deeper exploration.
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- Absorbance
- A dimensionless quantity defined as A = −log₁₀(T), where T is transmittance. Absorbance is linearly proportional to concentration via the Beer-Lambert law (A = εlc), making it the preferred representation for quantitative spectroscopy.→ absorbance transmittance reflectance, beer lambert law
- Absorption band
- A region of the spectrum where a molecule absorbs radiation. Each band corresponds to a specific molecular transition — vibrational (IR), electronic (UV-Vis), or rotational (microwave). In Raman spectroscopy, bands arise from inelastic scattering rather than absorption.
- Amide bands
- Characteristic infrared and Raman bands arising from the peptide bond (C=O and N-H vibrations). Amide I (~1650 cm⁻¹) primarily reflects C=O stretching and is sensitive to protein secondary structure. Amide II (~1550 cm⁻¹) arises from N-H bending coupled with C-N stretching.→ ftir interpretation
- Anti-Stokes scattering
- Raman scattering where the emitted photon has higher energy than the incident photon. The molecule starts in an excited vibrational state and transfers energy to the photon. Anti-Stokes lines are weaker than Stokes lines because fewer molecules occupy excited states at thermal equilibrium.→ raman interpretation
- Apodization
- A mathematical function applied to an interferogram before Fourier transformation to reduce spectral artifacts (sidelobes) around sharp peaks. Common functions include Happ-Genzel, triangular, and Blackman-Harris. Stronger apodization reduces sidelobes at the cost of spectral resolution.
- ATR (Attenuated Total Reflectance)
- An FTIR sampling technique where the infrared beam reflects inside a high-refractive-index crystal (diamond, ZnSe, germanium) while an evanescent wave penetrates 0.5–5 μm into the sample in contact with the crystal surface. ATR requires minimal sample preparation and is the most common FTIR sampling mode.→ ftir interpretation
- Auxochrome
- A substituent group (-OH, -NH₂, -OCH₃) that does not itself strongly absorb in the UV-Vis region but modifies the absorption of the chromophore it is attached to, typically by shifting the absorption wavelength or changing its intensity.→ uv vis spectroscopy explained
- Baseline correction
- A preprocessing step that removes the non-chemical background signal from a spectrum (sloping baselines, curved backgrounds from scattering or fluorescence). Methods include polynomial fitting, rubber-band correction, and asymmetric least squares.→ spectral-preprocessing
- Bathochromic shift
- A shift of an absorption band to longer wavelength (lower energy), also called a red shift. Caused by extended conjugation, increased solvent polarity (for π→π* transitions), or electron-donating substituents.→ uv vis spectroscopy explained
- Beer-Lambert law
- The fundamental equation of quantitative spectroscopy: A = εlc, where A is absorbance, ε is molar absorptivity (L mol⁻¹ cm⁻¹), l is path length (cm), and c is concentration (mol/L). Valid for dilute, non-scattering samples measured with monochromatic light.→ beer lambert law, beer-lambert-calculator
- Blackbody radiation
- Thermal electromagnetic radiation emitted by an idealized perfect absorber. The spectral distribution follows Planck's law and depends only on temperature. Relevant as the IR source in some spectrometers (globar) and as a background signal in high-temperature measurements.
- Bouguer-Lambert law
- The predecessor to the Beer-Lambert law, stating that the attenuation of light through a material is exponential with path length: I = I₀ × e^(−αl). Beer later added the concentration dependence.→ beer lambert law
- Calibration curve
- A graph plotting measured absorbance (or other spectral property) versus known concentration for a series of standards. The slope equals εl (from Beer-Lambert). Unknown concentrations are determined by interpolating from the curve.
- Chemometrics
- The application of mathematical and statistical methods to chemical data. In spectroscopy, it encompasses multivariate analysis techniques (PCA, PLS, classification) used to extract information from complex spectral datasets.→ introduction to chemometrics
- Chromophore
- The part of a molecule responsible for absorption of UV-Vis light. Contains electrons that undergo electronic transitions when excited. Examples: C=C, C=O, N=N, aromatic rings, and conjugated systems.→ uv vis spectroscopy explained
- Combination band
- An absorption band arising from the simultaneous excitation of two or more different vibrational modes. The frequency is approximately the sum (or, for difference bands, the difference) of the contributing fundamentals. Combination bands are prominent in the NIR region.→ nir spectroscopy explained
- Correlation chart
- A visual reference showing the characteristic wavenumber ranges for common functional groups. Used for quick peak assignment in FTIR and Raman spectroscopy.→ ftir spectrum table, raman common shifts
- D band
- A Raman band near 1350 cm⁻¹ in carbon materials, arising from a disorder-activated breathing mode of sp² carbon rings. The D/G intensity ratio (I_D/I_G) is a standard measure of disorder and defect density in graphene, carbon nanotubes, and carbon fibers.→ raman interpretation
- Derivative spectroscopy
- Taking the first or second mathematical derivative of a spectrum. First derivatives eliminate constant baseline offsets; second derivatives resolve overlapping peaks and remove linear baseline slopes. Savitzky-Golay is the standard algorithm for computing spectral derivatives.→ spectral-preprocessing
- Diffuse reflectance
- Light reflected in multiple directions from a rough or powdered surface after penetrating and being partially absorbed. Measured in DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy). The Kubelka-Munk function linearizes diffuse reflectance for quantitative analysis.→ absorbance transmittance reflectance
- DRIFTS
- Diffuse Reflectance Infrared Fourier Transform Spectroscopy. An FTIR technique for measuring powders, rough surfaces, and other scattering samples by collecting the diffusely reflected infrared light.→ absorbance transmittance reflectance
- Electromagnetic spectrum
- The full range of electromagnetic radiation, from gamma rays to radio waves. Spectroscopy techniques operate in specific regions: UV-Vis (190–800 nm), NIR (780–2500 nm), mid-IR (2500–25000 nm / 4000–400 cm⁻¹), Raman (scattered light from any excitation wavelength).→ which spectroscopy method
- Evanescent wave
- An exponentially decaying electromagnetic field that extends beyond the surface of total internal reflection. In ATR spectroscopy, the evanescent wave penetrates a few micrometers into the sample, enabling surface-sensitive analysis without transmission through the bulk.
- Extinction coefficient
- See molar absorptivity. The two terms are used interchangeably, though IUPAC recommends 'molar absorption coefficient.'→ beer lambert law
- Fermi resonance
- An interaction between a fundamental vibration and an overtone or combination band that are close in frequency and of the same symmetry. The two bands repel each other in frequency and share intensity, producing a characteristic doublet. Example: the CO₂ doublet at 1285/1388 cm⁻¹ in Raman.
- Fingerprint region
- The spectral region below ~1500 cm⁻¹ (IR) or 400–1800 cm⁻¹ (Raman) where complex patterns of C-C, C-O, C-N stretches and bending modes create a unique 'fingerprint' for each molecule. Peaks here are diagnostic but hard to assign to specific groups.→ ftir interpretation, raman interpretation
- Fluorescence
- Emission of light by a molecule after absorbing photons. In Raman spectroscopy, fluorescence is the most common interference — it produces a broad, intense background that can overwhelm the much weaker Raman signal. Using longer-wavelength lasers (785 nm, 1064 nm) reduces fluorescence.→ raman interpretation
- Fourier transform
- A mathematical operation that converts an interferogram (intensity vs. mirror displacement) into a spectrum (intensity vs. frequency/wavenumber). The basis of FTIR spectroscopy, which measures all frequencies simultaneously using an interferometer.
- FTIR
- Fourier Transform Infrared spectroscopy. An IR technique that uses an interferometer to measure all infrared wavelengths simultaneously, producing higher throughput, better wavelength accuracy, and faster measurements compared to dispersive IR instruments.→ ftir interpretation, ftir-peak-identifier
- Functional group
- A specific arrangement of atoms within a molecule that is responsible for characteristic chemical and spectroscopic properties. In FTIR, functional groups produce absorptions at predictable wavenumber ranges (e.g., O-H at 3200–3600 cm⁻¹, C=O at 1650–1800 cm⁻¹).→ ftir spectrum table
- G band
- A Raman band near 1580 cm⁻¹ in carbon materials, arising from the E₂g phonon mode of sp² carbon atoms. Present in graphite, graphene, and carbon nanotubes. The G band position shifts with strain, doping, and temperature.→ raman interpretation
- Group frequency
- A vibrational frequency characteristic of a particular functional group that appears at roughly the same position regardless of the rest of the molecule. Examples: C=O stretch near 1715 cm⁻¹, O-H stretch near 3400 cm⁻¹.→ ftir spectrum table, raman common shifts
- Hydrogen bonding
- An intermolecular (or intramolecular) interaction between an electronegative atom and a hydrogen atom bonded to another electronegative atom. In IR spectroscopy, hydrogen bonding broadens and shifts O-H and N-H stretching bands to lower frequencies, sometimes by hundreds of cm⁻¹.
- Hyperchromic effect
- An increase in the molar absorptivity (ε) of a UV-Vis absorption band, resulting in a more intense absorption. Often caused by changes in molecular geometry that alter the transition dipole moment.→ uv vis spectroscopy explained
- Hypsochromic shift
- A shift of an absorption band to shorter wavelength (higher energy), also called a blue shift. Caused by electron-withdrawing substituents or, for n→π* transitions, by increased solvent polarity.→ uv vis spectroscopy explained
- Interferogram
- The raw signal from an FTIR spectrometer — a plot of detector intensity versus mirror displacement (optical path difference). Fourier transformation converts it into the familiar frequency-domain spectrum.
- JCAMP-DX
- Joint Committee on Atomic and Molecular Physical Data – Data Exchange format. An IUPAC standard text-based format for spectral data exchange. Files use .jdx or .dx extensions and contain both metadata and spectral data in human-readable ASCII format.→ spectral file formats, convert spectral files
- KBr pellet
- A sample preparation technique for FTIR transmission measurements. The sample is ground with potassium bromide (KBr, which is IR-transparent) and pressed into a thin, transparent disc. Largely replaced by ATR for routine analysis.
- Kubelka-Munk function
- A mathematical transform for diffuse reflectance data: f(R) = (1-R)²/(2R) = K/S, where K is the absorption coefficient and S is the scattering coefficient. Analogous to the Beer-Lambert law for transmission, it linearizes the relationship between reflectance and concentration.→ absorbance transmittance reflectance
- Lambert's law
- States that the fraction of light absorbed by a material is independent of the incident intensity. More precisely: each equal thickness of material absorbs an equal fraction of the light passing through it, leading to exponential attenuation with path length.→ beer lambert law
- Laser
- Light Amplification by Stimulated Emission of Radiation. In Raman spectroscopy, lasers provide the monochromatic excitation light. Common wavelengths: 532 nm (green), 633 nm (red), 785 nm (NIR), and 1064 nm (Nd:YAG).→ raman interpretation
- Mean centering
- A preprocessing step that subtracts the mean spectrum from each spectrum in a dataset. This centers the data around zero, which is required for PCA and other multivariate methods to work correctly.→ introduction to chemometrics
- Molar absorptivity
- An intrinsic property of a substance at a given wavelength, symbol ε, with units of L mol⁻¹ cm⁻¹. It quantifies how strongly the substance absorbs light. Also called the molar extinction coefficient; IUPAC recommends 'molar absorption coefficient.' Used in the Beer-Lambert law: A = εlc.→ beer lambert law
- Monochromator
- An optical device that selects a narrow band of wavelengths from broadband light. Uses a diffraction grating or prism. Found in dispersive UV-Vis and IR instruments (but not FTIR, which uses an interferometer instead).
- MSC (Multiplicative Scatter Correction)
- A preprocessing method that corrects for baseline offsets and slope variations caused by scattering differences between samples (e.g., particle size effects in powders). Each spectrum is regressed against a reference spectrum and the slope/intercept are removed.→ spectral-preprocessing, introduction to chemometrics
- NIR (Near-Infrared)
- The spectral region from 780–2500 nm (12,800–4,000 cm⁻¹) between the visible and mid-IR regions. NIR spectroscopy measures overtone and combination bands of molecular vibrations. Requires chemometric calibration for quantitative analysis.→ nir spectroscopy explained
- Normalization
- Scaling spectra to make them comparable. Common methods: min-max normalization (scale to 0–1), vector normalization (divide by the Euclidean norm), and area normalization (scale so the integrated area equals 1).→ spectral-preprocessing
- Overtone
- A vibrational transition from the ground state to the second (first overtone), third (second overtone), or higher vibrational level. Overtones appear at approximately 2×, 3×, etc. the fundamental frequency, with rapidly decreasing intensity. They are the primary features in NIR spectroscopy.→ nir spectroscopy explained
- Path length
- The distance light travels through a sample, symbol l, typically measured in centimeters. Standard UV-Vis cuvettes have a 1 cm path length. In the Beer-Lambert law (A = εlc), path length is one of three factors determining absorbance.→ beer lambert law
- PCA (Principal Component Analysis)
- An unsupervised multivariate method that decomposes a data matrix into orthogonal components (principal components) ordered by variance explained. Used to visualize patterns, detect outliers, and reduce dimensionality before building predictive models.→ introduction to chemometrics
- Peak
- A local maximum in a spectrum corresponding to a specific absorption (IR), scattering (Raman), or emission event. Peak position, intensity, and width are the three primary observables used for spectral interpretation.
- PLS (Partial Least Squares)
- A supervised multivariate regression method that finds the spectral components most correlated with a target property (e.g., concentration). The standard method for building quantitative calibration models from spectral data, especially in NIR.→ introduction to chemometrics
- Polarizability
- A measure of how easily the electron cloud of a molecule is distorted by an external electric field. A vibration is Raman-active if it changes the polarizability of the molecule. Symmetric vibrations of polarizable bonds (C=C, S-S, aromatic rings) are strong Raman scatterers.→ raman interpretation
- Preprocessing
- Mathematical transformations applied to raw spectral data before analysis. Removes physical artifacts (baseline drift, noise, scatter) and enhances chemical information. Common steps: smoothing, baseline correction, normalization, derivatives.→ spectral-preprocessing, introduction to chemometrics
- Raman scattering
- Inelastic scattering of light by a molecule, where the scattered photon has a different energy than the incident photon. The energy difference corresponds to a vibrational mode of the molecule. Raman scattering is very weak (~1 in 10⁷ photons) compared to Rayleigh scattering.→ raman interpretation
- Raman shift
- The difference in wavenumber between the incident laser and the scattered light, measured in cm⁻¹. Raman shift is independent of the laser wavelength used, so spectra from different instruments are directly comparable.→ raman interpretation, raman common shifts
- Rayleigh scattering
- Elastic scattering of light where the scattered photon has the same energy as the incident photon. It is the dominant scattering process and carries no spectroscopic information. In Raman spectrometers, Rayleigh scattering is removed by a notch or edge filter.→ raman interpretation
- Reflectance
- The fraction of incident light reflected from a sample surface. May be specular (mirror-like, from smooth surfaces) or diffuse (scattered, from rough surfaces). Along with transmittance and absorbance, one of the three primary modes of light-matter interaction.→ absorbance transmittance reflectance
- Resolution
- The ability to distinguish two closely spaced spectral features. In FTIR, resolution is determined by the maximum mirror travel in the interferometer (typically 2–16 cm⁻¹ for routine work). Higher resolution requires longer acquisition time and produces more noise.
- Savitzky-Golay filter
- A digital smoothing algorithm that fits successive subsets of data points with a low-degree polynomial. It preserves peak shape and position better than simple moving averages. Also used to compute spectral derivatives. Named after Abraham Savitzky and Marcel Golay (1964).→ spectral-preprocessing
- Selection rules
- Quantum mechanical rules that determine whether a particular transition is allowed (produces a spectral feature) or forbidden. For IR: a vibration must change the dipole moment. For Raman: a vibration must change the polarizability.→ raman interpretation, ftir interpretation
- SERS
- Surface-Enhanced Raman Spectroscopy. Raman scattering enhanced by factors of 10⁶–10¹⁰ when molecules are adsorbed on nanostructured metal surfaces (gold, silver). Enables trace-level detection of analytes that are too dilute for conventional Raman.→ which spectroscopy method
- Signal-to-noise ratio (SNR)
- The ratio of the signal intensity to the noise level in a measurement. Higher SNR means cleaner spectra and more reliable peak detection. SNR improves with the square root of the number of scans (co-additions) and with longer integration times.→ signal-to-noise-calculator
- Smoothing
- A preprocessing step that reduces high-frequency noise in a spectrum. Common methods include Savitzky-Golay polynomial smoothing, moving average, and Gaussian convolution. Over-smoothing can distort peak shapes and reduce spectral resolution.→ spectral-preprocessing
- SNV (Standard Normal Variate)
- A row-wise preprocessing method that centers each spectrum to zero mean and scales to unit variance. Corrects for baseline offsets and multiplicative scatter effects caused by particle size differences. Applied independently to each spectrum.→ spectral-preprocessing, introduction to chemometrics
- SPC format
- A binary spectral data format originally developed for GRAMS software and widely used by Thermo Fisher. Supports single and multi-trace spectra with embedded metadata. Files use the .spc extension.→ spectral file formats, convert spectral files
- Stokes scattering
- Raman scattering where the emitted photon has lower energy than the incident photon. The molecule absorbs energy from the photon and transitions to a higher vibrational state. Stokes lines are the stronger component of the Raman spectrum and are what standard Raman spectra display.→ raman interpretation
- Stray light
- Unwanted light that reaches the detector without passing through the sample at the intended wavelength. Sets the practical upper limit of measurable absorbance (typically A ≈ 2–3). The dominant instrumental limitation for quantitative UV-Vis measurements.→ beer lambert law
- Transmittance
- The fraction of incident light that passes through a sample: T = I/I₀. Ranges from 0 (complete absorption) to 1 (no absorption). Percent transmittance (%T = 100 × T) is the traditional display mode for FTIR spectra. Related to absorbance by A = −log₁₀(T).→ absorbance transmittance reflectance
- UV-Vis spectroscopy
- Spectroscopy in the ultraviolet (190–400 nm) and visible (400–800 nm) regions. Measures electronic transitions in molecules. The primary application is quantitative concentration measurement via the Beer-Lambert law.→ uv vis spectroscopy explained
- Wavenumber
- The number of wave cycles per centimeter, symbol ν̃, measured in cm⁻¹ (reciprocal centimeters). Defined as ν̃ = 1/λ (cm). Directly proportional to energy (E = hcν̃). The preferred unit in FTIR and Raman spectroscopy because of its linear energy relationship.→ wavenumber vs wavelength, wavenumber-to-wavelength-calculator
- Zero-filling
- Adding zeros to the end of an interferogram before Fourier transformation. This interpolates additional data points in the resulting spectrum, producing smoother-looking peaks. It does not increase true spectral resolution but improves the visual appearance and peak position determination.