FT-IR Spectroscopy Definitions of Characteristics

100% Line: Calculated by taking the ratio of two background spectra under identical conditions. Ideally, the result is a flat line at 100% transmittance.

Absorbance: Units used to measure the amount of IR radiation absorbed by a sample. Absorbance is commonly used as the Y axis units in IR spectra. Absorbance is defined by Beer’s Law, and is linearly proportional to concentration.

Aliasing: If frequencies above the Nyquist Frequency are not filtered out, that energy will appear as spectral artifacts below the Nyquist Frequency. Optical and electronic anti-aliasing can be used to prevent this. Sometimes higher frequencies are said to be “folded” back, so the term “folding” is used.

Angular Divergence: The spreading out of an infrared beam as it travels through the FT-IR instrument. Angular divergence contributes to noise in high resolution spectra, and can be a limit to achievable resolution.

Apodization Functions: Functions used to multiply an interferogram to reduce the amount of side-lobes in a spectrum. Different types of apodization functions include boxcar, triangle, Beer-Norton, Hanning, and Bessel. The use of apodization functions unavoidably reduces the resolution of a spectrum.

ATR: Abbreviation which stands for Attenuated Total Reflectance, a reflectance sampling technique. In ATR, infrared radiation impinges on a prism of infrared transparent material of high refractive index. The total internal reflectance-based design assures that the light reflects off the surface of the crystal at least once before leaving it. The infrared radiation sets up an evanescent wave which penetrates a small distance above and below the crystal surface. Samples brought into contact with the surface will absorb the evanescent wave, giving rise to an infrared spectrum. This sampling technique is useful for liquids, polymer films, and semisolids.

Background Spectrum: A single beam spectrum acquired without a sample in the infrared beam. The purpose of a background spectrum is to measure the signal contribution of the instrument and environment to the spectrum. These effects are removed from a sample spectrum by taking the ratio of the sample beam spectrum to the background spectrum.

Baseline Correction: A spectral manipulation technique used to correct spectra with sloped or varying baselines. The user must draw a function parallel to the baseline, then this function is subtracted from the spectrum.

Boxcar Truncation: Without apodization, all points in an interferogram are given equal weight, up to the edges of the interferogram. If the resolution is less than the smallest line width in the spectrum, oscillations appear on the baseline on both sides of the peaks.

Centerburst: The sharp, intense part of an interferogram. The size of the centerburst is directly proportional to the amount of infrared radiation striking the detector.

Coadding: The process of adding interferograms together in order to improve the signal-to-noise ratio.

Collimation: The ideal input beam is a cylinder of light. No beam of finite dimensions can be perfectly collimated; at best there is a diffraction limit. In practice the input beam is a cone that is determined by the source size or aperture used. The degree of collimation can affect the S/N and the resolution.

Constructive Interference: A phenomenon that occurs when two waves occupy the same space and are in phase with each other. Since the amplitudes of waves are additive, the two waves will add together to give a resultant wave which is more intense than either of the individual waves.

Destructive Interference: A phenomenon that occurs when two waves occupy the same space. Since the amplitudes of waves are additive, if the two waves are out of phase with each other, the resultant wave will be less intense than either of the individual waves.

Diffuse Reflectance: The phenomenon that takes place when infrared radiation reflects off a rough surface. The light is transmitted, absorbed, scattered, and reflected by the surface. The light approaches the surface from one direction, but the diffusely reflected light leaves the surface in all directions. A reflectance sampling technique known as DRIFTS is based on this phenomenon.

Dispersive Instruments: Infrared spectrometers that use a grating or prism to disperse infrared radiation into its component wavenumbers before detecting the radiation. This type of instrument was dominant before the development of FT-IR spectroscopy.

DTGS: Deuterated tri-glycine sulfate pyroelectric detectors are the most common detectors used in FT-IR instruments. They are chosen for their ease of use, good sensitivity, wide spectral responsivity, and excellent linearity.

Dynamic Range: For an interferogram, it is the ratio of the large centerburst signal at ZOPD to the smallest recorded signal (which must be greater than the noise for any benefit from signal averaging). The A/D used must have sufficient precision to measure the entire range as any clipping or distortion of the largest signal affects the whole spectrum.

Felgett (multiplex) Advantage: An advantage of FT-IR instruments compared to scanning/single channel dispersive instruments. It is based on the fact that in an FT-IR all the wavenumbers of light are detected at once.

Fourier Transform: Calculation performed on an interferogram to turn it into an infrared spectrum.

Interferogram: A plot of infrared detector response versus optical path difference. The fundamental measurement obtained by an FT-IR is an interferogram. Interferograms are Fourier transformed to give infrared spectra.

Jacquinot or J Stop: An aperture placed in the beam to restrict the divergence angle to the maximum allowable with the selected resolution. When choosing lower resolution the S/N can be improved by opening the stop. Note that in many instances there is no physically separate stop but there will be some aperture, be it the source size, or the detector active area, that acts as the system J stop.

Jacquinot Advantage: This is the throughput advantage of FT-IRs over traditional spectrometers that require a slit aperture. The advantage varies as wavenumber and depends on resolution (because of slit width changes). In practice, any advantage will also depend on source dimensions.

Mirror Displacement: The distance that the mirror in an interferometer has moved from zero path difference.

Normalized: The process of dividing all the absorbance values in a spectrum by the largest absorbance value. This resets the Y axis scale from 0 to 1.

Nyquist Frequency: A term widely used in information theory, but here applies to the highest frequency, or shortest wavelength, that can be identified in an interferogram. It is the one for which there are exactly two points per cycle. The contribution of any higher frequency, signal or noise, can be represented by some lower frequency and so will appear aliased or folded into the spectrum.

Optical Distance: Physical distance multiplied by the index of refraction of the medium.

Optical Path Difference: The difference in the optical beam path that two light beams travel in an interferometer.

Phase Correction: A software procedure to compensate for not taking a data point exactly at ZOPD, and for frequency dependent variations caused by the beam splitter and signal amplification. The Mertz and Forman corrections are both used, with the Mertz applied to double-sided interferograms; this is considered the most accurate approach.

Resolution: A measure of how well an IR spectrometer can distinguish spectral features that are close together. For instance, if two features are 4 cm-1 apart and can be discerned easily, the spectrum is said to have at least 4 cm-1 resolution. Resolution in an FT-IR is mainly determined by the optical path difference.

Side-lobes: Spectral features that appear to the sides of an absorbance band as undulations in the baseline. Side-lobes are caused by having to truncate an interferogram, as a result of finite scan distance, and can be removed from a spectrum by multiplying the spectrum’s interferogram by an apodization function.

Single Beam Spectrum: The spectrum that is obtained after Fourier transforming an interferogram. Single beam spectra contain features due to the instrument, the environment, and the sample.

Smoothing: A spectral manipulation technique used to reduce the amount of noise in a spectrum. It works by calculating the average absorbance (or transmittance) of a group of data points called the “smoothing window,” and plotting the average absorbance (or transmittance) versus wavenumber. The size of the smoothing window determines the number of data points to use in the average, and hence the amount of smoothing.

Spectral Subtraction: A spectral manipulation technique where the absorbances of a reference spectrum are subtracted from the absorbances of a sample spectrum. The idea is to remove the bands due to the reference material from the sample spectrum. This is done by simply calculating the difference in absorbance between the two spectra, then plotting this difference versus wavenumber. The reference spectrum is often multiplied by a subtraction factor so that the reference material bands subtract out properly.

Transmission Sampling: A sampling method where the infrared beam passes through the sample before it is detected. Samples are typically diluted or flattened to adjust the absorbance values to a measurable range.

Wavelength: Distance between adjacent crests or troughs of a light wave.

Wavenumber: 1/wavelength, the units of wavenumbers are cm-1, and are most commonly used as the X axis unit in infrared spectra.
1 µm = 1,000 nm = 10,000 cm-1
5 µm = 5,000 nm = 2,000 cm-1

Zero Path Difference, or Zero Optical Path Difference: The mirror displacement at which the optical path difference for the two beams in an interferometer is zero. At ZPD, or ZOPD, the detector signal is often the largest, the centerburst.