Glossary of FT-IR Spectrometer Terms

Absorbance is the unit utilized for measuring the amount of IR radiation absorbed by a material. Absorbance is typically used as the Y axis units in IR spectra and varies in linear proportion to concentration. 100% Line is estimated by ratioing two background spectra acquired under identical conditions. The ideal result is a flat line at 100% transmittance.

Angular Divergence and Aliasing

Angular divergence is the spreading out of an IR beam when it passes through the FT-IR. It can be a limit to attainable resolution and contributes to noise in high resolution spectra. Aliasing is due to the appearance of energy as spectral artefacts below the Nyquist Frequency when frequencies above the Nyquist Frequency are not filtered out. This can be prevented using optical and electronic anti aliasing.

Attenuated Total Reflectance (ATR) and Apodization Functions

ATR is a reflectance sampling technique that involves impinging of an IR radiation on a prism of infrared transparent material of high refractive index. The total internal reflectance based design ensures the reflection of light off the crystal surface at least once before leaving it. Apodization Functions are used to multiply an interferogram to minimize the amount of sidelobes in a spectrum. There are different types of apodization functions available, including Bessel, Hanning, Beer-Norton, triangle, and boxcar. The application of apodization functions inevitably lowers the resolution of a spectrum.

Background Spectrum and Baseline Correction

Background spectrum is a single beam spectrum captured without sample in the IR radiation. It helps measuring the contribution of the device and environment to the spectrum. These effects are eliminated from a sample spectrum by ratioing the sample single beam spectrum to the background spectrum. Baseline Correction is a spectral manipulation technique for correcting spectra with sloped or varying baselines.

Boxcar Truncation and Centerburst

The absence of apodization gives equal weight to all points in the interferogram, up to the edges of the interferogram. Oscillations can be observed on the baseline on both sides of the peaks If the resolution is below the smallest linewidth in the spectrum. Centerburst is the sharp, intense region of an interferogram and its size is directly proportional to the amount of IR radiation hitting the detector.

Coadding and Collimation

Coadding is the process of integrating interferograms to attain an improved signal-to-noise ratio. A cylinder of light is the ideal input beam. Perfect collimation is not possible with beam of finite dimensions; at best there is a diffraction limit. The degree of collimation can influence the signal-to-noise ratio and the resolution.

Constructive Interference and Destructive Interference

Constructive interference is a phenomenon that takes place when two waves occupy the same space and are in phase with each other, yielding a wave that is more intense than either of the individual waves. Destructive interference is a phenomenon that takes place when two waves occupy the same space, but are out of phase with each other. This results in a wave that is less intense than either of the individual waves.

Diffuse Reflectance and Dispersive Instruments

Diffuse reflectance is a phenomenon that occurs during the reflection of IR radiation off a rough surface. The light incidents the surface from one direction, but the diffusely reflected light exits the surface in all directions. Dispersive Instruments such as infrared spectrometer employ a grating or prism for dispersion of infrared radiation into its component wavenumbers prior to the detection of the radiation.

DTGS and Duplicate Range

Deuterated tri-glycine sulfate pyroelectric detectors are the widely used detectors in FT-IR instruments. They are preferred for their ease of operation, wide spectral responsivity, good sensitivity, and outstanding linearity. Duplicate range for an interferogram is the ratio of the large centerburst signal at ZOPD to the smallest recorded signal. The A/D used must have adequate accuracy to measure the whole range as any clipping or distortion of the largest signal influences the entire spectrum.

Dynamic Range and Felgett (multiplex) Advantage

Dynamic Range for an interferogram is the ratio of the large centerburst signal at ZOPD to the smallest recorded signal. The A/D used must have adequate accuracy to measure the whole range as any clipping or distortion of the largest signal influences the entire spectrum. Felgett (multiplex) Advantage is a plus point of FT-IR instrument over scanning/single channel dispersive instruments. It is based on the fact of simultaneous detection of all the wavenumbers of light in an FT- IR.

Fourier Transform and Interferogram

Fourier Transform is the calculation carried out on an interferogram to convert it into an IR spectrum. Interferogram is a plot of infrared detector response versus optical path difference and is the fundamental measurement acquired by an FT-IR. It is Fourier transformed into an IR spectrum.

Jacquinot or J Stop and Jacquinot Advantage

Jacquinot or J Stop is an aperture positioned in the beam to limit the divergence to the maximum compatible with the chosen resolution. Jacquinot Advantage is the throughput advantage of FT-IRs over conventional spectrometers, wherein a slit aperture is essential.

Mirror Displacement and Normalized

Mirror Displacement is the distance travelled by the mirror in an interferometer from zero path difference. Normalized is the process of dividing all the absorbance values in a spectrum by the largest absorbance value to reset the Y axis scale to 0 to 1.

Nyquist Frequency and Optical Distance

Nyquist frequency is the highest frequency, shortest wavelength that can be observed in an interferogram. Optical Distance is the physical distance multiplied by the refractive index of the medium.

Optical Path Difference and Phase Correction

Optical path difference is the variation in optical distance that two light beams travel in an interferometer. Phase Correction is a software procedure to offset for not taking a data point accurately at ZOPD, and for frequency dependent variations due to the beam splitter and signal amplification.

Resolution and Sidelobes

Resolution is a measure of the ability of an IR spectrometer to differentiate closely spaced spectral features. Resolution in an FT-IR is primarily determined by the optical path difference. Sidelobes are spectral features that arise to the sides of an absorbance band as undulations in the baseline. Sidelobes appear due to the necessity to truncate an interferogram owing to finite scan distance and can be eliminated from a spectrum by multiplying the interferogram of the spectrum by an apodization function.

Single Beam Spectrum and Smoothing

Single beam spectrum is the spectrum derived subsequent to the Fourier transformation of an interferogram and consists of features caused by the instrument, the sample, and the environment. Smoothing is a spectral manipulation technique used for lowering the level of noise in a spectrum.

Spectral Subtraction and Transmission Sampling

Spectral Subtraction is a spectral manipulation method that involves subtraction of the absorbances of a reference spectrum from the absorbances of a sample spectrum to eliminate the bands caused by the reference material from the sample spectrum. Transmission Sampling is a sampling technique where the infrared beam traverses through the sample before getting detected. Samples are generally diluted or flattened to fine- tune the absorbance values to a quantifiable range.

Wavelength and Wavenumber

Wavelength is the interval between two adjacent crests or troughs of a light wave. Wavenumber is 1/wavelength and is expressed in cm-1. It is widely utilized as the X axis unit in infrared spectra.

Zero Path Difference (ZPD), or Zero Optical Path Difference (ZOPD)

In an interferometer, the mirror displacement upon which the optical path difference for the two beams is zero. The detector signal is often much larger at ZPD, ZOPD, and is called the centerburst.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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