Welcome to the Spectros Associates Infrared Spectroscopy Glossary page.  We have defined here all the important terms you need to know to understand infrared spectroscopy.  The definitions have been kindly provided by Dr. Brian C. Smith of Spectros Associates.  There are about 20 important terms defined on this page.  Please click the links below for more detailed glossaries on FTIR and on IR Spectral Interpretation.  Any term that appears in italics is defined elsewhere in the glossary.  Feel free to bookmark this page, and return to it whenever you have a question about infrared spectroscopy.

These terms are in the FTIR Glossary (click here for Adobe version )

These terms are in the Interpretation Glossary (click here for Adobe version )

DEFINITIONS

Absorbance: - Units used to measure the amount of infrared radiation absorbed by a sample. Absorbance is commonly used as the Y axis unit in infrared spectra . Absorbance is defined by Beer's law , and is linearly proportional to concentration. This is why spectra plotted in absorbance units should be used in quantitative analysis.
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ATR Beer's Law: - The equation that relates the absorbance of a sample to its concentration. Its form is A = e lc where the A is stands for absorbance, e for absorptivity, l is for pathlength, and c is for concentration. Beer's law is the equation used in FTIR quantitative analysis to perform calibrations and to predict unknown concentrations.
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Background Spectrum: - A single beam spectrum acquired with no sample in the infrared beam. The purpose of a background spectrum is to measure the contribution of the instrument and environment to the spectrum. These effects are removed from a sample spectrum by rationg the sample single beam spectrum to the background spectrum.
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Dipole Moment: - A measure of charge asymmetry, dipole moments are generated when two charges are held apart. The magnitude of a dipole moment is equal to the size of the charges times the distance between them.
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Electromagnetic Radiation: - Another term for light. Light consists of two waves, an electric wave (the electric vector ) and a magnetic wave that oscillate in planes perpendicular to each other, and perpendicular to the direction of travel of the light ray.
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Fourier Transform Infrared (FTIR): - A method of obtaining infrared spectra by first measuring the interferogram of the sample using an interferometer , then performing a Fourier transform on the interferogram to obtain the spectrum.
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Functional Groups : - A method of obtaining infrared spectra by first measuring the interferogram of the sample using an interferometer , then performing a Fourier transform on the interferogram to obtain the spectrum.
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Fundamental Transition : - A method of obtaining infrared spectra by first measuring the interferogram of the sample using an interferometer , then performing a Fourier transform on the interferogram to obtain the spectrum.
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Infrared Spectrometer: - An instrument that is used to obtain the infrared spectrum of a sample. The instrument typically consists of a source of infrared radiation, a sample compartment to allow the radiation to interact with a sample, a means of determining the intensity of radiation as a function of wavenumber, and a way of displaying the spectrum of the sample.
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Group Wavenumber : - An instrument that is used to obtain the infrared spectrum of a sample. The instrument typically consists of a source of infrared radiation, a sample compartment to allow the radiation to interact with a sample, a means of determining the intensity of radiation as a function of wavenumber, and a way of displaying the spectrum of the sample.
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Library Searching: - A process in which an unknown spectrum is compared to a collection of known spectra kept in a spectral library. The comparison gives a number called the hit quality index which represents how closely related two spectra are to each other. If a match is of high quality, it is possible to identify an unknown sample using library searching.
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Normal Modes: - The constituent vibrations of any mechanical system. The complex vibrational motion of any molecule can be resolved into its normal mode vibrations. .
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Resolution: - A measure of how well an infrared 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 be at least 4 cm -1 resolution. Resolution in FTIR is determined by optical path difference .
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Scan: - The process of measuring an interferogram with an FTIR. Typically, this involves moving the mirror in the interferometer back and forth once.
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Signal-to-noise Ratio: - The ratio of signal in a spectrum, usually measured as the intensity of an absorbance band, to noise measured at a nearby point in the baseline. SNR is a measure of the quality of a spectrum, and can be used to ascertain the quality of an infrared spectrometer if it is measured under controlled conditions.
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Spectral Subtraction: - The process by which a reference spectrum is subtracted from a sample spectrum . Ideally, the bands due to the reference material are removed from the sample spectrum.
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Transmittance: - The process by which a reference spectrum is subtracted from a sample spectrum . Ideally, the bands due to the reference material are removed from the sample spectrum.
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Wavelength: - The distance between adjacent crests or troughs of a light wave.
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Wavenumber: - Is defined as 1/ wavelength . The units of wavenumbers are cm -1 , and are most commonly used as the X axis unit in infrared spectra .
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