Infrared Infrared SpectroscopySpectroscopy

Department of Natural Department of Natural SciencesSciences

UM-DearbornUM-DearbornWritten by Bette KreuzWritten by Bette Kreuz

Produced by Ruth DusenberyProduced by Ruth Dusenbery20002000

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Infrared spectroscopy is an Infrared spectroscopy is an instrumental method of instrumental method of analysis that can be used to analysis that can be used to identify and quantify identify and quantify samples ranging from samples ranging from pharmaceuticals to diesel pharmaceuticals to diesel emissions.emissions.

Slide 1 Slide 1 IntroductionIntroduction

Info Slide 2Info Slide 2 All modern instruments All modern instruments

are Fourier Transform are Fourier Transform instruments. instruments.

In all transmission In all transmission experiments radiation experiments radiation from a source is directed from a source is directed through the sample to a through the sample to a detector. detector.

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CompartmentCompartmentIR SourceIR Source DetectorDetector

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The measurement of the The measurement of the type and amount of light type and amount of light transmitted by the sample transmitted by the sample gives information about gives information about the structure of the the structure of the molecules comprising the molecules comprising the sample.sample.

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Infrared Infrared SourceSource

DetectorDetector

Sample Sample CompartmentCompartment

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In the IR region of the In the IR region of the electromagnetic spectrum, electromagnetic spectrum, the absorption of radiation the absorption of radiation by a sample is due to by a sample is due to changes in the vibrational changes in the vibrational energy states of a energy states of a molecule. molecule.

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The pattern of absorption The pattern of absorption as a function of as a function of wavelength is called an IR wavelength is called an IR spectrum.spectrum.

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Info Slide 6Info Slide 6 To obtain an IR To obtain an IR

spectrum, the sample spectrum, the sample must be placed in a must be placed in a “container” or cell that “container” or cell that is transparent in the IR is transparent in the IR region of the spectrum. region of the spectrum.

Sodium chloride or salt Sodium chloride or salt plates are a common plates are a common means of placing the means of placing the sample in the light sample in the light beam of the instrument.beam of the instrument.

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IR transparent Salt PlatesIR transparent Salt Plates

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These plates are made of These plates are made of salt and must be stored in salt and must be stored in a a water freewater free environment environment such as the dessicator such as the dessicator shown here. shown here.

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DessicatorDessicator

Water-freeWater-freeEnvironmentEnvironment

for for Water-sensitiveWater-sensitive

Salt Plates.Salt Plates.

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The plates must also be The plates must also be handled with gloves to handled with gloves to avoid contact of the plate avoid contact of the plate with moisture from one’s with moisture from one’s hands.hands.

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To run an IR spectrum of a To run an IR spectrum of a liquid sample, a drop or liquid sample, a drop or two of the liquid sample is two of the liquid sample is applied to a salt plate.applied to a salt plate.

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A second salt plate is A second salt plate is placed on top of the first placed on top of the first one such that the liquid one such that the liquid forms a thin film forms a thin film “sandwiched” between the “sandwiched” between the two plates.two plates.

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The two plates are then The two plates are then secured in a sample holder secured in a sample holder that is compatible with the that is compatible with the particular instrument particular instrument being used.being used.

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The cell holder is then The cell holder is then placed in the beam of the placed in the beam of the instrument.instrument.

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The light beam traverses The light beam traverses the sample compartment, the sample compartment, as illustrated by the red as illustrated by the red line.line.

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Light PathLight Path(shown by red line)(shown by red line)

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The sample is then The sample is then scanned by the instrument scanned by the instrument utilizing predesignated utilizing predesignated parameters.parameters.

A relevant background A relevant background

scan should already have scan should already have been taken.been taken.

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Click Here to Start ScanClick Here to Start Scan

Info Slide 15Info Slide 15 A satisfactory spectrum has A satisfactory spectrum has

well defined peaks-but not well defined peaks-but not so intense as to cause so intense as to cause flattening on the bottom of flattening on the bottom of the peaks.the peaks.

Major peaks can be labeled Major peaks can be labeled using the peak function of using the peak function of

the softwarethe software

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Well-definedWell-definedpeaks are peaks are labeled with the labeled with the Wavenumbers Wavenumbers of the of the Absorption Absorption Maxima.Maxima.

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The spectrum can then be The spectrum can then be printed using the print printed using the print function of the software.function of the software.

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A sample of a printout of A sample of a printout of an IR spectrum.an IR spectrum.

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Sample of a Sample of a printout of printout of an IR an IR spectrum.spectrum.

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The salt plates are cleaned The salt plates are cleaned by rinsing into a waste by rinsing into a waste container with a suitable container with a suitable organic solvent-commonly organic solvent-commonly cyclohexane.cyclohexane.

NEVER WATER!NEVER WATER!

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CYCLOHEXANECYCLOHEXANESolventSolvent

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Cloudy plates must be Cloudy plates must be polished to return them to polished to return them to a transparent condition.a transparent condition.

To polish cloudy windows, To polish cloudy windows,

rotate salt plate on rotate salt plate on polishing cloth.polishing cloth.

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The clean plates and cell The clean plates and cell holder are stored in the holder are stored in the moisture free atmosphere moisture free atmosphere of a dessicator.of a dessicator.

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