IntroductiontoFT-IRSpectroscopy

CHEM4100Fall2017

1. Energysource2. Sampleholder3. MichelsonInterferometer4. Detector

https://www.slideshare.net/ashwinisomayaji7/ft ir-spectrophotometer

Energysources:HeatedmaterialsemitIRradiation

• TheNernstglower isacylinder(1-2mmdiameter,approximately20mmlong)ofrareearthoxides.Platinumwiresaresealedtotheends,andacurrentpassedthroughthecylinder.TheNernstglowercanreachtemperaturesof2200K.

• TheGlobar source isasiliconcarbiderod(5mmdiameter,50mmlong)whichiselectricallyheatedtoabout1500K.Watercoolingoftheelectricalcontactsisneededtopreventarcing.ThespectraloutputiscomparablewiththeNernstglower,execept atshortwavelengths(lessthan5 mm)whereit'soutputbecomeslarger.

• Theincandescentwiresource isatightlywoundcoilofnichromewire,electricallyheatedto1100K.ItproducesalowerintensityofradiationthantheNernstorGlobar sources,buthasalongerworkinglife.

2

https://www.slideshare.net/naveenkadian73/ir-23731388

https://www.slideshare.net/naveenkadian73/ir-23731388

ATRAttenuatedTotalReflectance

https://chem.libretexts.org/Core/Analytical_Chemistry/Instrumental_Analysis/Spectrometer/ATR-FTIR

MichelsonInterferometer

http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/irspec3.htm

MichelsonInterferometerItmodulatestheIRfrequenciessothatallfrequenciesofthesourcegothroughthesample.Ifthevelocityofthemirrorisknown,themodulatedfrequencyoftheradiationcanbedetermined.

fm =vxf(ir)1.5x1011

fm.=modulatedfrequencyf(ir)=frequencyofirradiation

v=velocityofthemirror.(usealasertodetermine itsexactpositionwithrespecttotime.)Thisproducesasignalwithrespecttotime(interferogram)FourierTransformationisusedtoconvertittoasignalwithrespecttofrequencyLasersareusedtodeterminetheexactpositionofthemirrorsatanytime.http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/irspec3.htm

Clickonthelink,toseethevideo:https://en.wikipedia.org/wiki/Laser

LightAmplificationbyStimulatedEmission

https://commons.wikimedia.org/wiki/File:Stimulated_Emission.svg

HeNelaser

https://chem.libretexts.org/Core/Analytical_Chemistry/Instrumental_Analysis/Lasers/Gas_Lasers

Pyroelectric detectors

• Pyroelectric detectors aremadefromasinglecrystallinewaferofapyroelectric material,suchastriglycerine sulfate.Whenanelectricfieldisappliedacrossapyroelectric material,electricpolarizationoccurs(thishappensinanydielectricmaterial).Inapyroelectricmaterial,whenthefieldisremoved,thepolarizationpersists.Thedegreeofpolarizationistemperaturedependent.So,bysandwichingthepyroelectric materialbetweentwoelectrodes,atemperaturedependentcapacitorismade.TheheatingeffectofincidentIRradiationcausesachangeinthecapacitanceofthematerial.Pyroelectric detectorshaveafastresponsetime.TheyareusedinmostFouriertransformIRinstruments.

SourcesofIRinstrumentinformation:

https://chemistry.oregonstate.edu/courses/ch361-464/ch362/irinstrs.htm

HowanFTIRSpectrometeroperates,includingtheFourierTransformseries:https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Spectroscopy/Vibrational_Spectroscopy/Infrared_Spectroscopy/How_an_FTIR_Spectrometer_Operates