Applications of IR Applications of IR spectroscopyspectroscopy

Yongsik LeeYongsik Lee2004. 62004. 6

IR spectrumIR spectrum

Sample handlingSample handling

►Most time-consuming part is sample preparation►Gases fill gas cell

transparent windows (NaCl/KBr) long path length (10 cm) - few molecules

► Liquids fill liquid cell solute in transparent solvent Not in water (attacks windows) short path length (0.015-1 mm) - solvents absorb

Solution sampleSolution sample

► SolventsSolvents Water, alcohols – NOWater, alcohols – NO Check the background absorptionCheck the background absorption

Demountable IR cell for liquidDemountable IR cell for liquid

► CellsCells 0.01 – 1.0 mm path length0.01 – 1.0 mm path length Narrower than UV/VIS Narrower than UV/VIS Sample concentration 0.1 – 10%Sample concentration 0.1 – 10%

► Demountable cells with Teflon spacersDemountable cells with Teflon spacers Variation in path lengthVariation in path length

Determination of cell Determination of cell thicknessthickness

►Determination of thickness(b)Determination of thickness(b) Empty cell in the light pathEmpty cell in the light path Interference fringeInterference fringe 2b/N = 2b/N = Number if interference fringes Number if interference fringes N between two kN between two k

nown wavelengthsnown wavelengths N = 2b/N = 2b/1 – 2b/1 – 2b/22 b = b = N /2(N /2(1-1-2)2)

Solid sampleSolid sample

►Solid samples make semi-transparent pellet with KBr

►Halide salts get transparent when pressured grind and mix with Nujol (hydrocarbon oil) to form

mull►Grind size < the radiation wavelength

1-2 drop(s) between NaCl plates.

Qualitative Analysis

►Step One : Identify functional groups (group frequency region) 1200 - 3600 cm-1

►Step Two : Compare with standard spectra containing these functional groups fingerprint region – sensitive to the

structure 600 – 1200 cm-1

Group frequency and Group frequency and fingerprint regionfingerprint region

Group frequenciesGroup frequencies

►Approximately calculated from masses and spring constants Variations due to coupling Compared to correlation charts/databases Bond force constant

Group frequecy table for organic grouGroup frequecy table for organic groupsps

Correlation chartCorrelation chart

Computer search systemComputer search system

► Spectra pattern searchSpectra pattern search PositionPosition Relative magnitudeRelative magnitude

► Sadtler IR collection (1980)Sadtler IR collection (1980) Over 130,000 spectraOver 130,000 spectra

► Spectra coding AlgorithmSpectra coding Algorithm Location of its strongest abs peakLocation of its strongest abs peak Then each additional strong band in 10 regionsThen each additional strong band in 10 regions 40 second for 25,000 compound search40 second for 25,000 compound search

Quantitative Analysis

► IR more difficult than UV-Vis because narrow bands (variation in e) complex spectra weak incident beam low transducer sensitivity solvent absorption

► IR mostly used for rapid qualitative but not quantitative analysis Beer’s law failure Long optical path-length required Regular FT-IR is worse than UV-VIS Exception - Tunable IR laser, quantum cascade laser, OPO

Reflectance spectroscopyReflectance spectroscopy

►TypesTypes Specular reflectionSpecular reflection Diffuse reflectionDiffuse reflection Internal reflectionInternal reflection Attenuated total reflectionAttenuated total reflection

DRIFTSDRIFTS

► Diffuse reflectance IR FT spectroscopyDiffuse reflectance IR FT spectroscopy Fine powder sampleFine powder sample Specular reflections from all randomly oriented surfaces Specular reflections from all randomly oriented surfaces

of the powderof the powder The intensity of the reflection is roughly independent of tThe intensity of the reflection is roughly independent of t

he viewing anglehe viewing angle► AnalysisAnalysis

Using mathematical modelsUsing mathematical models Kubelka and MunkKubelka and Munk Fuller and GriffithsFuller and Griffiths

Instrumentation of DRIFTSInstrumentation of DRIFTS

► Adaptor in cell compartmentAdaptor in cell compartment► Reference sampleReference sample

Finely grounded KBrFinely grounded KBr MirrorMirror

► ABS vs. DRIFTSABS vs. DRIFTS Peak locations are samePeak locations are same Relative intensities are differeRelative intensities are differe

ntnt Figure 17-10Figure 17-10

ATRATR

► Attenuated total reflectionAttenuated total reflection► Sample – wide variety of typesSample – wide variety of types

solids of limited solubilitysolids of limited solubility FilmsFilms ThreadsThreads PastesPastes AdhesivesAdhesives PowersPowers

► Principles of the methodPrinciples of the method At a certain angle, total At a certain angle, total

reflection can occurreflection can occur Depth of penetration when Depth of penetration when

reflected (< reflected (< ))► Evanescent wave can be Evanescent wave can be

absorbed by the sampleabsorbed by the sample

ATR instrumentationATR instrumentation

►Figure 17-11Figure 17-11►High refrative index ATR crystalHigh refrative index ATR crystal

Thallium bromideThallium bromide Thallium iodideThallium iodide Germanium and ZeSe plateGermanium and ZeSe plate

►Adjustment of incident angleAdjustment of incident angle►ATR crystal can be dipped into the liquidATR crystal can be dipped into the liquid

Applications of ATRApplications of ATR

17C Photoacoustic IR Spectroscopy17C Photoacoustic IR Spectroscopy

►HistoryHistory 1880 Alexander Graham Bell1880 Alexander Graham Bell Photo absorption effectPhoto absorption effect Chopped Photon -> sample -> microphoneChopped Photon -> sample -> microphone

Photoacoutstic (PA) IRPhotoacoutstic (PA) IR

►AdvantagesAdvantages Scattered & reflected light = no Scattered & reflected light = no

microphone signalmicrophone signal FT method is possibleFT method is possible

►CO2 laser PA IR spectroscopyCO2 laser PA IR spectroscopy Tunable CO2 laser sourceTunable CO2 laser source PA cellPA cell 10 gases (sensitivity 1 ppb) in 5 minutes10 gases (sensitivity 1 ppb) in 5 minutes

17D Near IR17D Near IR

► SpectrumSpectrum 770-2500 nm770-2500 nm 13000 – 4000 cm-113000 – 4000 cm-1 Overtone or combination of fundamental stretching modes Overtone or combination of fundamental stretching modes

► C-H, N-H, O-HC-H, N-H, O-H Weaker absorption than fundamental bandsWeaker absorption than fundamental bands

► DisadvantagesDisadvantages Low molar absorption coefficientLow molar absorption coefficient Detection limit 0.1%Detection limit 0.1%

► ApplicationApplication Mostly qualitative analysisMostly qualitative analysis Water, protein, low mw carbohydrates, food, petroWater, protein, low mw carbohydrates, food, petro

17G IR microspectrometry17G IR microspectrometry

► Introduced in 1980sIntroduced in 1980s► IR ABS or reflection spectraIR ABS or reflection spectra►Sample dimensions in 10 -500 Sample dimensions in 10 -500 mm► InstrumentInstrument

Ordinary optical microsocpeOrdinary optical microsocpe FT-IR with small IR beam sizeFT-IR with small IR beam size LN2 MCT (mercury/cadmium/telluride) PCLN2 MCT (mercury/cadmium/telluride) PC

Nicolet Magna 760 with Nic-Plan IR Nicolet Magna 760 with Nic-Plan IR Microscope Microscope

► Tabletop Optical Module (foreground), Tabletop Optical Module (foreground), ► Nicolet Magna 550 spectrometer, Nicolet Magna 550 spectrometer, ► Right Auxiliary Experimental ModuleRight Auxiliary Experimental Module► Olympus IX70 inverted microscope Olympus IX70 inverted microscope