Applications of IR spectroscopy Yongsik Lee 2004. 6

IR spectrum

Sample 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 sample Solvents Water, alcohols – NO Check the background absorption

Demountable IR cell for liquid Cells 0.01 – 1.0 mm path length Narrower than UV/VIS Sample concentration 0.1 – 10% Demountable cells with Teflon spacers Variation in path length

Determination of cell thickness Determination of thickness(b) Empty cell in the light path Interference fringe 2b/N = l Number if interference fringes DN between two known wavelengths DN = 2b/l1 – 2b/l2 b = DN /2(n1-n2)

Solid 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 fingerprint region

Group 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 groups

Correlation chart

Computer search system Spectra pattern search Position Relative magnitude Sadtler IR collection (1980) Over 130,000 spectra Spectra coding Algorithm Location of its strongest abs peak Then each additional strong band in 10 regions 40 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 spectroscopy Types Specular reflection Diffuse reflection Internal reflection Attenuated total reflection

DRIFTS Diffuse reflectance IR FT spectroscopy Analysis Fine powder sample Specular reflections from all randomly oriented surfaces of the powder The intensity of the reflection is roughly independent of the viewing angle Analysis Using mathematical models Kubelka and Munk Fuller and Griffiths

Instrumentation of DRIFTS Adaptor in cell compartment Reference sample Finely grounded KBr Mirror ABS vs. DRIFTS Peak locations are same Relative intensities are different Figure 17-10

ATR Attenuated total reflection Sample – wide variety of types solids of limited solubility Films Threads Pastes Adhesives Powers Principles of the method At a certain angle, total reflection can occur Depth of penetration when reflected (< l) Evanescent wave can be absorbed by the sample

ATR instrumentation Figure 17-11 High refrative index ATR crystal Thallium bromide Thallium iodide Germanium and ZeSe plate Adjustment of incident angle ATR crystal can be dipped into the liquid

Applications of ATR

17C Photoacoustic IR Spectroscopy History 1880 Alexander Graham Bell Photo absorption effect Chopped Photon -> sample -> microphone

Photoacoutstic (PA) IR Advantages Scattered & reflected light = no microphone signal FT method is possible CO2 laser PA IR spectroscopy Tunable CO2 laser source PA cell 10 gases (sensitivity 1 ppb) in 5 minutes

17D Near IR Spectrum Disadvantages Application 770-2500 nm 13000 – 4000 cm-1 Overtone or combination of fundamental stretching modes C-H, N-H, O-H Weaker absorption than fundamental bands Disadvantages Low molar absorption coefficient Detection limit 0.1% Application Mostly qualitative analysis Water, protein, low mw carbohydrates, food, petro

17G IR microspectrometry Introduced in 1980s IR ABS or reflection spectra Sample dimensions in 10 -500 mm Instrument Ordinary optical microsocpe FT-IR with small IR beam size LN2 MCT (mercury/cadmium/telluride) PC

Nicolet Magna 760 with Nic-Plan IR Microscope

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