Infrared Spectroscopy

Infrared spectroscopyChapter content TheoryInstrumentationMeasurement techniquesMid-infrared (MIR)– Identification of organic compounds– Quantitative analysis– Applications in food analysis Near-infrared (NIR)– Properties of the technique– Applications in food analysisInfrared spectroscopy measurement of IR radiation absorbed by or reflectedfrom a sample absorption of IR radiation is related to the changes ofvibrational or rotational energy states of molecules applications:– analysis of gaseous, liquid or solid samples– identification of compounds– quantitative analysis information deduced from IR spectrum:– functional groups of molecules, constitution of molecules– interaction among molecules1

Vibrational transitions fundamental (normal):change of vibrational quantum number v 1high probability high values of ε overtones:the difference of vibrational quantum number v 2; 3 lower probability low values of ε combination:simultaneous change of two or more vibrational numbersfor a polyatomic moleculeTypes of vibrations stretching: the length of chemical bond (the inter-nucleardistance) is changed– symmetric– anti-symmetric bending: the valence angle is changed– symmetric and anti-symmetric vibrations– vibrations in plane and vibrations out of plane2

Example: vibrations of a three-atomic non-linear moleculeand a group of three atoms in a multi-atom moleculeH2Osymmetric stretch anti-symmetric stretch scissoring bend --CH2rocking bendwagging bendtwisting bendWhich substances give a signal in IR spectrum?YES molecules that contain polarbondsi.e. molecules composed ofatoms of different elements organic compounds andinorganic compounds (H2O,CO2, NO2, HCl, salts.)NO pure chemical elements inmolecular or crystal state e.g. Ar, O2, O3, N2, Cl2,S8, silicon, graphite,diamond IR signal of a molecule is proportional to square of the change ofdipole moment that occurs during vibrational motion of the molecule.3

Spectral regions and corresponding analytical techniquesNear infrared region(near infrared spectroscopy, NIR)Mid infrared region(mid infrared spectroscopy, Mid IR, MIR)Far infrared region(far infrared spectroscopy, FIR)ν -1λ(µm)0.8 – 2.5(cm )12 500 – 4 0002.5 – 254 000 – 40025 – 1000400 – 10MIR – normal vibrational transitionsNIR – overtonesFIR – normal vibrations of weak bonds and bonds of heavy atomsInstrumentation for IR spectroscopyMain componentsof an instrumentTypes of instruments radiation source measuring (and simple instruments withreference) cell wavelength selector detector (transducer)a filter classical instrumentswith a monochromator instruments based on aninterferometer (FTIR)4

Double beam vs. single beam spectrometersSources of IR radiation for NIR: tungsten lamp for MIR:– Globar electrically heated (1100 C) silicon carbide rod–– it gives maximum intensity at λ 2 µm;at lower temp. – shift of maximum to a longer wavelength(600 C λmax 3,5 µm)lasers CO2, PbS – λmax 9–11 µm5

Transducers of IR radiation pyroelectric triglycine sulphate detectors– work at the normal temperature photoconductive detectors MCT (HgTe/CdTe)– work at the temp. of liquid nitrogen (-196 C)– high sensitivity– fast response– are used for MIR and FIR germanium bolometers– are used for FIR– work at the temp. of liquid helium (-271,7 C)Fourier transform IR spectrometers (FTIR)– based on Michelson interferometerThe beam from source is divided on splitterinto two halves;the first is reflected to the fixed mirror, whilethe second is transmitted to the movable one;the reflected beams are recombined in thesplitter again and an interference of wavesoccurs.For monochromatic radiation, a destructive interference of beams occurs,when δ (n 0,5) λδ is optical retardation ( two-times the difference between the distance of thefixed mirror to the splitter and the distance of the movable one to the splitter).The constructive interference occurs, when δ λ; 2 λ;3 λ; n λWhen polychromatic radiation passes through the interferometer andthe sample, the obtained record is an interferogram, which is convertedinto IR spectrum by Fourier transformation.6

Advantages of FTIR dispersion elements are not necessary more energy enters the sample fast spectrum recording ( 1s) high resolution (up to 0.01 cm-1)Measurement techniques of IR spectroscopySample types and sample preparationCells for transmission measurements have windows made of NaCl, KBr, CaF2, ZnSe, AgCl, TlBr/TlI pro MIR CsBr, polyethylene (for FIR) glass, quartz glass (for NIR)Gaseous samplesCells are firstly evacuated and then filled with a sample;cell path length ranges from 10 cm to 80 m (multiple reflection ofthe beam inside the cell, outer size of cell is up to 1 m)7

Liquid samplesDemountable cellfor liquid samplesFor aqueous solution it is necessary to use windows made of CaF2or ZnSe (insoluble in water).The measurement is possible only within the interval of1400–1000 cm-1, used e.g. for the analysis of sugars in fruit juicesbased on the measurement of thin films (10–50 µm).Liquid samplesSolvents for samples applicable in MIRSolventCCl4CHCl3CS2Measurement in wavenumber region4000–1600 cm-1 and 1500–850 cm-14000–1250a cm-1 and 1150–850 cm-14000–1650b cm-1 and 1400–500 cm-1a – with exception of strong bands at 3050 and 940 cm-1b – with exception of strong bands at 2350 and 2200 cm-1Pure liquid samples (e.g. oils) and gels can be measured in a verythin film (1 µm) by transmission technique of by ATR technique.8

Solid samples preparation of KBr discs (pellets):1-15 mg of sample 200 mg KBr – the well powderedmixture is pressed under vacuum to form a pellet which isinserted into a holder in the spectrometer(disadvantage: KBr absorbs some water, the bands ofwhich may interfere with the sample spectrum) preparation of Nujol mull:powdered sample is dispersed in mineral (paraffin) oil toform a suspension(disadvantage: bands C-H a C-C bonds of the sample areoverlapped with those of paraffin oil)Measurement techniques in IR spectroscopyTransmission techniquethe measurement of transmittance, absorbanceT I / I0 10-εbcI0 / I 10 εbcAλ log10 (1/T) - log10 T log10 (I0/I) ελ . b . c9

Reflection techniquesthe measurement of the radiation reflected from samplereflectance R I / I0optical density OD log10 (1/R) -log10 RDiffusionSpecularreflection:reflection:common techniquein NIR for solidpowdered samplesSpecial arrangement:measurement of interactance – the use of optical cablesATR technique (attenuated total reflectance, or internal reflectance)n1n2n1 a sample (liquid, semi-solid, solid) is placed in a layer on the surface of the crystal; pressure is applied for solid samples toachieve good contact between the sample and the crystalthe crystal must have high refractive index n2 (higher than that ofsample n1)the beam enters the crystal at the incidence angle θ higher thancritical angle θc arcsin (n1/n2); under this condition, the beam istotally caught by the crystal, i.e. a complete internal reflectionoccurs and the beam travels in the crystal; after one or multiplereflections, the beam leaves the crystal and reaches the detectorat the crystal-sample interface, the beam actually penetrates a veryshort distance ( 2 µm) into the sample and the absorption ofspecific wavelengths occurspenetration depth depends on the wavelength:dp λ/2π [sin2θ-(n1/n2)2]0,5[µm]10

Advantages of ATRRequirements for a sample and a crystal good adherence of the sample tosimple preparation of samplebefore measurement non-transparent samplescan be analysedthe crystal mechanical strength of the crystal (whenthe sample is pressed against the crystal) inertness of the crystal against samples removal of sample residue from thecrystal using various solventsKind of crystaln at 1000 cm-1Spectral range-1diamond30 000 – 200 cm2.4sapphire50 000 – 1 7801.74NaCl40 000 – 5901.49ZnSe20 000 – 4542.4Ge5 500 – 6004.0Si8 300 – 6 6003.4TlBr/TlI20 000 – 2502.37Mid-infrared spectroscopy (Mid IR) the mid IR spectra consist of the bands corresponding mainly to normal (fundamental) vibrationsthe number of vibrations of a molecule composed of N atomsis 3N-6 (for non-linear molecules) or3N-5 (for linear molecules)absorption bands are stronger, when two or more vibrationshave the same frequency (wavenumber); such vibrations arecalled degenerated vibrationsstrong signals are caused by vibrations of polar bonds,especially multiple polar bonds, such as C Osingle bonds of a low polarity (especially C-C) give veryweak signals (and totally non polar bonds give no signal)11

Identification of organic compounds using Mid IRMid IR region is divided into two sub-regions: region of characteristic vibrations of functional groups2.5–8 µm (4000–1250 cm-1)contains the characteristic bands of individual bonds andfunctional groups that correspond mainly to stretchingvibrations fingerprint region8–25 µm (1250–400 cm-1)contains the bands corresponding mainly to bendingvibrationsthe spectrum in this region characterises each molecule asan integral wholeExample of spectra of isomeric monoterpenesthe spectra are very similar in in theregion of characteristic vibrations(even almost identical around 3000 cm-1)but very different in the fingerprint region12

Steps of identification process1. searching for functional groups on the basis of characteristicvibrations (using tables)2. confrontation with the results of other tests elementary analysis of the compound stoichiometric formula of the compound determination of molecular mass (from mass spectrum) molecular formula of the compound calculation of unsaturation indexU 1 0.5 (2. number of C number of N P – number of H– number of halogen atoms)U 0 no multiple bond, no ringU 1 1 double bond or 1 ringU 2 2 double bonds or 1 triple bond or 1 double 1 ringU 3 3 double bonds or 3 rings or 1 double 1 triple bondor 2 double bonds 1 ring or 1 double bond 2 ringsor 1 triple 1 ringSteps of identification process3.4.5.drawing of all possible structures that correspond to thepresence of groups, molecular formula and unsaturationindexcomparison of the measured IR spectrum with thespectra of the suggested compounds found in an atlas ora database of spectra identificationverification of identity using other spectral methods(MS, NMR)13

Trends in wavenumbers of carbon chemical bondseffect ofmass of X atomin C–X bond:effect ofcarbon hybridization:C-C bondC–H bondC–H2900 cm-1sp2 C–H 3100C–C1200sp C–HC–O1100C–Cl750C–Br600C–I500C–C1200 cm-1sp3 C–HC C1650C C21502900 cm-13300Look on IR spectrum: identification of functional groups1. carbonyl groupsSTARTstrong band1820-1660 cm-1(C O group)NO2KETONENOYESwide band3550-3200 cm-1(sometimes overlapswith the band ofC-H groups)YESCARBOXYLICACIDNOmediumbandca 3400 cm-1(N-H group)YESAMIDENOstrong band1300-1100 cm-1(C-O bond)YESESTERor LACTONENOtwo strong bands1810 and 1760 cm-1(C O groups)YESANHYDRIDEtwo weak bandsNO 2850 and 2750 cm-1at the right side ofC-H bandYESALDEHYDE214

OC OOOC-HC OC-OLook on IR spectrum: identification of functional groups2. other functional groups2NOband 1300-1000 cm-1(C-O bond)mediumto strong band(s)at 3400 cm-1(N-H bond)YESYESwide bandat3400-3300 cm-1(O-H bond)NOstrong bandbellow 800 cm-1(C-X bond)YESNO2 strong bands1600-1530 cm-1and1390-1300 cm-1NOYESNOYESALCOHOLor PHENOLETHERAMINEHALIDENITRO-COMP.315

CH3CH3COHCH3pure tert. butanoldiluted solution of tert. butanol in CHCl3OHC-OOHOHO-CH3OHC CC-O16

Look on IR spectrum: identification of functional groups3. double bond and aromatic ring3sharp bandat ca 1650 cm-1(C C bond)** the peak can be overlapped with the signal of carbonyl groupYESmediumband(s)1600-1450cm-1YESALKEN Eor ARENEband(s)at ca 3030 cm-1NOYES890-680 cm-1optionally 2C-HBrN-HC-Brbenzene ring17