APPLICATIONS OF IR SPECTROSCOPY

Presented by:SHRUTI PANDEY

1st M.Pharm

QUALITATIVE ANALYSIS

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1. Identification of Substances

• To compare spectrums.• No two samples will have identical IR

spectrum.• Criteria: Sample and reference must be tested

in identical conditions, like physical state, temperature, solvent, etc.

• Disadvt: Enantiomers cannot be distinguished (spectrum are identical).

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The “Fingerprint” Region (1200 to 700 cm-1) :

• Small differences in structure & constitution of molecule result in significant changes in the peaks in this region.

• Hence this region helps to identify an unknown compound.

Computer Search Systems:

• Newer IR instruments offer computer search systems to identify compounds from stored infrared spectral data.

• The position and magnitudes of peaks in the spectrum is compared with profiles of pure compounds stored.

• Computer then matches profiles similar to that of the analyte and result is displayed.

2. Determination of Molecular Structure

• Used along with other spectroscopic techniques.

• Identification is done based on position of absorption bands in the spectrum.

• Eg.: C=O at 1717 cm-1.• Absence of band of a particular group

indicates absence of that group in the compd.

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3. Studying Progress of Reactions

• Observing rate of disappearance of characteristic absorption band in reactants; or

• Rate of increasing absorption bands in products of a particular product.

• Eg.: O—H = 3600-3650 cm-1

C=O = 1680-1760 cm-1

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4. Detection of Impurities

• Determined by comparing sample spectrum with the spectrum of pure reference compound.

• Eg.: ketone impurity in alcohols.• Detection is favoured when impurity possess a

strong band in IR region where the main substance do not possess a band.

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5. Isomerism in Organic Chemistry

(i) Geometrical Isomerism:• trans isomers give a simpler spectrum than

cis due to symmetry.

(ii) Conformers (Rotational Isomers):• Identified with the help of high resolution IR

spectrometers.

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Contd…

• E.g.: Ethanol normal OH – 3636 cm-1 weak band – 3622 cm-1

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(iii) Tautomerism:Existence of 2 or more chemical compds capable of intercovertion , usually by exchanging a hydrogen atom between the 2 atoms.

e.g.: Thiocarboxylic acid

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6. Functional Group Isomerism

• Isomerism shown by compounds having same molecular formula but different functional groups.

Eg: CH3–O–CH3 and CH3–CH2–OH

(Diethyl ether) (Ethanol)

OH = 3500-3100 cm-1

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7. Shape of Symmetry of a Molecule

• E.g.: Nitrogen dioxide, NO2

If linear --> only 2 bands should be present.If bent --> 3 bands should be present.Actual spectrum shows 3 peaks at 750, 1323 and 1616 cm-1.

• Similarly, IR spectrum was used to determine structures of XeF2, XeF4 & XeF6 linear, square planar and octahedral resp.:

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8. Identification of Functional Groups

Due to the presence of functional group region.E.g.:

(3500-3100 cm-1) (1700 cm-1)

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9. Presence of Water in Sample

• If lattice water is present, spectra will contain 3

characteristic bands at 3600-3200 cm-1, 1650

cm-1 and 600-300 cm-1.

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10. Measurement of Paints & Varnishes

• Measured by ‘reflectance analysis’• Advt: Measure IR absorbance of paints on

appliances or automobiles without destroying the surface.

• Make and year of car can be determined from IR spectral analysis.

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11. Examination of Old Paintings & Artifacts

• Help to determine fake “masterpieces”.• Varnish & paints from old items (statues, canvas,

etc.) are analysed by IR spectroscopy.• Presence of new paint traces implies the

“masterpiece” is fake.

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12. In Industry1. Determine impurities in raw materials (to ensure

quality products).2. For Quality Control checks; to determine the %

of required product.3. Identification of materials made in industrial

research labs,or materials of competitors.E.g.: Impurity in bees wax (with petroleum wax)

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13. Analysis of Petroleum HCs, Oil & Grease contents

• These contain C–H bonds.Absorption at 3100-2700 cm-1.

• ‘Freons’—Fluorocarbon-113; do not contain C–H bond.

• Thus, quantity of HCs, oil & grease in freons is determined by measuring C–H absorption at 2930 cm-1.

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14. Quantitative Analysis of Multicomponent Mixtures of Sulfur-oxygen

Anions by ATR Spectroscopy

• FTIR-ATR help to determine sulfur-oxygen anions in aqueous solutions.S–O stretching band at 1350-750 cm-1.

• ATR uses water resistant cells,have short & reproducible effective path length.

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15. Characterization of Heterogenous Catalysts by Diffuse Reflectance

Spectroscopy

• Diffuse Reflectance Spectroscopy help to determine

nature of molecules attached to catalyst surfaces.

E.g.: characterization of olefin polymerization

catalysis with silica gel; diff. types of Si–OH bonds are

determined.

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16. Analysis of Multilayered Polymeric Film using FTIR Spectroscopy

• Determine identities of polymer materials in

multilayered film.

• FTIR helps in quick characterization.

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Other Applications1. Determination of unknown contaminants in

industry using FTIR.2. Determination of cell walls of mutant & wild

type plant varieties using FTIR.3. Biomedical studies of human hair to identify

disease states (recent approach).4. Identify odour & taste components of food.5. Determine atmospheric pollutants from

atmosphere itself.25

QUANTITATIVE ANALYSIS

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QUANTITATIVE ANALYSIS• Based on the determination of one of the

functional groups.E.g.: concn of hexanol in hexane-hexanol mixture.

A = -log I1/I0 = abc (Beer-Lambert’s law)

A = AbsorbanceI0 = Intensity of radiation before entering the sampleI1 = Intensity of radiation after leaving the samplea = Absorptivity of the solutionb = Initial path length of the sample cellc = concn. of the solution

If ‘b’ & ‘a’ are const., then ‘A’ α ‘c’27

2 methods to determine ‘A’ and conc. ‘c’:

1. Cell-in cell-out Method:Std. calibration curve method

2. Baseline Method: selection of suitable absorption band P0 & P are measured

Abs, log (P0/P) plotted against conc; determine unknown

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Baseline Method:

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Advantages:1. Common possible errors are eliminated.2. Same cell is used for all determinations.3. All measurements are done on points defined by

the spectrum; hence no dependence on λ intensity.

4. Eliminate changes in instrument sensitivity and source intensity.

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Using KBr Pellets (Disk Technique): Uniform pellets of similar weight & thickness Known wts. of KBR + known qty of test Calibration curve plotted Disks are weighed and thickness measured

Using Internal Std. (pot. thiocyanate): Dried, ground with KBr to make a conc of 0.2% by wt

of thiocyanate. Calibration curve plotted. Ratio of thiocyanate absorption at 2125 cm-1 to a

chosen band of test is plotted vs conc.31

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THANK YOU