Simply, it is the absorption measurement of different IR frequencies by a sample positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is to determine the chemical functional groups in the sample. Different functional groups absorb characteristic frequencies of IR radiation. Using various sampling accessories, IR spectrometers can accept a wide range of sample types such as gases, liquids, and solids. Thus, IR spectroscopy is an important and popular tool for: * Identification of unknown materials. * Determination of the quality or consistency of a sample. Determination of the amount of components in a mixture. * Identification of all types of organic and many types of inorganic compounds. * Determination of functional groups in organic materials. * Determination of the molecular composition of surfaces. * Identification of chromatographic effluents. * Quantitative determination of compounds in mixtures. * Non-destructive method. * Determination of molecular conformation (structural isomers) and stereochemistry (geometrical isomers). * Determination of molecular orientation (polymers and solutions). By nterpreting the infrared absorption spectrum, the chemical bonds in a molecule can be determined. FTIR spectra of pure compounds are generally so unique that they are like a molecular "fingerprint". While organic compounds have very rich, detailed spectra, inorganic compounds are usually much simpler. For most common materials, the spectrum of an unknown can be identified by comparison to a library of known compounds. Finger print regionFor every compound a very complicated series of absorptions occur between wave numbers 500 to 1500 due to a variety of bending and stretching within the molecule.
This region is called the finger print region. Each compound has a unique set of troughs within the fingerprint region that can be used to identify the molecule. It is hard to identify individual troughs due to specific bonds in the fingerprint region. Attenuated Total Reflectance (ATR)In Attenuated Total Reflectance (ATR) spectroscopy all that is required for analysis is that the sample of interest be brought into contact with the ATR crystal. The infrared beam is passed into the ATR element such that its angle of incidence exceeds the “critical” angle.
Under this condition total internal reflection of the beam occurs and a standing evanescent wave is established at the ATR crystal/sample interface. The amplitude of this wave decays rapidly with increasing distance from the reflecting interface thus sample concentration and thickness are not a concern for these measurements. Minimal to no sample preparation is required for this technique and a wide variety of solids and some liquids (dependent upon crystal material) can be analyzed using ATR[Ref 3]Zinc Selenide (ZnSe) ComponentsZinc Selenide (ZnSe) is the most popular material for infrared application and it is chemically inert.
Due to very wide transmission range covering 0. 6 to 20 m m CVD grown ZnSe high optical quality material is used to manufacture optical components (windows, mirrors, lenses etc. ) for high power IR lasers. [Ref 4]| Materials and Methods. | Material: FTIR instrument, Liquid sample. Method:1. The background of the environment (water vapour, covalent bond) was measured before placing sample. 2. Sample placed onto the cell and pressed sample collection on the instrument. 3.
Sample cell cleaned using ethanol and cotton wool(water cannot be used )4. Results collected and interpreted. | Results. | Compound no:| Identified as:| Aromatic or Aliphatic | 1| Ester| Aliphatic| 2| Alcohol/Phenol| Aliphatic| 3| Hydrocarbon| Aromatic| 4| Keton| Aromatic| | Discussion. | 1. Compound identified as Ester : C-H absorption around 3000cm-1 C=O or Carbonyl peak, Strong absorption in 1820-1660 cm-1 C=O Strong intensity absorption near 1300-100 cm-1 Not Aromatic 2.
Compound indentified as Alcohol/Phenol:C-H absorption around 3000cm-1NO C=O Strong absorption in 1820-1660 cm-1O-H Broad absorption near 3300-3600 cm-1Aromatic C-H occurs to the left of C-H region (3000cm-1) and aliphatic to the right. 3. Compound indentified as Hydrocarbon :C-H absorption around 3000cm-1NO C=O or Carbonyl peak, Strong absorption in 1820-1660 cm-1MAJOR absorption near 3000cm-1 C-H region and only one other absorption at 1450-1375cm-1 4.
Compound indentified as Keton:C-H absorption around 3000cm-1 C=O or Carbonyl peak, Strong absorption in 1820-1660 cm-1No OH broad absorption near 3400-2400 cm-1 (Acid)No C-O single bond absorption near 1300-1000 cm-1(Ester)No C-H absorption near 2850-2750 cm-1 on the right hand side of the C-H absorptionFinally compounds were assigned to aromatic or aliphatic groups follows: Medium to strong absorptions in the region 1650/1450cm-1 the presence of an aromatic ring. Please see table in Results section.
Consult the C-H region (3000cm-1)aromatic C-H occurs to the left of the 3000cm cm-1and aliphatic to the right. [Ref 1]| Conclusions. | The interpretation of infrared spectra involves the correlation of absorption bands in the spectrum of an unknown compound with the known absorption frequencies for types of bonds. This table will help users become more familiar with the process. Significant for the identification of the source of an absorption band are intensity (weak, medium or strong), shape (broad or sharp), and position (cm-1) in the spectrum.
CHARACTERISTIC INFRARED ABSORPTION FREQUENCIES| Bond| Compound Type| Frequency range, cm-1| C-H| Alkanes| 2960-2850(s) stretch| | | 1470-1350(v) scissoring and bending| | CH3 Umbrella Deformation| 1380(m-w) - Doublet - isopropyl, t-butyl| C-H| Alkenes| 3080-3020(m) stretch| | | 1000-675(s) bend| C-H| Aromatic Rings| 3100-3000(m) stretch| | Phenyl Ring Substitution Bands| 870-675(s) bend| | Phenyl Ring Substitution Overtones| 2000-1600(w) - fingerprint region| C-H| Alkynes| 3333-3267(s) stretch| | | 700-610(b) bend| C=C| Alkenes| 1680-1640(m,w)) stretch| C? C| Alkynes| 2260-2100(w,sh) stretch|
C=C| Aromatic Rings| 1600, 1500(w) stretch| C-O| Alcohols, Ethers, Carboxylic acids, Esters| 1260-1000(s) stretch| C=O| Aldehydes, Ketones, Carboxylic acids, Esters| 1760-1670(s) stretch| O-H| Monomeric -- Alcohols, Phenols| 3640-3160(s,br) stretch| | Hydrogen-bonded -- Alcohols, Phenols| 3600-3200(b) stretch| | Carboxylic acids| 3000-2500(b) stretch| N-H| Amines| 3500-3300(m) stretch| | | 1650-1580 (m) bend| C-N| Amines| 1340-1020(m) stretch| C? N| Nitriles| 2260-2220(v) stretch| NO2| Nitro Compounds| 1660-1500(s) asymmetrical stretch| | | 1390-1260(s) symmetrical stretch| - variable, m - medium, s - strong, br - broad, w – weak[Ref 2]| Recommendations. | 1. Do not use cells as follows :glass, quartz, plastic | References. | 1. Manual of instrumentation laboratory practice part 2,Author:Jesus Frias Celayeta,Ph. D. ,2012. Source :Galway-Mayo Institute of Technology . School of Science Dr. Kathleen Lough and Dr. Gay Keaveney2. http://wwwchem. csustan. edu/Tutorials/INFRARED. HTM accessed 02/12/12. 3. http://www. sciner. com/Opticsland/ZnSe. htm accessed 03/12/12. 4. http://www. mri. psu. edu/facilities/MCL/techniques/FTIR/FTIRdesc. asp|
