Infra Red Spectroscopy in Chemistry - Essay Example

INFRA RED SPECTROSCOPY ASSIGNMENT Part A: Infra red spectroscopy can be used as both a qualitative and quantitative tool? Discuss what is meant by this statement, giving examples in each case. Which of the two processes is more difficult to achieve and why? Infra-red (IR) spectroscopy is an experimental tool for obtaining information about chemical structure. Molecules can absorb infra-red light of frequencies that match the energy of changes in molecular vibrations. Infra-red spectroscopy is therefore often also called vibration spectroscopy. Infra-red spectroscopy has been a ‘workhorse’ (Introduction to Fourier Transform Infra-red Spectrometry, 2001, p.3) tool for both a qualitative and quantitative material analysis in the laboratories over more than seventy five years. When infra-red radiation strikes a sample; some of the radiation is absorbed by the sample and others simply pass through it. The absorbed radiation is converted into energy of molecular vibration by the sample molecules. The frequency of absorption of radiation depends on the masses of the atoms in the molecule, the force constants of the bonds, and the geometric structure of the molecule. Hence, the resulting spectrum is an exact replica of sample contents and its molecular structure as supported by Beer-Lambert Law (Duckworth, 1998) which states that there is a linear relationship between how much light is absorbed by a sample and the product of the concentration of the absorbing species and its path length. A close examination of infra-red spectrum fingerprints reveals that absorption peaks correspond to the frequency of vibration between the bonds of the atoms of sample material while the magnitude of peaks corresponds to the amount of material present. Since each sample has a unique molecular structure just like a fingerprint so no two unique molecular structures produce the same infra-red spectrum (leaving few exceptions). Hence the information provided by infra-red spectra about the composition as well as the structure of a molecule of a sample material clearly paves a way for both qualitative a “positive identification” and (with an aid of modern software algorithms) quantitative i. e. “amount” (Introduction to Fourier Transform Infrared Spectrometry, 2001, p.3) analysis of material, an invaluable tool for QA/QC or contamination analysis applications. Infra-red Spectroscopy is used both in in-lines and on-lines material analysis. For example, Ethanol content is the single most important parameter in alcoholic beverage industries. ‘For many economic and regulatory reasons, convenient, accurate, and fast quantitative determination of ethanol in mixtures is important. With the increased importance of quality assurance in industries, Near-IR spectroscopy has become an important quantitative tool for solvent characterization’ (Liquid Phase Quantitative Analyses Using Axsun’s Targeted-Range Near-Infrared Spectrometers, 2001, p.1). Another example for both qualitative and quantitative analysis is the study of ink components on paper using Synchrotron Reflectance Infrared (SR-IR) Spectroscopy as a rapid, direct, non-destructive method (Wilkinson, et al., n. d.). Although IR spectroscopy is a superior analytical tool for both qualitative and quantitative material analysis, compared to conventional analytical methods yet it has few limitations and has to rally on conventional methods like NMR, chromatography and mass spectrometry. In qualitative analysis the positive identification of a material is done by matching the sample IR spectrum with some reference spectrum. This reference spectrum is obtained on the basis of information provided by other analysis methods. Also in many cases where exact match to the spectrum of an unknown material cannot be found, analysis have to be done by combining ‘with the data from other analysis such as NMR or mass spectrometry; a positive identification or high-confidence level tentative identification can often be achieved’. Based on Lambert’s law IR quantitative analysis method ‘is generally considered to provide only semi- quantitative analysis of common samples’ (Hsu, n. d. , p. 270). The margin of error (1 to 2%) and detection and sensitivity limits (0.01% to 2%) both have instrumental and sample procedural restrains. The Lambert’s law linear relation between the intensities of absorption bands and the concentration of each component is defined for a homogeneous mixture or solution. As homogeneity of sample particularly for solids varies from sample to sample hence a deviation in Lambert’s can be observed. Also during (pellets or wafers) sampling process a partial or entire rupture in the structure of nonporous material occurs (Hullmet, 1998, pp. 42-45). These facts results a nonlinear relationship for plots of absorbance against concentration of a sample; a clear violation of Lambert’s law. This makes quantitative analysis a bit more difficult method to have desired spectra. Part B: Infra red spectroscopy provides information concerning the vibrational modes within a molecule. Discuss the two possible types of vibration, using carbon dioxide as an example. Use sketches to illustrate your answer. IR spectroscopy provides information about the vibrational modes within a molecule. Since ‘each molecule has a certain natural vibrational frequencies’ (Phan, H. n. d.); so when a molecule is exposed to an IR radiation; and if the frequency of radiant matches with the natural vibrational frequency of the molecule; the molecule absorbs the radiation. This absorption of energy results the vibrational and rotational transitions within the molecule known as stretching and scissoring or bending modes of vibration. In (symmetrical or asymmetrical) stretching mode of vibration the inter-atomic distances within a molecule vary while bending or scissoring mode corresponds to a change in relative position of atoms with respect to original bond axis. As both modes of vibration change the inter- and intra-geometry of a molecule hence it causes a change in dipole moment of the molecule. The CO2 molecule vibrational modes can be illustrated by fig.1 as depicted by online edition for students of organic chemistry lab courses at the University of Colorado, Boulder, Dept. Chemistry and Biochemistry (2002 p.159). Figure 1: Stretching and bending vibrational modes for CO2 In 3D Cartesian coordinate plane the degree of freedom for a polyatomic linear molecule like CO2 or non-linear molecule like H2O is 3n where n stands for total number of atoms presents in a molecule. Since in 3D, three degree of freedom are required for transitional motion in entire molecular space while three degree corresponds to rotational transition. Hence total of 6 degree of freedom are required to describe the both rotational and vibrational transition of a molecule the remaining fundamental vibrations. Also for linear shape molecule only 2 degrees of freedom is sufficient to rotational transition the net number of fundamental vibrations for nonlinear and linear molecules are 3n – 6 and 3n – 5 respectively. As the probability of oxygen atoms in CO2 is 180º apart because of double covalent bond hence CO2 has a linear symmetry and has four fundamental vibrations. The [linear] asymmetrical stretch of CO2 gives a strong band in the IR at 2350 cm–1. The two scissoring or bending vibrations are equivalent and therefore, have the same frequency and are said to be degenerate, appearing in an IR spectrum at 666 cm–1. A symmetrical vibration along the axis produces no change in the center and therefore is infra-red inactive. (Online edition of lab courses at the University of Colorado, 2001, p.158-159).Then asymmetrical stretching gives a charge separation and thus is capable of interacting with radiation of the same frequency. Part C: Infra red spectra can be obtained from samples in the solid, liquid and gas phase. Discuss briefly the techniques employed to run an infra red spectrum for each type of sample. Infra red spectra can be obtained from samples in the solid, liquid and gas phase. As IR Spectroscopy is the oldest and most widely used general analytical method hence a lot sampling techniques are also developed and utilized for material analysis. Infra-red spectra of opaque materials can be obtained either by reflection or emission of IR radiation or by dissolving or diluting them in a suitable solvents (such as CCl3 or CS2 or ethylene, n-hexane or n-heptane for the region between 4000 – 625 cm–1). Liquid cells (BaF2, AgCl, or KRS-5) as (shown in fig. 2 (NUANCE, 2006)) are used to obtain the spectra of aqueous samples and dilute solutions of solid and liquid. For IR spectra of semi- or non-volatile liquids, a thin film of approximately 0.01mm thickness squeezed between two salts palate (like NaCl or Silver chloride and Barium fluoride) is used. Figure 2: Liquid Cells IR sample Deposition or smeared technique is also employed for non-volatile liquids and solids. In this method a sample is first dissolved in a reasonably volatile solvent. A few drops of the resulting solution are placed on ‘the highly polished salt plates (such as NaCl, AgCl or KBr)’ (NUANCE, 2006). Both the plates are clamped together and a sandwich like structure in made as shown in fig. 3. A thin layer of sample material acts as sample. Solids sample are obtained after evaporation off the solvent. Figure 3: Sandwich like IR Sample The infra-red spectra of solids having high melting points can be obtained by using pellets and mulls technique. For both methods a fine mixture of sample (about 2μm or less in size) is first prepared. In pellets technique an amount of sample from 0.5 to 1.0 mg material is first mixed with a 100 mg moisture free KBr. Then this mixture is ground to a fine pasty like powder with particle size of less than 5 mm in diameter. ‘Grinding and mixing can be done with an agate mortar and pestle, a vibrating ball mill (Wig-L-Bug from Crescent Dental Manufacturing), or by lyophilization’ (Hsu, n. d., p. 261). Large particles can cause a scattering effect and hence a slope base line of spectra. Finally this fine mixture is pressed into an evacuable die at sufficiently high pressure and suitable disks or pellets are made prepared as sample. This technique can be illustrated by following fig. 4 (NUANCE, 2006). Figure 4: Different steps in Pellets sampling technique. In mulls method the fine ground sample is suspended into a mulling agent. ‘The common mulling agents include mineral oil or Nujol (a high-boiling hydrocarbon oil), Fluorolube (a chlorofluorocarbon polymer), and Hexachlorobutadiene’ (Hsu, n. d. , p. 261). A 1 to 2 drops of mulling agent like Nujol to be added to 1 to 5 mg of sample because if too much of amount is added to the sample being analysed the IR will show readings of the nujol and not the sample as ‘the information in the C-H stretching region is lost because of the absorptions of the mulling agent’ (Infra red spectroscopy, n. d. , pp. 75-75). Then a thin film of this suspension is placed between two salt plates and used as a sample. A standard magnetic holder (as give in fig. 5) is mostly used for holding a sample. Figure: 5 Standard Magnetic sample holder Attenuated total reflectance (ATR) sampling technique ‘is one of the most Versatile sampling technique’ (Hsu, n. d. , pp. 249-263) and employed a large number of highly absorbing solids and liquids material. It based on ‘evanescent wave’ (Hsu, S. n.d. p.261) phenomena. When a beam of radiation enters from a medium of higher refractive index to a medium of lower refractive index at an angle greater than critical angle total reflection occurs, yet before total reflection, the incident beam penetrates few μm into the denser medium causing an attenuation or reduction in intensity of radiation. This penetration is termed as the evanescent wave. The resulting ATR-IR spectrum has same parameters as the conventional IR spectrum, except a relative change in intensities of corresponding bands. Except these methods ‘Seculars reflectance, a non-destructive technique for measuring thin coatings on selective, smooth substrates without sample preparation, gas cells method, to examine gases or low-boiling liquids, diffuse reflectance technique for acquiring IR spectra of powders and rough surface solids, and many more other methods are available to have desired sample spectra. Part D: Discuss what is meant by subtraction of spectra. In you answer considers the use of a background scan and any other situation where you may be required to carry out a subtraction. In many off-line qualitative and quantitative analysis of materials using Fourier transform infra-red (FT-IR) spectrometer, a ‘background scan’ (Taking an IR, 2007); i.e. a scan with no sample in the beam is done. The purpose of the background scan is to remove any contaminating peaks from the spectra. Then after normal scanning of sample, the sample spectra are subtracted from the background spectra by using modern computer software. The subtraction of spectra is also used in many industrial QC/QA applications where sample spectra are compared with patent sample of spectrum. Part E: Spectral libraries are databases of infra red spectra. What use are they? Another important application of infra-red spectroscopy is spectral libraries which are databases of infra-red spectra? Based on Attenuated Total Reflective (ATR) techniques, a large number of standard infra-red spectra are store in these libraries. These libraries are used for identifying materials and to compare sample analyzed with ATR to ATR Spectral library databases. Comparing samples analyzed with ATR to ATR Spectral library databases is the most accurate method of library searching because of the analogous parameters by which the spectra were obtained. Results are not only more reliable but more beneficial when qualifying samples or identifying unknowns. (ATR Spectral Libraries HIGH QUALITY GENERAL PURPOSE DATABASES, 2005). Infrared spectroscopy correlation table Serial # Compound Peak Type of bond Wave # cm–1 Comments 1 Banzamide C6H5CONH2 A C-H 2894.7 Broad blunt peak or NACL Plates B 2360.1 Short/ small sharp peak C C=O 1616.1 Small sharp peak D N-H 1540.0 Likely to be N-H E N-H 1570.0 Likely to be N-H F N-H 1564.0 Likely to be N-H G C-H 1451.5 C-H I C-O 1373.9 C-O J Benzene ring 768.2 Benzene ring small / very short peak K Benzene ring 797.5 Benzene ring small / very short peak L C-N 1020.0 C-N M C-N 1119.0 C-N 2 Butan-2-ol CH3CH2CH(OH)CH3 A O-H 3354.2 Broad peak B C-H 2944.2 Sharp/ small / narrow triple C 2351.7 Relative short peak compare to A &B D C-H2 1457.4 Small/ relatively sharp peak E C-H3 1373 Small/ relatively sharp peak F C-O 1110.7 relatively broad 3 Nitronbenzoic Acid (4-Nitrobenzoic Acid) O2NC6H4COOH A O-H 2911.5 Likely to be O-H bond broad peak B 2351.7 A CO2 peak due to contamination with IR machine C C=O 1678.8 Medium peak D -NO2 1457.4 Small & sharp peak E -NO2 1373.9 Small & sharp peak F -NO2 1290.0 Small & sharp peak G C-N 1144.1 Small & sharp peak I C-N 1069.0 Medium thin not, very sharp peak J Benzene ring 797.5 Small Infrared spectroscopy correlation table Serial # Compound Peak Type of bond Wave # Comments 4 CH3-(CH2)6-CH3 Octane CH3(CH2)6CH3 A C-H 2919.8 Typical of two peaks B 2653.0 Relatively long short sharp C 2351.7 Short small sharp peak D C-H2 1461.6 Short not very sharp peak E C-H3 1378.1 Short not very sharp peak 5 Propanoic Acid CH3CH2COOH A O-H 2978.3 Likely to be O-H bond B CO2 Peak due to contamination 2351.7 CO2 each compound produced a carbon dioxide peak due to some contamination with the IR machine C C=O 1712.2 Medium peak D 155.7 Small & sharp peak E 1536.8 Small & sharp peak F C-H2 1465.8 Small & sharp peak G C-C 1415.7 Small & sharp peak H C-H3 1240.2 Medium thin not & very sharp peak I 1877.3 Small & sharp peak 6 Phenylethanone (1-phenylethanone) C6H5COCH3 A C-H 2928.8 Small / thin peak B 2351.7 Double sharp peak relatively small C C=O 1683.3 Long relatively thin peak D C=C 1595.3 Small / thin /sharp peak E 1449.1 Thin sharp peak F C-H3 1357.2 Relatively small / sharp peak G 1265.3 Long / small peak Sampling Techniques Depending upon the physical state of sample material following important sampling techniques can be used. 1. Liquid Cells Liquid cells are small cells consists of a metal frame, an IR transmitting window and gasket that determine the length of the cell. In these cells a neat drop of film of sample is placed and squeezed between two plates and spectrum is obtained. Depending on the states of sample different liquid cells are used. For example for opaque liquid and solid samples spectra are obtained by dissolving them in liquid cells of carbon tetrachloride, tetrachloroethylene chloride, n-hexane or n-heptane. The first two solvent have toxic nature and should be carefully handled. Polar solvents are rarely used because of their polar nature. For aqueous samples thin cells of of BaF2, AgCl, or KRS-5(a mixed thallium bromide–thallium iodide) are used. ‘Sodium chloride disks are the most popular and economical choice for no aqueous liquids. Silver chloride or barium fluoride plates may be used for samples that dissolve or react with NaCl plates’. 2. Pellets and Solid Nujol Moll Pellets are frame like structures and Nujol is high-boiling hydrocarbon oil. For spectra of solids having high melting point pellets and nujol molls are used. For both methods a fine mixture of sample (about 2μm or less in size) is first prepared. In pellets technique an amount of sample from 0.5 to 1.0 mg material is first mixed with a 100 mg moisture free KBr. Then this mixture is ground to a fine pasty like powder with particle size of less than 5 mm in diameter. ‘Grinding and mixing can be done with an agate mortar and pestle, a vibrating ball mill (Wig-L-Bug from Crescent Dental Manufacturing), or by lyophilization’ (Hsu, n. d., p.261). Large particles can cause a scattering effect and hence a slope base line of spectra. Finally this fine mixture is pressed into an evacuable die at sufficiently high pressure and suitable disks or pellets are made prepared as sample. In mulls method the fine ground sample is suspended into a mulling agent such as Nujol. One or two drops of mulling agent like Nujol to be added to 1 to 5 mg of sample because if too much of amount is added to the sample being analysed the IR will show readings of the nujol and not the sample as ‘the information in the C-H stretching region is lost because of the absorptions of the mulling agent’ (Infra red spectroscopy, n. d. , pp. 75-75). Then a thin film of this suspension is placed between two salt plates and used as a sample. 3. Light pipe Flow cells The light pipe flow cell is a piece of glass tube 10 to 20 cm long having 1mm inner diameter with a gold coated inner side with IR transmitting window on each side. ‘The light pipe is connected to effluent port of the gas chromatograph by a heated transfer line. Eluents from a capillary gas chromatograph flow through the transfer line into the light pipe, wherethe IR spectra are acquired in real time with a rate up to 20 spectra per second’ (Hsu, n. d. , pp. 261-265).. References ATR Spectral Libraries HIGH QUALITY GENERAL PURPOSE DATABASES (2005). Smiths Detection Inc. [Internet] Available from: [Accessed 11 February, 2007] AXSUN Technologies Inc. (2005) Liquid Phase Quantitative Analyses Using Axsun’s Targeted-Range Near-Infrared Spectrometers: Application note [Internet], pp. 1-2. Available from: [Accessed 10 February 2007]. Cantu, A. A. (n. d.) Use of Synchrotron Reflectance Infrared Spectromicroscopy as a Rapid, Direct, Non-Destructive Method for the Study of Inks on Paper. [Internet], pp. 1-5. Washington: U.S. Secret Service, Department of Treasury. Available from: < www-als.lbl.gov/als/compendium/AbstractManager/uploads/01107.pdf> [Accessed 12 February 2007] Duckworth, J. H. (1998) Spectroscopic Quantitative Analysis in Applied Spectroscopy: A Compact Reference for Practitioners. Academic Press. Hsu, S. C. -P. (n. d.). Handbook of Instrumental Techniques for Analytical Chemistry: Infrared Spectroscopy 15, 243-277. [Internet] Mallinckrodt Baker Division, Mallinckrodt, Inc. Available from: < www.prenhall.com/settle/chapters/ch15.pdf> [Accessed 10 February, 2007] Hellmut, G., Karge, J., & Beck, J. (1998) MOLECULAR SIEVE CHARATERIZATION. pp. 43-46. [Internet] Springer Science. Available from: [Accessed 16 Febraury 2007] Keck Interdisciplinary Surface Science Centre. (2006). How to prepare IR samples? NUANCE [Internet] North-western University, Campus Evanston, IL. Available from:< www.nuance.northwestern.edu/KeckII/ftir4.asp > [Accessed 17 February 2007] Online edition for students of organic chemistry lab courses (2002) Infrared Spectroscopy: Theory [Internet]. Boulder, University of Colorado Dept of Chemistry & Biochemistry. Available from: netLiberary < http://www.netLiberary.com> [Accessed 10 February 2007]. Phan, H. (n. d.) FUNDAMENTAL INFRARED SPECTROSCOPY p. 3. Taking an IR (2005). Journal of Vacuum Science and Technology B [Internet] Available from: < http://www.avs.org./literature.jvst.aspx > [Accessed 11 February, 2007] Thermo Nicolet Corporation. (2001) Introduction to Fourier Transform Infrared Spectrometry. Thermo Nicolet [Internet], pp. 3-7. Available from: < www.mmrc.caltech.edu/mmrc_html/FTIR/FTIRintro.pdf > [Accessed 10 February 2007]. Wilkinson, T. J., Perry, D. L., Martin, M. C., McKinney, W. R. (n. d.) Use Of Synchrotron Reflectance Infrared Spectromicroscopy as a Rapid, Direct, Non-Destructive Method for the Study of Inks on Paper. [Internet], pp.1-3. Berkeley: Lawrence Berkeley National Laboratory Available from: < www-als.lbl.gov/als/compendium/AbstractManager/uploads/01107.pdf> [Accessed 12 February 2007]. Read More