Ftir Report

The Hong Kong Polytechnic University Department of Applied Biology and Chemical Technology

ABCT458 Advanced Analytical Techniques Lab Report Experiment: FTIR Reflectance Measurements Using A FTIR Spectrophotometer

Mar 25, 2008

Group 1B Name (Student Number)

Duty

NI Qing (05995048d)

Group Leader

MAN Lai Fan (07515458d)

Introduction

YEUNG Shing Hin (07533415d) Theoretical Section LI Lai Ping (07546484d)

Experimental Section

FUNG Yat Sing (07524431d)

Results & Discussion

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Table of Contents 1. Introduction........................................................................................................................................................1 2. Theory................................................................................................................................................................2 A. Attenuated Total Reflection (ATR)...................................................................................................................2 B. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT).........................................................2 3. Procedure............................................................................................................................................................2 Part 1.................................................................................................................................................................2 Part 2.................................................................................................................................................................3 4. Results and Discussion.......................................................................................................................................3 (1) Comparison of the Milk Contents (ATR).......................................................................................................3 (2) Identification of Unknown Polymers with DRIFT spectroscopy....................................................................4 5. Conclusion..........................................................................................................................................................7 6. References..........................................................................................................................................................7 7. Appendices.........................................................................................................................................................8

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1. Introduction Fourier transform infrared spectroscopy (FTIR) is a measuring technique of getting the infrared spectra by measuring the interferogram (time-domain signal) of the samples with the use of interferometer and then the interferogram is Fourier transformed to give the spectrum (frequency-domain functions). Fourier transform is a data processing tool and infrared spectroscopy is the study of interaction of infrared radiation with matter with the resultant spectrum showing the absorption due to various distinctive functional groups, giving a molecular fingerprint of the studied compound. FTIR can be applied for the identification of the unknown, determination of the quality of the sample and the components of a mixture. There are mainly two common types of infrared sampling techniques-they are the transmission techniques and the reflectance techniques. For the transmission mode, the infrared radiation beam passes through the sample directly. And it is the most popular way of obtaining the infrared spectra because of its various advantages such as it is relatively inexpensive, having high signal to noise ratio and universal –can work on various kinds of samples such as polymers, gases, liquids and solids that can be ground into a powder with alkaline metal halide such as KBr pellet or suspended in an oil. In the reflectance technique, the infrared light beam is bounced off the sample rather than passing through the sample. The reflectance technique can solve the thickness problem that the transmission mode may have. Hence, the method is less time consuming as there is no need for the sample to be in the proper concentration, thickness or the proper amount of light passing through and hence no dilution or grinding of the sample is required. And the method is non-destructive. Hence, the technique can be applied to case where it is difficult to study with the transmission technique. The attenuated total reflection (ATR) and the Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) are studied in this experiment. DRIFTS can be used to examine powder and solid, even very large and intractable samples such as pieces of plastic and furniture. ATR can be applied to examine polymer films and semisolid samples such as shampoo, toothpaste and tomato sauce etc. 1

In this experiment, various milk and polymer samples are studied with the reflectance measurement techniques using the FTIR spectrophotometer.[1-3] 2. Theory A. Attenuated Total Reflection (ATR) There is a phenomenon called total reflection that a light beam will be totally reflected when it travels from a more refractive medium to less refractive one with incident angel greater than critical angel. Critical angel, θ c = sin − 1 η 2 where is η1 the refractive index of more refractive η1 medium and η2 is the refractive index of less refractive medium. In ATR, a highly refracting prism is used. Although the IR light beam is expected totally refracted, evanescent wave will get through to the shallow surface of the less refractive medium and absorbed by the sample that contact to the prism. B. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT) Diffuse reflection and specular reflection are the components observed of reflection from solid surface. In DRIFT, light shines onto the powdered sample, specular component, where the light is reflected directly off, was blocked and diffuse reflectance, where the light penetrated, transmitted and reflected within the sample, was collected and analyzed. Both of the methods can use in sample that is not pulverized or palletized easily which cause difficulty in analysis by typical methods.[1-3] 3. Procedure Part 1 (The power of the PC had turned on and the spectrometer operating software had loaded)

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The parameters such as resolution were set on the spectrometer operating software. We had made sure that the surface of the ATR prism was cleaned. The background was then collected by operating the spectrometer. After that, a few drops of water were added on the surface of the prism by dropper. The spectrum of water was collected. The water was removed from the prism surface by absorbing it away with soft tissue paper. The fullcream milk samples were then added to the surface of prism by the same steps of adding water. And the spectrum of full-cream milk was collected by operating the spectrometer. Finally, the operating software was operated to subtract the water spectrum from the milk spectrum. The spectrum of full-cream milk after spectral subtraction of water was collected. The same procedures were done for the skimmed as well as reduced-fat milk. Part 2 The sample was inserted into the sample holder and a blank spectrum of the gold plated surface was recorded. A piece of Si-C abrasive paper was adhered to the surface of the platen and then placed into the sample holder to obtain the spectrum of the Si-C abrasive paper. The platen was screwed onto the probe and the Si-C paper was rubbed against the surface of sample (PMMA) for a few seconds. The platen was unscrewed and placed onto the sample holder. The spectrum of the sample was obtained. The same procedure was done on another sample (polystyrene). After recording all the sample spectra, the operating software was operated to convert the y-axis of the spectrum from reflectance into absorbance and then substrate the blank from the sample spectra. The OMNIC software was operated to search the major absorption bands from a library search functions. 4. Results and Discussion (1) Comparison of the Milk Contents (ATR) Four spectra of the milk samples are attached. Three of them are for each of the fullcream milk, reduced-fat milk, and skim milk, and the other one summarizes all of them for comparison purpose.

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Because the three milk samples differ in fat content, it is useful to identify the characteristic peaks that result from fat. In this experiment, the absorption at about 2874 and 1747 cm-1 are used. They correspond to the C-H bond of fatty acid and C=O of ester linkage.[4] The absorptions at these two frequencies decrease from full-cream milk, to reducedfat milk, and to skim milk. This is in agreement with the fat content in the three milk samples. It is noted, however, that some absorptions still remain even in the skim milk sample. This may be due to the residue fat, or may be due to the hydrocarbon and carbonyl (peptide bond) in the protein. (2) Identification of Unknown Polymers with DRIFT spectroscopy The powders ground from a “plastic” tray (Sample 1) and a bucket cover (Sample 2) were put into the FT-IR spectrophotometer for spectrum acquisition. The spectra were then compared by the reference spectrum obtained from automated library search. Spectrum of sample 1 had closest resemblance with that of poly-(methyl methacrylate), with a similarity of 91.96%. Sample 2 is closest to polystyrene, with a matching score of 91.85%. The primary source of error is due to a diminished reflectance observed at lower frequency side. The phenomenon will be discussed later.

poly-(Methyl Methacrylate)

poly-Styrene

Diagram 1: Polymer structures

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The major peaks in the spectra are identified and summarized below (in cm-1) poly-(Methyl Methacrylate) 2900~3000

1730

Stretching of C-H ( Saturated) Stretching of ester C=O

poly-Styrene 2820~2920

3000~3100

1600

Stretching of C-H (Saturated) Stretching of C-H (Aromatic) Stretching of C=C (Aromatic)

Table 1: Identification of the major peaks in the two spectra

(3) Answers to the Questions 1. What are the causes of the prominent absorption features in the background spectrum around 1200 to 2000 cm-1, 2300 to 2400 cm-1 and 3500 to 4000 cm-1? Ans. The absorption around 2300 to 2400 cm-1 is due to the asymmetric stretching of CO2, the absorption around 1200 to 2000 cm-1 is due to the vibrational and rotational motion of water, the absorption around 3500 to 4000 cm-1 is the overtone of the absorption peaks caused by water.

2.Why is there a sudden drop off of transmission to zero at frequencies below ~650 cm-1? Ans. The IR absorption of ZnSe crystal below about 650 cm-1 causes the sudden drop off of transmission to zero at frequencies below ~650 cm-1.

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3. Can you distinguish the different types of milk by looking at the IR spectrum alone? Ans. By just looking at the IR spectrum alone, the types of milk cannot be distinguished, because the peak caused by IR absorption of fat cannot reflect the fat content in milk, in fact, the peak of IR spectrum caused by fat is still present in the spectrum of skim milk (this may caused by the trace amount of residual fat or other components having the same absorption frequency), it’s impossible to distinguish the type of milk by just looking at the IR spectrum alone.

4. Why do diffuse reflectance spectra obtain with the Si-C paper scrubbing technique show a general decrease in reflectance towards the lower frequency end? Ans. In the diffuse reflectance spectroscopy, the reflected light from the soild sample was measured. However, in most cases, such light often comes along with specular reflection, which bounces off the surface instead of penetrating into the depth of the sample. It can interfere with diffuse reflectance, and cause distortion in the spectrum. Some papers have described this phenomenon in detail with advanced knowledges[5-7]. However, for the time being, it might be explained in the following simple way. Since diffuse reflection requires the light to penetrate into the sample, it inevitably travels more path than specular reflection. If the difference in path length is equal to integer of wavelength,constructive Diagram 2: Interference between two kinds of reflections

interference occurs. If it is equal to “integer and half”

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wavelength, destructive interference occurs. So depending on the wavelength, the reflectance will have slightly (sometimes maybe severely) altered. In the DRIFT spectrum, the low frequency side is closer to destructive interference, thus the observed reflectance is smaller.

5. Is background correction a necessity for the identification of the chemical nature of the sample in diffuse reflectance measurements? Ans. Background correction is needed for identification of the chemical nature of the sample in diffuse reflectance measurement. The container used to contain the sample for the test, e.g. the Si-C paper used in this experiment, would not totally reflect the incidence light by specular way, it also reflects the light by diffuse reflectance, i.e. it also absorbs the incidence light as the sample for test. To eliminate the effect of diffuse reflectance of the container, background correction is needed in order to make the spectrum of the sample given out more accurate.

5. Conclusion The ATR infrared spectra of the three types of milk – full-cream, reduced-fat, and skim – were obtained. The relative fat content was justified from the peaks of spectra. Also, the compositions of the tray and bucket cover were identified with DRIFT spectroscopy. They correspond to poly-(Methyl Methacrylate) and polystyrene.

6. References [1] Skoog D.A, West D.M. Principles of Instrumental Analysis, 5th ed. Saunders, Philadelphia, 1998 Chapters 16~17 [2] Kincaid J.R, Instrumental Analysis, 2nd ed. 1986 Chapter 8 [3] Smith B.C. Fundamentals of Fourier Transform Infrared Spectroscopy, CRC Press, Chapter 4 7

[4] Fernando A.I, Salvador G., Miguel de la G., Analytica Chemica Acta 513(2004), 401-402 [5] Hembree D.M.(Jr), Smyrl H.R., Journal of Applied Spectroscopy 1989 43(2) 267-274 [6] Schmitt J.M., Kumar G., Applied Spectroscopy 1996 50(8) 1066-1072

[7] KORTE E.H., STAAT H., Journal of Molecular Structure 1990 218 381-386

7. Appendices Appendices I~IV are the spectra of three milks (indicated by the title on the upper left corner) and a graph including all the three spectra for absorption. The correct order are from top to bottom: Full-cream, reduced-fat, and skim milk. There is a mistake in the original print-out of this graph. Appendices V~VI are the spectra for part B, the DRIFT technique for identification of polymers

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