The Hong Kong Polytechnic UniversityDepartment of Applied Biology and Chemical Technology

ABCT458Advanced 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 Contents1. 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.

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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, 1 2

1

sincηθη

−= where is η1 the refractive index of more refractive

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 full-

cream 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 full-

cream 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 reduced-

fat 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) poly-Styrene

2900~3000Stretching of C-H

( Saturated)2820~2920

Stretching of C-H

(Saturated)

1730 Stretching of ester C=O 3000~3100Stretching of C-H

(Aromatic)

1600Stretching 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

interference occurs. If it is

equal to “integer and half”

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Diagram 2: Interference between two kinds of reflections

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. ConclusionThe 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

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[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. AppendicesAppendices 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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