INFRARED SPECTROPHOTOMETRY

A Project Paper in Organic Chemistry

2nd Semester, AY 2012-2013

Submitted to:

Mrs. Romelisa Ibale

Chemistry Teacher

Submitted by:

Group 7 – Chem 84 EA

Abarquez, Rea Mae

Abonete, Rhonalou

Aguilar, Casey Jane

Empeynado, Dana Kaye

Responte, Mary Eurydice

March 1, 2013

INFRARED SPECTROSCOPY

Infrared spectroscopy (IR spectroscopy) is the spectroscopy that deals with

the infrared region of the electromagnetic spectrum, that is light with a

longer wavelength and lower frequency than visible light. It covers a range of techniques,

mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can be

used to identify and study chemicals. A common laboratory instrument that uses this

technique is a Fourier transform infrared (FTIR) spectrometer.

Infrared spectroscopy is a technique based on the vibrations of the atoms of a

molecule. An infrared spectrum is commonly obtained by passing infrared radiation through

a sample and determining what fraction of the incident radiation is absorbed at a particular

energy. The energy at which any peak in an absorption spectrum appears corresponds to

the frequency of a vibration of a part of a sample molecule.

THEORIES

Infrared light imposed on a molecule will not create electronic transitions but it does

contain enough energy to interact with a molecule causing vibrational and rotational

changes. For example, the molecule can absorb the energy contained in the incident light

and the result is a faster rotation or a more pronounced vibration. The possible rotations are

around the axis of symmetry for a given molecule or either of the two perpendicular axis.

Vibrations can be in the form of a bend or a stretch for each bond. Illistrated below are

possible vibrational motions for a three atom molecule (all are in the plane unless explicitly

stated):

(+ means out of screen and - is into screen)

Infrared spectroscopy exploits the fact that molecules absorb specific frequencies

that are characteristic of their structure. These absorptions are resonant frequencies, i.e.

the frequency of the absorbed radiation matches the transition energy of the bond or group

that vibrates. The energies are determined by the shape of the molecular potential energy

surfaces, the masses of the atoms, and the associated vibronic coupling.

In particular, in the Born–Oppenheimer and harmonic approximations, i.e. when

the molecular Hamiltonian corresponding to the electronic ground state can be

approximated by a harmonic oscillator in the neighborhood of the equilibrium molecular

geometry, the resonant frequencies are associated with the normal modes corresponding to

the molecular electronic ground state potential energy surface. The resonant frequencies

are also related to the strength of the bond and the mass of the atoms at either end of it.

Thus, the frequencies of the vibrations are associated with a particular normal mode of

motion and a particular bond type.

Number of vibrational modes

In order for a vibrational mode in a molecule to be "IR active," it must be associated

with changes in the dipole. A permanent dipole is not necessary, as the rule requires only a

change in dipole moment.

A molecule can vibrate in many ways, and each way is called a vibrational mode. For

molecules with N atoms in them, linear molecules have 3N – 5 degrees of vibrational modes,

whereas nonlinear molecules have 3N – 6 degrees of vibrational modes (also called

vibrational degrees of freedom). As an example H2O, a non-linear molecule, will have

3 × 3 – 6 = 3 degrees of vibrational freedom, or modes.

Simple diatomic molecules have only one bond and only one vibrational band. If the

molecule is symmetrical, e.g. N2, the band is not observed in the IR spectrum, but only in the

Raman spectrum. Asymmetrical diatomic molecules, e.g. CO, absorb in the IR spectrum.

More complex molecules have many bonds, and their vibrational spectra are

correspondingly more complex, i.e. big molecules have many peaks in their IR spectra.

The atoms in a CH2X2 group, commonly found in organic compounds and where X

can represent any other atom, can vibrate in nine different ways. Six of these involve only

the CH2 portion: symmetric and antisymmetric stretching, scissoring, rocking, wagging and twisting.

Special effects

The simplest and most important IR bands arise from the "normal modes," the

simplest distortions of the molecule. In some cases, "overtone bands" are observed. These

bands arise from the absorption of a photon that leads to a doubly excited vibrational state.

Such bands appear at approximately twice the energy of the normal mode. Some vibrations,

so-called 'combination modes," involve more than one normal mode. The phenomenon

of Fermi resonance can arise when two modes are similar in energy; Fermi resonance

results in an unexpected shift in energy and intensity of the bands etc.

Molecular Frequencies

Classical electromagnetic theory: an oscillating dipole is an emitter or absorber of

radiation

Infrared spectroscopy exploits the fact that molecules have specific frequencies at

which they rotate or vibrate

How to determine if a compound is IR active:

o Ex. CO2

o The change in the dipole moment from the symmetric stretch of the left

oxygen will be offset by the symmetric stretch of the right oxygen, therefore

is IR inactive.

IR Spectra

IR spectrum lies between the microwave and visible parts of the electromagnetic

spectrum

o Far-IR has low energy and may be used for rotational spectroscopy

o Mid-IR may be used to study the fundamental vibrations and associated

rotational-vibrational structure

o Near-IR can excite overtone or harmonic vibrations

Absorbance

• Absorption/Transmission results from coupling of a dipole vibration with the electric

field of the infrared radiation

• Both frequencies must be the same and the net dipole moment must be oscillating

• The absorption due to a particular dipole oscillation generally not affected greatly by

other atoms present in the molecule.Thus the absorption occurs at approximately

the same frequency for all bonds in different molecules (including polymers).

• More complex

molecules have

many bonds, and

vibrations can be

conjugated, leading to IR absorptions at characteristic frequenciesthat may be

related to chemical functional groups

• Absorbance is determined by the equation of A = log(I0 / I1)

o Where I0 and I are the intensities of radiation before and after transmission

through the sample

Concentration

The change in absorption from the baseline to the maximum absorption is directly

related to the Beer Lambert equation: A=ɛbc

A is the absorbance; epsilon is the molar absorptivity; l is the path length; and c is

the concentration

For IR, the path length and molar absorptivity will be constants, while the

absorbance will be determined using IR spectroscopy

Hydrogen Bonding

Hydrogen bonding between chemicals can cause an IR spectrum shift

o This is because the molecular interaction of the hydrogen atom, the atom it is

bonded to, and the atom that is pulling on the hydrogen will all have slight

changes in their dipole frequencies.

INSTRUMENTATION

The main parts of IR spectrometer are as follows:

1. radiation source

2. sample cells and sampling of substances

3. Monochromators

4. detectors

5. recorder

1. IR radiation sources

IR instruments require a source of radiant energy which emits IR radiation which

must be steady, intense enough for detection and extend over the desired

wavelength. Various sources of IR radiations are as follows.

a) Nernst glower

b) Incandescent lamp

c) Mercury arc

d) Tungsten lamp

e) Glober source

f) Nichrome wire

2. Sample cells and sampling of substances

IR spectroscopy has been used for the characterization of solid, liquid or gas

samples.

i. Solid - Various techniques are used for preparing solid samples such as

pressed pellet technique, solid run in solution, solid films, mull technique etc.

ii. Liquid – samples can be held using a liquid sample cell made of alkali

halides. Aqueous solvents cannot be used as they will dissolve alkali halides. Only

organic solvents like chloroform can be used.

iii. Gas – sampling of gas is similar to the sampling of liquids.

3. Monochromators – Various types of monochromators are prism, gratings and filters.

Prisms are made of Potassium bromide, Sodium chloride or Caesium iodide. Filters

are made up of Lithium Fluoride and Diffraction gratings are made up of alkali

halides.

4. Detectors – Detectors are used to measure the intensity of unabsorbed infrared

radiation. Detectors like thermocouples, Bolometers, thermisters, Golay cell, and pyro-

electric detectors are used.

5. Recorders – Recorders are used to record the IR spectrum.

The components of an IR machine are the IR source, beam splitter, monochromator,

a transducer, an analog to digital converter and a digital machine to quantify the

readout. The IR light exits the source and becomes split into to beams, one to be directed to

the sample the other to a reference. The intensity of the beam is measured by the intensity

emitted divided by the intensity observed, also known as the Transmittance. All frequencies

are measured in wave number, cm-1. To make a sample with a liquid, the liquid is placed

between two pure salt sheets of NaCl and for a solid it is pressure pressed with KBr to

incorporate both into one sheet. The reason for using salt to suspend the molecule is

because the salt structures form a lattice that is strongly ionically bonded and will not

absorb IR light because it lacks the vibrational capability. The Background scan or reference

tends to be air.

Below 1500 cm-1 the spectra have very high sensitivity and this region is known as

the fingerprint region where C-C bond stretching and bending motions overlap, making it

difficult to predict functional groups.

APPLICATIONS

Infrared spectroscopy is widely used in industry as well as in research. It is a simple

and reliable technique for measurement, quality control and dynamic measurement. It is

also employed in forensic analysis in civil and criminal analysis.

Some of the major applications of IR spectroscopy are as follows:

Identification of functional group and structure elucidation

Entire IR region is divided into group frequency region and fingerprint region. Range

of group frequency is 4000-1500 cm-1 while that of finger print region is 1500-400 cm-1.

In group frequency region, the peaks corresponding to different functional groups can be

observed. According to corresponding peaks, functional group can be determined.

Each atom of the molecule is connected by bond and each bond requires different IR

region so characteristic peaks are observed. This region of IR spectrum is called as finger

print region of the molecule. It can be determined by characteristic peaks.

Identification of substances

IR spectroscopy is used to establish whether a given sample of an organic substance

is identical with another or not. This is because large number of absorption bands is

observed in the IR spectra of organic molecules and the probability that any

two compounds will produce identical spectra is almost zero. So if

two compounds have identical IR spectra then both of them must be samples of the same

substances. IR spectra of two enatiomeric compound are identical. So IR spectroscopy fails

to distinguish between enantiomers.

For example, an IR spectrum of benzaldehyde is observed as follows.

C-H stretching of aromatic ring- 3080 cm-1

C-H stretching of aldehyde- 2860 cm-1 and 2775 cm-1

C=O stretching of an aromatic aldehyde- 1700 cm-1

C=C stretching of an aromatic ring- 1595 cm-1

C-H bending- 745 cm-1 and 685 cm-1

No other compound then benzaldehyde produces same IR spectra as shown above.

Studying the progress of the reaction

Progress of chemical reaction can be determined by examining the small portion of

the reaction mixture withdrawn from time to time. The rate of disappearance of a

characteristic absorption band of the reactant group and/or the rate of appearance of the

characteristic absorption band of the product group due to formation of product is

observed.

Detection of impurities

IR spectrum of the test sample to be determined is compared with the standard

compound. If any additional peaks are observed in the IR spectrum, then it is due to

impurities present in the compound.

Quantitative analysis

The quantity of the substance can be determined either in pure form or as a mixure

of two or more compounds. In this, characteristic peak corresponding to the drug substance

is chosen and log I0/It of peaks for standard and test sample is compared. This is called base

line technique to determine the quantity of the substance.

ADVANTAGES AND DISADVANTAGES

Non-Destructive

One of the primary advantages is that infrared spectroscopy causes no damage.

Several other forms of mechanical sight can detect particles through other

spectrums, but many of their methods use radiation. For example, X-ray technology

requires precautions so that the radiation doesn't cause damage to people in the

area. However, infrared radiation is harmless and won't damage the environment or

the area being viewed.

Sensitivity

One downside of using infrared spectroscopy is that it requires very sensitive and

properly tuned instruments. Any basic infrared instrument can see the infrared

spectrum, but being able to focus on it well enough to make sense of what's being

seen requires tools that are well tuned. Also, the better tuned and focused a set of

tools happens to be, the more expensive it will be to buy and maintain in the long

term.

Preparation

A major advantage of infrared spectroscopy is that the samples being viewed don't

require any sort of special preparation. Some tests may require a subject to be

bathed in radiation or have radioactive dye put into it, but infrared spectroscopy

doesn't require that. The detection instruments simply need to be set up so they can

"look" at the subject. The readings can be taken without doing anything special to

the subject at hand.

JOURNALS

Infrared Analysis as a Tool for Assessing Degradation in Used Engine Lubricants

Used oil samples are complex mixtures of a large number of different chemical

compounds and include compounds derived from the original formulation of base oil and its

additives, oil degradation products and oil contaminants.

As a result of this a used oil spectrum is complex and essentially the net sum of the

spectra of all the individual compounds making up the sample.

In fact, because of this complexity, the spectrum of a used oil alone is of limited

value, and it must be compared against the spectrum of the unused oil to be of significant

analytical value.

The relationship between absorbance, transmittance, and concentration. (Figure 3)

Comparison of used and unused diesel lubricant. (Figure 4)

Figure 4 shows transmittance spectra from two oil samples that are superimposed

on a common spectral grid. Spectrum A is that of new oil (original fill) and Spectrum B is that

of the same oil, degraded by a period of usage in a diesel engine. Apart from the

displacement of transmittance values, caused by the presence of soot in sample B, there

appears to be little difference between the two samples and it would be reasonable to

expect the assumption that minimal degradation has occurred.

This situation changes dramatically when a differential spectrum is viewed. Figure 5

shows the Difference Absorbance Spectrum of the same two samples in which very obvious

differences are apparent. A DIFFERENTIAL or DIFFERENCE SPECTRUM is obtained by

subtracting the absorbance spectrum of one sample form that of the other. This process is

carried out by the spectrometer’s internal microprocessor. Data for each sample is collected

and converted into a numerical format which is subsequently subtracted to yield the

difference data.

Difference Spectrum including soot. (Figure 5)

Difference spectrum excluding soot showing important spectral regions. (Figure 6)

Difference data may be used for further calculations or be converted back to a

graphical representation. Once in numerical form spectral data may be manipulated

mathematically to yield vast amounts of information in a short time period.

Figure 6 shows the same difference spectrum that has been further enhanced by

"soot correction". The soot loading is estimated from absorbance values determined at two

specified wavenumbers and the values applied to a mathematical model. The mathematical

model determines the shape of the "soot curve" to be subtracted.

The data in this corrected form now contains all the information about the

"differences" that exist between the new and the used oil, and may be considered as being

a spectrum of the degradation products that exist in the used oil. To convert this data into a

meaningful form, the numerical data of the corrected difference spectrum is examined in

various spectral regions by software routines that calculate numbers representative of the

degree to which types of degradation and contamination has occurred.

Typical spectral regions of interest and the degradation processes they represent are

detailed in table 2 and are represented graphically in Fig 6.

At WearCheck, used engine oil samples are run on the Perkin Elmer FTIR and the

resulting spectrum matched and compared to a new oil spectrum contained in a "new oil"

library. The difference spectrum generated from this match is processed by the oil analysis

software and "SEVERITY INDICES" are calculated for the various degradation processes.

Severity indices are reported rather than concentrations owing to the complex and variable

nature of the compounds that are being measured in each process. In the case of the

simpler contaminants, such as Water and Glycol, concentration may be expressed directly.

The software in use at WearCheck calculates and reports indices for SOOT, OXIDATION,

SULPHATES and NITRATES. Water and Glycol are also reported but are quantified where

necessary by additional tests. Fuel dilution may also be measured but owing to the

complexity of local supplies of fuel, the method is unreliable.

TABLE 2CHARACTERISTIC INFRARED ABSORPTION BANDS THAT ARE USEFUL IN MONITORING OIL DEGRADATION PROCESSESDegradation Process Spectral Region Centre (cm-1)OXIDATION (CARBONYL) 1720NITRATION 1630, 1553SOOT CONTAMINATION 3800, 1980WATYER CONTAMINATION 3450, 1640, 770SULPHATE FORMATION 1160, 606GLYCOL 3370, 1087, 1043FUEL(AROMATIC) 3052, 1605, 874, 811, 748

Where regular sampling is undertaken severity indices are particularly useful in

monitoring engine trends, determining oil change periods and determining the onset of

potential problems. If oil is used over a prolonged period with minimal top-up, the severity

indices would be expected to increase as concentrations of degradation products build up to

unacceptable levels. The interpretation of these values however, requires considerable skill

as numerous considerations such as engine type, engine conditions and operating

environment must be taken into account.

The diagnostic department at WearCheck has considerable experience in

interpreting infrared results and successfully identifying many potential problems in engines

that would otherwise require extensive testing to detect.

At WearCheck, Infrared analysis is a valued technique in used oil analysis.

FT-NIR Spectroscopy in the Ink and Paint Industry

Identity control of incoming raw materials

The identification and quality control of incoming materials is a straightforward

application, which can be performed directly on the platform without any further time

consuming tests.Organic solvents have different functional groups like alkyl-, hydroxyl-,

aromatic CH-links or others. These show characteristic absorption bands, which support the

automatic identification.

Figure 1. NIR spectra of different solvents used

frequently in the ink & paint industry.

Figure 1 shows the spectra of butyl acetate, acetone, cyclohexanone, methyl ethyl

ketone, 1-butanol, 2-propanol, ethanol and ethyl acetate. All solvents are commonly used in

the ink and paint industry. It can easily be seen that the spectra are sufficiently different for

the establishment of identification.

In this example a transmittance fiber optic probe had been used. With such an

sampling option it is possible to do the measurements directly in a barrel or even in the tank

lorry.

Figure 2. Measuring liquids directly in the original

containers using a transmittance fibre optic probe.

It features a solid construction with integrated remote control for measurements

even of highly corrosive liquids. With the remote control it is possible to start the

measurement directly without the need of operating a PC. Additionally the result (OK or

NOT OK) is indicated at the device. Thepath length (1.5 mm) of the probe is optimized for

the investigation of various liquids.

Quality control of resins

Resins are complex polymers which often are dissolved in different solvents. Today

water based systems are increasingly used. The quality control of such systems by classical

chemical methods is time consuming and rather expensive. With NIR spectroscopy this can

be done very easily and fast. The example shown below demonstrates how to check the

quality of the phenolic resin. Bad qualities can be rejected at a very early stage of the

production.

Figure 3. Spectra of phenolic resins.

The measurements shown below had been performed in the transflectance mode.

This can be done with a special attachment to the standard reflectance fiber optic probe,

which is mounted directly on the reflectance probe head.

With this sampling option there is a small gap between the end of the fiber optic

probe and a diffuse reflector. The path length can be optimized for the application. This gap

is filled with the sample. The radiation passes the sample, is reflected and passes the sample

again. In this way a transmission measurement is performed using the indirection of

reflectance.

Quantitative analysis of resins

Additionally it is not only possible to differentiate between various qualities but also

to quantify the components of resins. As an example the quantitative determination of

formaldehyde, phenol and water in a phenolic resin is given. The measurements had been

done with the transmission fiber optic probe described already above. Only a few scans had

been accumulated, which means that the result is obtained within less than 30 seconds

only. The standard error of prediction (SEP) is about 1.5. This shows that the amount of the

components can be determined with high accuracy.

(a) (b)

Figure 4. Calibration and validation results for the

estimation of phenol (a) and water (b) content.

Determination of NCO content

As mentioned above classical analytical methods often involve wet chemical

procedures. Titration is a common technique for the determination of NCO content. NIR

spectroscopy is an indirect method, which means that a calibration is needed before it can

be used for the analysis of unknown samples. The standards used must be well

characterized by classical analytical techniques. The uncertainties of these techniques are

inherently transferred to the chemometric methods used. However the following example

(Du Pont, Wuppertal) demonstrates that in fact it will deliver comparable results [4].

Figure 5. Comparison of NIR and titration results

for the estimation of NCO content.

Analysis of pigments

At the beginning it had been mentioned that pigments are very important for the

quality of the paints. Therefore of course they are subject of an extensive quality control as

well. The next example will address this segment.

Figure 6. NIR spectra of copper phthalocyanine pigments.

Copper phthalocyanine pigments are often used as blue colour. They exist in two

crystalline modifications, the so called alpha and beta forms. It is necessary to identify both

forms, because of their different properties. In total 80 spectra of both forms had been

measured in diffuse reflectance using the standard fiber optic probe described above. It is

easily possible to set up a calibration for the differentiation of both forms. The following

cluster plot demonstrates the excellent separation of both forms.

Figure 7. Cluster plot for the differentiation of alpha and beta

forms of Cu-phthalocyanine pigments.

The identification of pigments in paper coatings by infrared spectroscopy

Infrared spectra of seven inorganic materials commonly used as pigments are shown

in Figure 1. Infrared spectra of four protein materials used as binders are shown in Figure 2.

Figures 3 and 4 show the infrared spectra of three celluloses and four starches.

Figure 1.The infrared spectra of seven pigments.

Pigments

Table 2 summarises the characteristic features of the infrared spectra of the seven

pigments of Figure 1. All spectra show evidence of the presence of water (a strong broad

band at ~3400 cm-1 and a weaker band at 1635 cm-1). It is very difficult to avoid moisture and

traces of water are usually present in a sample. Although the KBr is dried in an oven and

kept in a dessicator, traces of water are still evident in the infrared spectrum.

Compound Wavenumbers (cm-1 ) and description

Zinc oxide (ZnO) 427(s), 510(vs). These are the only peaks due to ZnO

Titanium dioxide

(TiO2 )

528(vs), 690(vs). These bands are stronger and

broader than those in the ZnO spectrum

Kaolin (china clay) 431(w), 470(m),540(s), 912(s,sharp), 1030(vs,broad),

3620(m)/3695(s), sharp doublet

Talc (talcum, steatite) 450(s), 1020(vs, broad), 3695(w, sharp)

CaSO4

(gypsum, satin white)

600(s)/665(m), doublet, 1130(vs, broad),

1618(m)/1683(w) sharp doublet

BaSO4 (blanc fixe,

barite, baryta)

610(m)/665(s), doublet, 982(w, sharp), 1081/1117

(vs, doublet), 1191(vs)

Whiting (CaCO3) 771(m), 875(s), two very sharp peaks, 1420(vvs),

a very broad band, 1800(vw), 2510(w) two sharp

peaks

Water (H2O) 1625(w), ~3400(S,broad).

These bands are present in all spectra

Table 2. Characteristic features of infrared spectra of some pigments.

The two oxide pigments ZnO and TiO2 and talc have very simple spectra. Kaolin has a more

complex spectrum, with several characteristic peaks. The most notable feature of the

infrared spectrum of a coating containing kaolin is a sharp doublet at 3620/3695 cm -1, which

is due to stretching vibrations of OH groups in the kaolin structure [10]. Confirmation of the

presence kaolin is found from the presence of three peaks of decreasing intensity at 540,

470 and 430 cm-1 and a strong broad band centred near 1030 cm-1.

The two compounds CaSO4 and BaSO4 have broad absorption bands near 1100 cm-1 and

several other sharp characteristic peaks in their spectra. CaCO3 occurs in two polymorphic

forms, calcite and aragonite. The infrared spectrum of the pigment, whiting has a very

strong, very broad band centred near 1420 cm-1. This feature is characteristic of the calcite

form of CaCO3.

Binders

Commonly used binders include gelatin, casein, hideglue and soya. These materials

are all proteins and as can be seen in Figure 2, their infrared spectra are all very similar. The

very strong, very broad absorption between 3700 and 2700 cm-1 and the very strong band

centred near 1700 cm-1 are characteristic of proteins.

Figure 2. The infrared spectra of four protein binders.

Celluloses

Celluloses are carbohydrates (polysaccharides) and the main features of their

spectra are due to the numerous OH groups in the structure. The very broad, very strong

absorption between 3700 and 3200 cm-1 seen in Figure 3 is due to stretching vibrations of

these groups and the OH stretching modes of water. The very strong absorption between

1200 and 1000 cm-1 is attributed to stretching of the many C-OH and C-O-C bonds in the

structure. Subtle differences in the spectra can be seen between 3000 and 2800 and 1500

and 1300 cm-1.These differences arise from the different arrangements of the methyl and

methylene groups in the various celluloses.

Figure 3.The infrared spectra of three celluloses

Starches

Starches are also carbohydrates (polysaccharides) of the general formula

(C6H10O5)n and the main features of their spectra are very similar to those of cellulose. Starch

occurs in plant cells as structural granules, which consist of concentric shells containing

linear amylose, and highly branched amylopectin polymers. The very broad, very strong

absorption between 3700 and 3200 cm-1 seen in Figure 4 is due to stretching vibrations of

OH groups and the very strong absorption between 1200 and 1000 cm-1 is attributed to

stretching of the C-OH and C-O-C bonds in the structure. The infrared spectra of the four

starches shown in Figure 4 indicate that the composition of starch is the same regardless of

the source.

Figure 4.The infrared spectra of four starches

Sample Spectra

Figure 5 shows the infrared spectra of three different papers. A variety of pigment

materials are evident in these spectra. Figure 6 shows the spectra of 3 different wallpapers.

It is clear from the absence of absorption near 1720 cm -1 that the pre-1970 wallpaper has no

plastic coating. Figure 7 compares the spectra of two different coloured card stocks. Figure 8

compares the printed and unprinted sides of a wrapping paper. Figures 9 and 10 show

infrared spectra of two old papers. Detailed interpretation of these sample spectra is given

below.

Magazine and erasable typing papers

All three spectra of Figure 5 show peaks due to kaolin. However, there is clearly less

of this pigment in the erasable typewriter paper than in the two magazine papers. The

strong band at 1420 cm-1 and the sharp peak at 875 cm-1 in the infrared spectrum of the

"Country Life" paper show that the coating contains whiting (CaCO3). The sharp peaks near

1720 cm-1 in the spectrum of this paper and that of the typewriter paper indicate that the

coatings also contain a synthetic polymer material.

Figure 5. The infrared spectra of the coatings on an erasable typewriter paper

and papers from Maclean’s and Country Life magazines.

Wallpapers

Figure 6 compares the infrared spectra of two modern plastic coated wallpapers

("patterned" and "coloured") with a pre-1970 product. Features of both of kaolin and

whiting (CaCO3) pigments are clearly seen in the spectra. The sharp doublet at 3620/3695

cm-1 and the three peaks between 540 and 430 cm -1 identifies kaolin and the strong broad

band near 1420 cm-1and the sharp peak at 875 cm-1 is characteristic of calcium carbonate.

There is also evidence for the presence of an organic polymer (peak near 1720 cm -1 ).

However, the spectra show that, the coating on the "patterned" wallpaper does not contain

kaolin, while the pre-1970 product contains neither whiting, nor polymer.

Figure 6. The infrared spectra of the coatings on three wallpapers.

Coloured card stocks

Figure 7 shows that the coatings on these two materials both contain kaolin,

(doublet at 3620/3695 cm-1 and three peaks between 540 and 430 cm-1 ), and a synthetic

polymer (peak near 1720 cm-1 ). The coating on the "brown" card also contains whiting as

indicated by the strong broad band near 1420 cm-1 and the sharp peak at 875 cm-1.

Figure 7. The infrared spectra of the coatings on two card stocks.

Printed wrapping paper

Figure 8 compares the infrared spectra of the printed side and the unprinted side of

a wrapping paper of German origin. It is clear from the spectrum that the coating on the

printed side contains both kaolin and whiting. In addition, there is evidence for the presence

of a polystyrene-containing polymer. This evidence includes three weak peaks in the CH

stretching region above 3000 cm-1, the sharp peak at 755 cm-1 and the pattern of very weak

absorbances between 2000 and 1700 cm-1 . These features are all present in the infrared

spectrum of polystyrene. The spectrum of the unprinted side is essentially that of cellulose

with a very small amount of kaolin.

Figure 8. The infrared spectra of both sides of a printed wrapping paper.

A 1897 postcard

Figure 9 shows the infrared spectrum of the coating scraped from an 1897 Canadian

postcard (celebrating the Jubilee year of Queen Victoria’s reign). A spectrum of kaolin is

included to show that a very small amount of this pigment present. The spectrum of the

postcard is essentially that of cellulose.

Figure 9. The infrared spectrum of the coating on a 1897

postcard, compared with the spectra of kaolin and cellulose

An old furniture catalogue (circa 1920)

The spectra of Figure 10 show that the coating on the paper taken from a furniture

catalogue (circa 1920) contains two pigments BaSO4 and CaCO3. Although CaCO3 appears to

be present in the coating (the strong broad band near 1420 cm-1 , and the sharp peak at

875 cm-1 ), it is interesting that the calcite peaks at 771 and 1800 cm -1 are absent from the

spectrum of the paper coating. This may be due to the form of the "whiting" pigment used

at the time.

Figure 10. The infrared spectrum of the coating on the paper of a circa 1920 furniture catalogue compared with the spectra of

calcium carbonate and barium sulphate..

Comparison of infrared ATR and transmission spectra

Figures 11 and 12 show the infrared spectra of samples of a coated wallpaper and a

Christmas card recorded using both the ATR and the KBr pellet methods. In these Figures

baselines have been fitted to the ATR spectra. It is seen that apart from the poorer signal-to-

noise in the ATR spectra, the main features of the spectra are similar. The same conclusions

concerning the pigments present in the coatings can be drawn from either spectrum. By

reference to the infrared spectra of the pigments of Figure 1, it can be seen that both

coatings contain kaolin and calcium carbonate. However there are also some obvious

differences, which can be attributed to the differences between the transmission and ATR

techniques. ATR examines only the top few microns of the surface. On the other hand, the

sample for the KBr pellet is manually scraped from the surface and undoubtedly contains

some sub-surface material.

Figure 11. A comparison of the infrared ATR and transmission spectra of the coating on a plastic-coated wallpaper.

Figure 12. A comparison of the infrared ATR and transmission spectra of the coating on a Christmas card.