FTIR Analysis

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FTIR Analysis: What Is It?

The most common type of infrared spectroscopy is FTIR (Fourier transform infrared) analysis. All infrared spectroscopies work on the principle that some infrared (IR) radiation is absorbed when it passes through a sample. First and foremost, it does not destroy the sample. Second, it is much faster than older techniques. Third, it is significantly more sensitive and precise.

How does FTIR analysis work?

The FTIR instrument passes infrared radiation ranging from 10,000 to 100 cm-1 through a sample, with some absorbed and some passing through. The sample molecules convert the absorbed radiation into rotational and/or vibrational energy. The resulting signal at the detector appears as a spectrum, typically ranging from 4000 cm-1 to 400cm-1, representing the sample’s molecular fingerprint. Because each molecule or chemical structure produces a distinct spectral fingerprint, FTIR analysis is an excellent tool for chemical identification.

HOW TO INTERPRET FTIR SPECTRUM?

The absorbed bands in the spectrum are only marginally discrete and degenerative. The specific “peak” of energy at a given wavenumber can shift due to other chemical and matrix factors (as well as by the way the incident energy is introduced). As a result, we don’t simply have a “look up” table that tells us where a specific band of energy belongs. The spectrum must be interpreted as a whole system, requiring the most experienced analysts in all spectrographic techniques to accurately characterize the functionality presented.
Although FTIR analysis is typically used to identify materials, it can also be used to quantify specific functional groups when the chemistry is understood and standard reference materials are available. The absorbance intensity will be proportional to the amount of functionality present in the sample.

It is necessary to determine which groups and bonds correspond to which peaks when reading the spectrum. Here is a simple reference tables for the various groups.

Table 1
Frequency Range	Absorption (cm-1)	Appearance	Group	Compound Class	Comments
4000-3000 cm-1	3700-3584	medium, sharp	O-H stretching	alcohol	free
	3550-3200	strong, broad	O-H stretching	alcohol	intermolecular bonded
	3500	medium	N-H stretching	primary amine
	3400
	3400-3300	medium	N-H stretching	aliphatic primary amine
	3330-3250
	3350-3310	medium	N-H stretching	secondary amine
	3300-2500	strong, broad	O-H stretching	carboxylic acid	usually centered on 3000 cm-1
	3200-2700	weak, broad	O-H stretching	alcohol	intramolecular bonded
	3000-2800	strong, broad	N-H stretching	amine salt
3000-2500 cm-1
3000-2500 cm-1	3333-3267	strong, sharp	C-H stretching	alkyne
	3100-3000	medium	C-H stretching	alkene
	3000-2840	medium	C-H stretching	alkane
	2830-2695	medium	C-H stretching	aldehyde	doublet
	2600-2550	weak	S-H stretching	thiol
2400-2000 cm-1
2400-2000 cm-1	2349	strong	O=C=O stretching	carbon dioxide
	2275-2250	strong, broad	N=C=O stretching	isocyanate
	2260-2222	weak	CΞN stretching	nitrile
	2260-2190	weak	CΞC stretching	alkyne	disubstituted
	2175-2140	strong	S-CΞN stretching	thiocyanate
	2160-2120	strong	N=N=N stretching	azide
	2150		C=C=O stretching	ketene
	2145-2120	strong	N=C=N stretching	carbodiimide
	2140-2100	weak	CΞC stretching	alkyne	monosubstituted
	2140-1990	strong	N=C=S stretching	isothiocyanate
	2000-1900	medium	C=C=C stretching	allene
	2000		C=C=N stretching	ketenimine
2000-1650 cm-1
2000-1650 cm-1	2000-1650	weak	C-H bending	aromatic compound	overtone

	1870-1540
	1818	strong	C=O stretching	anhydride
	1750
	1815-1785	strong	C=O stretching	acid halide
	1800-1770	strong	C=O stretching	conjugated acid halide
	1775	strong	C=O stretching	conjugated anhydride
	1720
	1770-1780	strong	C=O stretching	vinyl / phenyl ester
	1760	strong	C=O stretching	carboxylic acid	monomer
	1750-1735	strong	C=O stretching	esters	6-membered lactone
	1750-1735	strong	C=O stretching	δ-lactone	γ: 1770
	1745	strong	C=O stretching	cyclopentanone
	1740-1720	strong	C=O stretching	aldehyde
	1730-1715	strong	C=O stretching	α,β-unsaturated ester	or formates
	1725-1705	strong	C=O stretching	aliphatic ketone	or cyclohexanone or cyclopentenone
	1720-1706	strong	C=O stretching	carboxylic acid	dimer
	1710-1680	strong	C=O stretching	conjugated acid	dimer
	1710-1685	strong	C=O stretching	conjugated aldehyde
	1690	strong	C=O stretching	primary amide	free (associated: 1650)
	1690-1640	medium	C=N stretching	imine / oxime
	1685-1666	strong	C=O stretching	conjugated ketone
	1680	strong	C=O stretching	secondary amide	free (associated: 1640)
	1680	strong	C=O stretching	tertiary amide	free (associated: 1630)
	1650	strong	C=O stretching	δ-lactam	γ: 1750-1700 β: 1760-1730
1670-1600 cm-1
1670-1600 cm-1	1678-1668	weak	C=C stretching	alkene	disubstituted (trans)
	1675-1665	weak	C=C stretching	alkene	trisubstituted
	1675-1665	weak	C=C stretching	alkene	tetrasubstituted
	1662-1626	medium	C=C stretching	alkene	disubstituted (cis)
	1658-1648	medium	C=C stretching	alkene	vinylidene
	1650-1600	medium	C=C stretching	conjugated alkene
	1650-1580	medium	N-H bending	amine
	1650-1566	medium	C=C stretching	cyclic alkene
	1648-1638	strong	C=C stretching	alkene	monosubstituted
	1620-1610	strong	C=C stretching	α,β-unsaturated ketone
1600-1300 cm-1
1600-1300 cm-1	1550-1500	strong	N-O stretching	nitro compound
	1372-1290
	1465	medium	C-H bending	alkane	methylene group
	1450	medium	C-H bending	alkane	methyl group
	1375
	1390-1380	medium	C-H bending	aldehyde
	1385-1380	medium	C-H bending	alkane	gem dimethyl
	1370-1365
1400-1000 cm-1
1400-1000 cm-1	1440-1395	medium	O-H bending	carboxylic acid
	1420-1330	medium	O-H bending	alcohol
	1415-1380	strong	S=O stretching	sulfate
	1200-1185
	1410-1380	strong	S=O stretching	sulfonyl chloride
	1204-1177
	1400-1000	strong	C-F stretching	fluoro compound
	1390-1310	medium	O-H bending	phenol
	1372-1335	strong	S=O stretching	sulfonate
	1195-1168
	1370-1335	strong	S=O stretching	sulfonamide
	1170-1155
	1350-1342	strong	S=O stretching	sulfonic acid	anhydrous
	1165-1150				hydrate: 1230-1120
	1350-1300	strong	S=O stretching	sulfone
	1160-1120
	1342-1266	strong	C-N stretching	aromatic amine
	1310-1250	strong	C-O stretching	aromatic ester
	1275-1200	strong	C-O stretching	alkyl aryl ether
	1075-1020
	1250-1020	medium	C-N stretching	amine
	1225-1200	strong	C-O stretching	vinyl ether
	1075-1020
	1210-1163	strong	C-O stretching	ester
	1205-1124	strong	C-O stretching	tertiary alcohol
	1150-1085	strong	C-O stretching	aliphatic ether
	1124-1087	strong	C-O stretching	secondary alcohol
	1085-1050	strong	C-O stretching	primary alcohol
	1070-1030	strong	S=O stretching	sulfoxide
	1050-1040	strong, broad	CO-O-CO stretching	anhydride
1000-650 cm-1
1000-650 cm-1	995-985	strong	C=C bending	alkene	monosubstituted
	915-905
	980-960	strong	C=C bending	alkene	disubstituted (trans)
	895-885	strong	C=C bending	alkene	vinylidene
	850-550	strong	C-Cl stretching	halo compound
	840-790	medium	C=C bending	alkene	trisubstituted
	730-665	strong	C=C bending	alkene	disubstituted (cis)
	690-515	strong	C-Br stretching	halo compound
	600-500	strong	C-I stretching	halo compound
900-700 cm-1
900-700 cm-1	880 ± 20	strong	C-H bending	1,2,4-trisubstituted
	810 ± 20
	880 ± 20	strong	C-H bending	1,3-disubstituted
	780 ± 20
	(700 ± 20)
	810 ± 20	strong	C-H bending	1,4-disubstituted or
				1,2,3,4-tetrasubstituted
	780 ± 20	strong	C-H bending	1,2,3-trisubstituted
	(700 ± 20)
	755 ± 20	strong	C-H bending	1,2-disubstituted
	750 ± 20	strong	C-H bending	monosubstituted
	700 ± 20			benzene derivative

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Table 2.

Compound Class	Group	Absorption (cm-1)	Appearance	Comments
acid halide	C=O stretching	1815-1785	strong
alcohols	O-H stretching	3700-3584	medium, sharp	free
	O-H stretching	3550-3200	strong, broad	intermolecular bonded
	O-H stretching	3200-2700	weak, broad	intramolecular bonded
	O-H bending	1420-1330	medium
aldehyde	C-H stretching	2830-2695	medium	doublet
	C=O stretching	1740-1720	strong
	C-H bending	1390-1380	medium
aliphatic ether	C-O stretching	1150-1085	strong
aliphatic ketone	C=O stretching	1725-1705	strong	or cyclohexanone or cyclopentenone
aliphatic primary amine	N-H stretching	3400-3300	medium
alkane	C-H stretching	3000-2840	medium
	C-H bending	1465	medium	methylene group
	C-H bending	1450	medium	methyl group
	C-H bending	1385-1380	medium	gem dimethyl
	C-H stretching	3100-3000	medium
	C=C stretching	1678-1668	weak	disubstituted (trans)
	C=C stretching	1675-1665	weak	trisubstituted
	C=C stretching	1675-1665	weak	tetrasubstituted
	C=C stretching	1662-1626	medium	disubstituted (cis)
	C=C stretching	1658-1648	medium	vinylidene
	C=C stretching	1648-1638	strong	monosubstituted
	C=C bending	995-985	strong	monosubstituted
	C=C bending	980-960	strong	disubstituted (trans)
	C=C bending	895-885	strong	vinylidene
	C=C bending	840-790	medium	trisubstituted
	C=C bending	730-665	strong	disubstituted (cis)
alkyl aryl ether	C-O stretching	1275-1200	strong
alkyne	C-H stretching	3333-3267	strong, sharp
	CΞC stretching	2260-2190	weak	disubstituted
	CΞC stretching	2140-2100	weak	monosubstituted
allene	C=C=C stretching	2000-1900	medium
amine	N-H bending	1650-1580	medium
	C-N stretching	1250-1020	medium
amine salt	N-H stretching	3000-2800	strong, broad
anhydride	C=O stretching	1818	strong
	CO-O-CO stretching	1050-1040	strong, broad
aromatic amine	C-N stretching	1342-1266	strong
aromatic compound	C-H bending	2000-1650	weak	overtone
aromatic ester	C-O stretching	1310-1250	strong
azide	N=N=N stretching	2160-2120	strong
benzene derivative		700 ± 20
carbodiimide	N=C=N stretching	2145-2120	strong
carbon dioxide	O=C=O stretching	2349	strong
carboxylic acid	O-H stretching	3300-2500	strong, broad	usually centered on 3000 cm-1
	C=O stretching	1760	strong	monomer
	C=O stretching	1720-1706	strong	dimer
	O-H bending	1440-1395	medium
conjugated acid	C=O stretching	1710-1680	strong	dimer
conjugated acid halide	C=O stretching	1800-1770	strong
conjugated aldehyde	C=O stretching	1710-1685	strong
conjugated alkene	C=C stretching	1650-1600	medium
conjugated anhydride	C=O stretching	1775	strong
conjugated ketone	C=O stretching	1685-1666	strong
cyclic alkene	C=C stretching	1650-1566	medium
cyclopentanone	C=O stretching	1745	strong
ester	C-O stretching	1210-1163	strong
esters	C=O stretching	1750-1735	strong	6-membered lactone
fluoro compound	C-F stretching	1400-1000	strong
halo compound	C-Cl stretching	850-550	strong
	C-Br stretching	690-515	strong
	C-I stretching	600-500	strong
imine / oxime	C=N stretching	1690-1640	medium
isocyanate	N=C=O stretching	2275-2250	strong, broad
isothiocyanate	N=C=S stretching	2140-1990	strong
ketene	C=C=O stretching	2150
ketenimine	C=C=N stretching	2000
monosubstituted	C-H bending	750 ± 20	strong
nitrile	CΞN stretching	2260-2222	weak
nitro compound	N-O stretching	1550-1500	strong
none		3330-3250
none		1870-1540
none		1750
none		1720
none		1372-1290
none		1375
none		1370-1365
none		1200-1185
none		1204-1177
none		1195-1168
none		1170-1155
none		1165-1150		hydrate: 1230-1120
none		1160-1120
none		1075-1020
none		1075-1020
none		915-905
none		810 ± 20
none		780 ± 20
none		(700 ± 20)
none		(700 ± 20)
phenol	O-H bending	1390-1310	medium
primary alcohol	C-O stretching	1085-1050	strong
primary amide	C=O stretching	1690	strong	free (associated: 1650)
	N-H stretching	3500	medium
secondary alcohol	C-O stretching	1124-1087	strong
secondary amide	C=O stretching	1680	strong	free (associated: 1640)
secondary amine	N-H stretching	3350-3310	medium
sulfate	S=O stretching	1415-1380	strong
sulfonamide	S=O stretching	1370-1335	strong
sulfonate	S=O stretching	1372-1335	strong
sulfone	S=O stretching	1350-1300	strong
sulfonic acid	S=O stretching	1350-1342	strong	anhydrous
sulfonyl chloride	S=O stretching	1410-1380	strong
sulfoxide	S=O stretching	1070-1030	strong
tertiary alcohol	C-O stretching	1205-1124	strong
tertiary amide	C=O stretching	1680	strong	free (associated: 1630)
thiocyanate	S-CΞN stretching	2175-2140	strong
thiol	S-H stretching	2600-2550	weak
vinyl / phenyl ester	C=O stretching	1770-1780	strong
vinyl ether	C-O stretching	1225-1200	strong
α,β-unsaturated ester	C=O stretching	1730-1715	strong	or formates
α,β-unsaturated ketone	C=C stretching	1620-1610	strong
δ-lactam	C=O stretching	1650	strong	γ: 1750-1700 β: 1760-1730
δ-lactone	C=O stretching	1750-1735	strong	γ: 1770
1,2,3,4-tetrasubstituted
1,2,3-trisubstituted	C-H bending	780 ± 20	strong
	C-H bending	880 ± 20	strong
1,2-disubstituted	C-H bending	755 ± 20	strong
	C-H bending	880 ± 20	strong
1,4-disubstituted or	C-H bending	810 ± 20	strong

What does FTIR serve as a tool for?

Unknown materials (e.g., films, solids, powders, or liquids) and contamination on or in a material (e.g., particles, fibers, powders) can be identified and characterized by FTIR analysis.

FTIT sample preparation:

The ability to introduce and observe energy from a specific matrix is required for proper FTIR analysis. To properly analyse the sample, we have many sample preparation and introduction techniques available in the laboratory. Transmission was the only available method of analysis in the early days of infrared spectroscopy. For transmission analysis, the sample had to be made translucent to laser and infrared energy by directly inserting it into the optical path, casting a thin film on a salt crystal, or mixing a powder version of the sample with a salt and casting.
Today, however, we can use not only transmission techniques but also reflectance techniques. We generally rely on variations of ATR (Attenuated Total Reflectance) techniques to introduce and observe energy because we can focus and manipulate the incident beam with optics. ATR involves the use of an internal reflectance phenomenon to propagate incident energy.

The beam is introduced into a crystal at an incident angle, allowing internal reflectance “bounces” at the bottom and top of the crystal before leaving on the opposite side. The sample is brought into contact with the crystal at the top, causing energy interaction at the crystal and sample interface, which is where the bounce positions are located. The more bounce positions there are, the greater the energy transfer (and thus the better the spectral response), but single bounce systems are used when a very small area needs to be analyzed.
A HATR (Horizontal Attenuated Total Reflectance) will be typically used for liquid and paste samples, which will involve placing the sample on a crystal plate or trough in a horizontal position so that gravity acts to make intimate contact with the cell. The depth of penetration into the sample can be varied by using different crystals. For example, we will use a germanium crystal for rubber analysis to limit the effect of highly IR absorbing materials in rubber (specifically carbon black), but the zinc selenide crystal is the crystal of choice for durability, moisture resistance, and penetration depth in normal everyday samples.

What are Advantages and Disadvantages of FTIR analysis?

Advantages:

Identify compounds as small as 10 to 20 microns
In many cases, the FTIR analysis does not harm the sample and does not alter it.
FTIR analysis measures absorbance bands across the mid-Infrared spectrum simultaneously, allowing for a large amount of analytical information to be obtained quickly.
Because wavelength measurement is absolute, analytical techniques such as spectral subtraction are quick and highly accurate.

Disadvantages:

Because FTIR analysis exposes the sample to all mid-infrared frequencies at the same time, noise in one part of the radiation from the infrared source will spread throughout the spectrum.
FTIR analysis can be affected by changes in atmospheric conditions, making it difficult to use on highly sensitive samples or samples that must be studied over a long period of time.
Because FTIR is a bulk analysis method, it is best suited to locating and identifying broad categories of substances in a compound; however, it is less capable of identifying trace amounts of materials in mixtures with other materials.

Different types of FTIR Analysis

Transmission FTIR: In this method, the sample is placed between two transparent windows, and the infrared light passes through the sample. The amount of light absorbed by the sample at each frequency is measured, and this data is used to construct a spectrum of the sample’s chemical composition. This method is useful for analyzing samples that are transparent to infrared radiation.

Attenuated Total Reflection (ATR) FTIR: In this method, the sample is placed in contact with a crystal surface, and the infrared light is absorbed by the sample at the surface. The amount of light absorbed by the sample at each frequency is measured, and this data is used to construct a spectrum of the sample’s chemical composition. This method is useful for analyzing samples that are not transparent to infrared radiation, such as liquids or solids.

Diffuse Reflectance FTIR: In this method, the sample is ground into a powder and placed on a reflective surface, and the infrared light is reflected off the sample. The amount of light absorbed by the sample at each frequency is measured, and this data is used to construct a spectrum of the sample’s chemical composition. This method is useful for analyzing samples that are difficult to prepare in a thin, transparent film, such as powders or rough surfaces.

Fourier Transform Raman Spectroscopy (FT-Raman): In this method, the sample is irradiated with a laser, and the scattered light is analyzed by a Fourier transform spectrometer. The frequency of the scattered light is shifted due to interactions with the sample’s chemical bonds, and this shift is used to construct a spectrum of the sample’s chemical composition. This method is useful for analyzing samples that are not easily analyzed by FTIR, such as inorganic materials or samples with strong fluorescence.

Infrared Microscopy: In this method, a microscope is used to focus the infrared beam on a small area of the sample, allowing for spatially resolved measurements. The amount of light absorbed by the sample at each frequency is measured, and this data is used to construct a spectrum of the sample’s chemical composition. This method is useful for analyzing samples that have spatially varying chemical composition, such as polymer blends or biological tissues.

Reference

ANALYSIS OF INFRARED SPECTROSCOPY (IR)

strengths and limitations of fifferent types of FTIR

Each type of FTIR analysis has its own strengths and limitations, and the choice of which method to use depends on the specific sample and analysis requirements.

Transmission FTIR is a commonly used method and is suitable for analyzing samples that are transparent to infrared radiation. It is also a relatively simple and straightforward method, and the data can be easily compared to reference spectra in a library.

ATR-FTIR is useful for analyzing samples that are not transparent to infrared radiation, and it can be used with a wide range of sample types, including liquids, solids, and powders. ATR-FTIR is also relatively easy to use, and the data can be compared to reference spectra in a library.

Diffuse Reflectance FTIR is useful for analyzing samples that are difficult to prepare in a thin, transparent film, such as powders or rough surfaces. It is also a relatively simple method and can be used to analyze a wide range of sample types.

FT-Raman spectroscopy is useful for analyzing samples that are not easily analyzed by FTIR, such as inorganic materials or samples with strong fluorescence. It can also be used to provide complementary information to FTIR analysis.

Infrared microscopy is useful for analyzing samples that have spatially varying chemical composition, such as polymer blends or biological tissues. It provides spatially resolved measurements and can be used to analyze small or complex samples.

In summary, the choice of which type of FTIR analysis to use depends on the specific sample and analysis requirements. Each method has its own advantages and limitations, and the most suitable method should be selected based on the nature of the sample and the analysis needs.

Is there any free FTIR database?

There are several free online databases of IR spectra, including:

NIST Chemistry WebBook
SDBS (Spectral Database for Organic Compounds)
Sigma-Aldrich IR Spectra Library
Spectral Database for Inorganic Compounds (SDBS)
Organic Compounds Database (ThermoFisher Scientific)
AIST Spectral Database for Organic Compounds (SDBS)

These databases can be useful for identifying unknown compounds and comparing spectra.

Learn more about step-by-step analysis of FTIR results here.

Limited offer: Get your FTIR data analyzed for just $14!

We provide interpretation for raw data obtained from the following analytical methods:

Fourier-transform infrared spectroscopy (FTIR)
Raman Spectroscopy
Inductively coupled plasma (ICP)
UV–visible (UV-Vis) spectrophotometry
X-ray powder diffraction (XRD)
Thermal gravimetric analysis (TGA)
Vibrating-sample magnetometer (VSM)
Electrochemical Impedance Spectroscopy (EIS)
Scanning Electron Microscopy (SEM)
Transmission electron microscopy (TEM)
Energy Dispersive X-ray Spectrometry (EDS)

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