Fourier Transform Infrared
(FT-IR) Spectroscopy
Theory and Principles
Introduction to Infrared Spectroscopy
• What is infrared spectroscopy?
• Theory and applications
• Why FT-IR?
• The 'good' infrared spectrum
Molecular Spectroscopy
High Energy /
• All molecules absorb energy
Short Wavelength in different ways. There are
numerous wavelengths of
light in the electromagnetic
spectrum and the study of
each energy region can offer
specific information about
the structure, composition
Low Energy / and electronic configuration
Long Wavelength of the molecule.
Molecular Vibrations
n Absorption
d
ne
n
b
na nc
Incident white light interacts with all vibrational modes of the
sample at once and emerges with missing lines characteristic of
the molecule
An Infrared Active Vibration
d-
d+
Small change
in equilibrium state
polarizability
d-
d+
A molecule such as H2O will absorb infrared light when the vibration
(stretch or bend) results in a molecular dipole moment change
Energy levels in Infrared Absorption
Excited states
hn
Infrared Absorption and Emission
h(n2 - n1) (overtone)
n3
h(n1 - n0 ) h(n1 - n0)
n2
Ground
n1 (vibrational)
n0 states
Infrared absorption occurs among the ground vibrational states, the
energy differences, and corresponding spectrum, determined by the
specific molecular vibration(s). The infrared absorption is a net
energy gain for the molecule and recorded as an energy loss for the
analysis beam.
Infrared Spectroscopy
• As the atomic bonds in the molecule stretch and bend, they
absorb infrared energy, resulting in the infrared spectrum.
Symmetric Stretch Antisymmetric Stretch Bend
Infrared Spectroscopy
• A molecule can be characterized (identified) by its molecular
vibrations, based on the absorption and intensity of specific
infrared wavelengths.
Infrared Spectroscopy
• For isopropyl alcohol, CH(CH3)2OH, the infrared absorption
bands identify the various functional groups of the molecule.
Capabilities of Infrared Analysis
• Identification and quantitation of organic solid, liquid or
gas samples.
• Analysis of powders, solids, gels, emulsions, pastes, pure
liquids and solutions, pure and mixed gases.
• Infrared used for research, methods development, quality
control and quality assurance applications.
• Samples range in size from single fibers only 20 microns
in length to atmospheric pollution studies involving large
areas.
Applications of Infrared Analysis
• Pharmaceutical research
• Forensic investigations
• Polymer analysis
• Lubricant formulation and fuel additives
• Foods research
• Quality assurance and control
• Environmental and water quality analysis methods
• Biochemical and biomedical research
• Coatings and surfactants
• Etc.
Infrared Quantitative Analysis
• Sample concentration is directly related to
infrared absorption intensity
Beer’s Law
A = abc
Where:
A = Absorption intensity
a = absorption coefficient for a specific IR peak
b = sample pathlength
c = sample concentration
Quantitative Analysis Applications
0 . 2 3
0 . 2
0 . 1 5
A b s
0 . 1
0 . 0 5
1 6 6 0 1 6 0 0 1 5 5 0 1 5 1 0
W a v e n u m b e r [ c m - 1 ]
Quantitative Analysis - CLS
The FT-IR Instrument
Mirror Displacement and Monochromatic Light
As the moving mirror moves, light of a single wavelength
cycles from zero to maximum intensity.
The Interferogram
When all the infrared wavelengths are processed through
the interferometer, the result is an interferogram.
FT-IR Advantages
• Fellgett's (multiplex) advantage - an FT-IR collects all
resolution elements with a complete scan (mirror
movement) of the interferometer. Successive scans of
the FT-IR instrument are coadded and averaged to
enhance the signal-to-noise of the spectrum.
• Theoretically, an infinitely long scan would average out
all the noise in the baseline.
• As the number of scans increases, so does the signal-to-
noise (SNR). The signal-to-noise doubles as the square
of the number of scans: i.e. 1, 4, 16, 64, 256, ….
• The dispersive instrument collects data one wavelength
at a time and collects only a single spectrum. There is
no good method for increasing the signal-to-noise of the
dispersive spectrum.
FT-IR Advantages
• Jacquinot advantage - an FT-IR uses a combination of
circular apertures and interferometer travel to define
resolution.
• More energy is available for a standard infrared scan and
thus various accessories can be used to solve sample
handling problems.
• The dispersive instrument uses a rectangular slit to control
resolution and cannot increase the signal-to-noise for high
resolution scans. Accessory use is limited for a dispersive
instrument.
FT-IR Advantages
• Connes advantage - an FT-IR uses a HeNe laser as an
internal wavelength standard. The infrared wavelengths
are calculated using the laser wavelength, itself a very
precise and repeatable 'standard'.
• Wavelength assignment for the FT-IR spectrum is very
repeatable and reproducible and data can be compared to
digital libraries for identification purposes.
• Caution: When the laser is replaced, the FT-IR must be
validated versus a known standard such as polystyrene.
FT-IR Application Advantages
• Opaque or cloudy samples
• Energy limiting accessories such as diffuse reflectance or
FT-IR microscopes
• High resolution experiments (as high as 0.001 cm-1
resolution)
• Trace analysis of raw materials or finished products
• Depth profiling and microscopic mapping of samples
• Kinetics reactions on the microsecond time-scale
• Analysis of chromatographic and thermogravimetric
sample fractions
FT-IR Terms and Definitions
• Resolution - the separation
of the various spectral
wavelengths, usually defined
in wavenumbers (cm-1). A
setting of 4 to 8 cm-1 is
sufficient for most solid and
liquid samples. Gas analysis
experiments may need a
resolution of 2 cm-1 or
higher. Higher resolution
experiments will have lower
signal-to-noise on a scan-
per-scan basis.
FT-IR High Resolution
FT-IR Terms and Definitions
• Apodization - a
Apodization
mathematical operation
to reduce unwanted
oscillation and noise
contributions from the
interferogram. Common
apodization functions
include Beer-Norton,
Cosine and Happ-
Genzel.
FT-IR Terms and Definitions
• Scan mode - Either single
beam or ratio. Single
beam can be a scan of the
background (no sample)
or the sample. Ratio
mode always implies the
sample spectrum divided
by, or ratioed against, the
single beam background.
FT-IR Terms and Definitions
• Scan(s) - a complete cycle of movement of the
interferometer mirror. The number of scans collected
affects the signal-to-noise ratio (SNR) of the final
spectrum.
• Scan speed or optical path velocity - the rate at which the
interferometer mirror moves. For a DTGS detector, the
SNR decreases as the scan speed increases.
• Scan range - spectral range selected for the analysis. The
most useful spectral range for mid-infrared is 4000 to 400
cm-1.
The 'Good' FT-IR Spectrum -
What are the criteria?
• The spectrum baseline should be relatively flat.
• The highest transmission point of the spectrum should be
between 95 and 100 %T.
• There should be little noise in the spectrum. Most FT-IR
instruments will obtain a sufficient SNR within a minute of
scanning the sample (16 to 50 scans depending on model).
• The strongest band in the spectrum should be greater than
50 %T and should fall no lower than between 3 and 10 %T.
• Sample preparation is the most important aspect of
collecting a useful FT-IR spectrum.
A 'Good' FT-IR Spectrum
