Introduction
Fourier Transform Infrared (FTIR) spectrometry is designed to overcome the limitations of
dispersive equipment. The lengthy scanning procedure was a significant constraint. It is
preferable to measure all IR frequencies at the same time. The approach employed a basic optical
device known as an interferometer to generate a signal that contained all infrared frequencies
“embedded” inside it. The signal is normally recorded in a few seconds
When exposed to infrared radiation, sample molecules selectively absorb radiation of specific
wavelengths which causes the change of dipole moment of sample molecules. Consequently, the
vibrational energy levels of sample molecules transfer from ground state to excited state. The
frequency of the absorption peak is determined by the vibrational energy gap.
Most molecules are infrared active except for several homonuclear diatomic molecules such as
O2, N2 and Cl2 due to the zero dipole change in the vibration and rotation of these molecules. It
is capable to analyze all gas, liquid and solid samples. The common used region for infrared
absorption spectroscopy is 4000 ~ 400 cm-1 because the absorption radiation of most organic
compounds and inorganic ions is within this region.
Principle of FTIR
The Michelson interferometer consists of a beam splitter, a moving mirror, and a stationary
mirror. The beam splitter divides the light beam into two halves, which are reflected by the
moving and fixed mirrors before being recombined by the beam splitter.
As the moving mirror makes reciprocating movements, the optical path difference to the fixed
mirror changes, causing the phase difference to shift over time. Interference light is created In the
Michelson interferometer by recombining the light beams. An interferogram records the intensity
of the interference light, with the optical path difference recorded along the horizontal axis.
The Components of FTIR Spectrometers
A common FTIR spectrometer consists of a source, interferometer, sample compartment,
detector, amplifier, A/D convertor, and a computer. The source generates radiation which passes
the sample through the interferometer and reaches the detector. Then the signal is amplified and
converted to digital signal by the amplifier and analog-to-digital converter, respectively.
Eventually, the signal is transferred to a computer in which Fourier transform is carried out
A typical Michelson interferometer consists of two perpendicular mirrors and a beamsplitter. One
of the mirror is a stationary mirror and another one is a movable mirror. The beamsplitter is
designed to transmit half of the light and reflect half of the light. Subsequently, the transmitted
light and the reflected light strike the stationary mirror and the movable mirror, respectively.
When reflected back by the mirrors, two beams of light recombine with each other at the
beamsplitter.
Because each material is a unique atom combination, no two compounds create the same infrared
spectrum. An infrared spectrum is a sample’s fingerprint, with absorption peaks corresponding to
the frequency of vibrations between the bonds of the atoms that make up the material.
Most interferometers use a beamsplitter, which separates the entering infrared beam into two
optical beams. The signal that emerges from the interferometer is the consequence of these two
beams “interfering” with one another since one beam has a fixed route and the other’s path is
variable due to the movement of its mirror. An interferogram is the resulting signal. All
frequencies are thus measured concurrently when the interferogram is measured. As a result of
the interferometer’s employment, measurements are exceedingly quick.
The measured interferogram signal cannot be analyzed directly because the analyst requires a
frequency spectrum for identification. It is necessary to have a method of “decoding” the
individual frequencies. This is possible because of a well-known mathematical technique known
as the Fourier transformation.
Different parts of FTIR instrumentation include:
The Source: A broadband emitter, such as a mid-IR ceramic source, a far-infrared mercury lamp,
or a near-infrared halogen lamp, is used as the light source.
The Interferometer: The interferometer, which consists of a beamsplitter, a stationary mirror, and
a moving mirror, is the heart of an FTIR spectrometer. The beamsplitter is a semi-transparent
mirror that divides a collimated light beam into two optical channels. Half of the light is
transferred to the moving mirror and half is reflected to the stationary mirror. The moving and
stationary mirrors reflect the two light beams, which are recombined at the beamsplitter before
going through the sample chamber and onto the detector.
The sample: Depending on the type of analysis being performed, the beam enters the sample
compartment and is either transmitted through or reflected off the surface of the sample. This is
where certain frequencies of energy that are unique to the sample are absorbed.
Detector: FTIR detectors are used to measure and convert the transmitted or reflected light from
a sample into an electrical signal. The sensitivity and wavelength range of the data that can be
captured is determined by the type and material of the detector.
The detector converts the beam into photons, which are then translated into measurable electric
signals that the computer can read. The following are some examples of common detectors:
