Fundamentals of ATR-FTIR Spectroscopy and Its Role …
5
Several researchers have focused their attention towards utilizing the simplistic
non-invasive approach of ATR-FTIR spectroscopy for a wide domain of applications in various fields of science. It has been combined with several theoretical
algorithms for improving point-of-care diagnostics in the health care sector with
sensitive detection efficiency for different bio-analytes and disease biomarkers [13–
15]. By tuning the IR incident angle and the subsequent penetration depth, the ATRFTIR spectroscopy can be used to probe specific components of the layered surfaces
over the ATR crystal, for example, thin biofilms or peptide layers, and electrolyte
interphases [9, 16]. The polarization (parallel or perpendicular) of the incident IR
beam can be used in the ATR-FTIR spectrometer to extract the molecular orientation of the molecules [17, 18]. The concept of ATR-FTIR spectrometer has also been
explored for various IR imaging modalities like histopathology, live cells and tissues,
identifying material surface properties, etc. [19–21].
In this book chapter, we aim to address the fundamental aspects of the ATRFTIR vibrational spectroscopic technique. Primary sections include the working
principles, the instrumental details, and the theoretical aspects of the ATR-FTIR
spectroscopy. Some of the application areas of the ATR-FTIR technique are also been
covered by the comprehensive review of the work done in this field by emphasizing
the broader perspective of the ATR-FTIR spectroscopic tool for conducting in-situ
molecular-level characterizations of various molecular systems.
2 Working Principle and Instrumental Details
2.1 FTIR Spectrometer
Modern IR spectroscopic instruments are widely equipped with the FTIR design to
accelerate the scanning and data collection process. The fundamental unit of the FTIR
spectrometer constitutes the Michelson interferometer. The detailed optical layout of
the ATR-FTIR spectrometer is depicted in Fig. 1 along with the path ways of generating the IR spectrum [1, 2, 22]. This device introduces a path difference to the IR
beams (emanated from an IR light source) through the involvement of a beam splitter
at the center, two optical mirrors (stationary and motorized), and an efficient detector
system. IR source utilized most commonly for the mid-infrared beam generation in
FTIR-based instruments is the silicon carbide rod, and it is also called globar source.
The IR beam from the source is split by the beam splitter and projected towards both
the stationary and the motorized mirrors. The beam reflected from the motorized
moving mirror induces a path difference with respect to the second beam which
is being reflected from the stationary mirror. The resultant IR beams with an effective path difference undergo interference process (constructive and destructive). This
recombined IR radiation is then allowed to interact with the sample assembly and
the resultant output is collected by a DTGS (deuterated triglycine sulfate) detector
as a Fourier transform spectra of the sample response.
5
Several researchers have focused their attention towards utilizing the simplistic
non-invasive approach of ATR-FTIR spectroscopy for a wide domain of applications in various fields of science. It has been combined with several theoretical
algorithms for improving point-of-care diagnostics in the health care sector with
sensitive detection efficiency for different bio-analytes and disease biomarkers [13–
15]. By tuning the IR incident angle and the subsequent penetration depth, the ATRFTIR spectroscopy can be used to probe specific components of the layered surfaces
over the ATR crystal, for example, thin biofilms or peptide layers, and electrolyte
interphases [9, 16]. The polarization (parallel or perpendicular) of the incident IR
beam can be used in the ATR-FTIR spectrometer to extract the molecular orientation of the molecules [17, 18]. The concept of ATR-FTIR spectrometer has also been
explored for various IR imaging modalities like histopathology, live cells and tissues,
identifying material surface properties, etc. [19–21].
In this book chapter, we aim to address the fundamental aspects of the ATRFTIR vibrational spectroscopic technique. Primary sections include the working
principles, the instrumental details, and the theoretical aspects of the ATR-FTIR
spectroscopy. Some of the application areas of the ATR-FTIR technique are also been
covered by the comprehensive review of the work done in this field by emphasizing
the broader perspective of the ATR-FTIR spectroscopic tool for conducting in-situ
molecular-level characterizations of various molecular systems.
2 Working Principle and Instrumental Details
2.1 FTIR Spectrometer
Modern IR spectroscopic instruments are widely equipped with the FTIR design to
accelerate the scanning and data collection process. The fundamental unit of the FTIR
spectrometer constitutes the Michelson interferometer. The detailed optical layout of
the ATR-FTIR spectrometer is depicted in Fig. 1 along with the path ways of generating the IR spectrum [1, 2, 22]. This device introduces a path difference to the IR
beams (emanated from an IR light source) through the involvement of a beam splitter
at the center, two optical mirrors (stationary and motorized), and an efficient detector
system. IR source utilized most commonly for the mid-infrared beam generation in
FTIR-based instruments is the silicon carbide rod, and it is also called globar source.
The IR beam from the source is split by the beam splitter and projected towards both
the stationary and the motorized mirrors. The beam reflected from the motorized
moving mirror induces a path difference with respect to the second beam which
is being reflected from the stationary mirror. The resultant IR beams with an effective path difference undergo interference process (constructive and destructive). This
recombined IR radiation is then allowed to interact with the sample assembly and
the resultant output is collected by a DTGS (deuterated triglycine sulfate) detector
as a Fourier transform spectra of the sample response.