Fundamentals of ATR-FTIR Spectroscopy and Its Role …
7
source and the detector. In this way, a similar atmospheric condition that is maintained
within the spectrometer is sensed by the sampling accessory.
The ATR accessory can have different types of sampling assemblies mounted with
the ATR crystal for facilitating the evaluation of the sample. The flat plate assembly
is utilized to analyze solid samples like films, powders, etc., by simply pressing the
sample over the face of IRE. Trough crystal assembly accommodates the crystal in
a lower position to the accessory surface creating a trough, where both liquid and
solid samples can be placed bearing a direct contact with the crystal surface. Another
sampling assembly is a liquid flow cell that provides a sealed channel with the ATR
crystal, and an external temperature and flow controller devices [1, 2, 5, 22]. These
sampling assemblies can be mounted over the horizontal ATR accessory.
2.3 Internal Reflection Elements for ATR-FTIR Spectrometer
The internal reflection elements (IRE), also called as ATR crystals, are widely used
for undertaking the ATR-FTIR spectroscopic process. These elements must have
a higher refractive index than the sample in contact for IR probing, which is the
requirement for satisfying the TIR condition. Then the material has to be transparent
in the specified wavelength region of scanning with IR incident beams. In order
to achieve the above-mentioned requirements, different types of materials are used
for fabricating the IREs. The most common materials being utilized for this purpose
include Zinc selenide (ZnSe), Germanium (Ge), Silicon (Si), Diamond, and Thallium
bromide/iodide (KRS-5). These IREs differ among themselves in their refractive
index values (n 1 ) and specific critical angle values (θ c ). Moreover, the choice of the
crystal or IRE depends on their overall dimensions, durability, material strength,
working pH and sample sensitivity. For inference, ZnSe and Ge provide a costeffective and wider pH range stability for sample probing and are commonly used
for spectrometry, while the diamond crystal is a highly robust material but its usage
is limited by its high cost and limited dimensional area of probing [1, 5, 22, 24, 25].
Details about the other IRE or ATR crystal materials used in the mid-infrared range
are elaborated in Table 1 [22, 24, 25]. Considering the different geometries of IREs
for the ATR technique, the internal reflection could be conducted by either single or
multiple reflection geometries. The optical layout of both the geometries are depicted
in Fig. 2 (panel a and b). Both the schemes differ in terms of number of reflections
of the incident IR beam that generates an equivalent number of evanescent waves
and dictates the corresponding attenuation by the sample due to absorption. Multiple
reflections provide a much intense signal in comparison to the single reflection mode
of the IREs due to its multiscale provision of light-matter interaction option. Although
a single reflection mode IRE provides a weaker signal, it could conveniently probe
the samples having a smaller amount. The active area of the IRE for carrying out a
single reflection process could be achieved either by the type of IRE geometry or
by using the highly focusing optics inside the ATR accessory to maintain the beam
diameter at the focal spot (Fig. 1).
7
source and the detector. In this way, a similar atmospheric condition that is maintained
within the spectrometer is sensed by the sampling accessory.
The ATR accessory can have different types of sampling assemblies mounted with
the ATR crystal for facilitating the evaluation of the sample. The flat plate assembly
is utilized to analyze solid samples like films, powders, etc., by simply pressing the
sample over the face of IRE. Trough crystal assembly accommodates the crystal in
a lower position to the accessory surface creating a trough, where both liquid and
solid samples can be placed bearing a direct contact with the crystal surface. Another
sampling assembly is a liquid flow cell that provides a sealed channel with the ATR
crystal, and an external temperature and flow controller devices [1, 2, 5, 22]. These
sampling assemblies can be mounted over the horizontal ATR accessory.
2.3 Internal Reflection Elements for ATR-FTIR Spectrometer
The internal reflection elements (IRE), also called as ATR crystals, are widely used
for undertaking the ATR-FTIR spectroscopic process. These elements must have
a higher refractive index than the sample in contact for IR probing, which is the
requirement for satisfying the TIR condition. Then the material has to be transparent
in the specified wavelength region of scanning with IR incident beams. In order
to achieve the above-mentioned requirements, different types of materials are used
for fabricating the IREs. The most common materials being utilized for this purpose
include Zinc selenide (ZnSe), Germanium (Ge), Silicon (Si), Diamond, and Thallium
bromide/iodide (KRS-5). These IREs differ among themselves in their refractive
index values (n 1 ) and specific critical angle values (θ c ). Moreover, the choice of the
crystal or IRE depends on their overall dimensions, durability, material strength,
working pH and sample sensitivity. For inference, ZnSe and Ge provide a costeffective and wider pH range stability for sample probing and are commonly used
for spectrometry, while the diamond crystal is a highly robust material but its usage
is limited by its high cost and limited dimensional area of probing [1, 5, 22, 24, 25].
Details about the other IRE or ATR crystal materials used in the mid-infrared range
are elaborated in Table 1 [22, 24, 25]. Considering the different geometries of IREs
for the ATR technique, the internal reflection could be conducted by either single or
multiple reflection geometries. The optical layout of both the geometries are depicted
in Fig. 2 (panel a and b). Both the schemes differ in terms of number of reflections
of the incident IR beam that generates an equivalent number of evanescent waves
and dictates the corresponding attenuation by the sample due to absorption. Multiple
reflections provide a much intense signal in comparison to the single reflection mode
of the IREs due to its multiscale provision of light-matter interaction option. Although
a single reflection mode IRE provides a weaker signal, it could conveniently probe
the samples having a smaller amount. The active area of the IRE for carrying out a
single reflection process could be achieved either by the type of IRE geometry or
by using the highly focusing optics inside the ATR accessory to maintain the beam
diameter at the focal spot (Fig. 1).