434
K. malek et al.
it does not exceed 2–3 μm in cells and tissues [43]. Compared to transmission or
transflection modes, AtR FtIR spectra do not show artefacts due to mie scattering
and the sample preparation is generally straight forward. Potential applications of
AtR FtIR imaging have been reported for both cells and tissues [45–48]. the other
advantage of this technique is that normalisation for the sample path length is usually not required because the depth of penetration is constant throughout the medium.
15.3.2 Linear and FPA Detectors
IR microspectrometers operate with the use of a mercury-Cadmium-telluride
(mCt) detector, which is more sensitive to small amounts of energy delivered
through the aperture than the deuterated L-alanine doped triglycine sulphate
(dLatgS) detector typically used for benchtop FtIR spectrometers. A single point
mCt detector is a hg 1–x Cd x te alloy, in which detection of IR radiation begins with
the excitation of an electron from the valence band into the conduction band. the
minimum photon energy required is equal to the bandgap, E g . the bandgap of a
mCt device is a function of its composition ratio “x” of Cdte to hgte, and the temperature of the material [49, 50]. It also affects the cut-off wavenumber. the single
and linear mCt detectors employed in IR microscopes operate in the photoconductive mode with the cut-off at ca. 650 and 720 cm
−1
, respectively, whereas a matrix
of mCt detectors (mCt FPA, mCt Focal Plane Array) acts as a photodiode and
enables recording an IR spectrum from 850 cm
−1
[49, 50]. the latter belongs to the
third-generation mCt devices—hyperspectral array, generating a hypercube image.
For such an application, standard second-generation hgCdte arrays are enhanced
with optical coatings and/or proximal filters [50].
Single point mCt detectors, which operate in IR microscopes, usually have a
size of 50 × 50, 100 × 100, or 250 × 250 μm
2
. A smaller area of the mCt element is
usually utilized, since only a part of the detector is illuminated by the infrared beam,
and the remaining part only contributes noise. A linear mCt detector consists of a
16 × 1 MCT array (400 μm × 25 μm). For instance, the PerkinElmer microspectrometer system coupled with a Spectrum Spotlight infrared microscope permits 1:1 or
4:1 imaging, resulting in sample areas of 25 × 25 or 6.25 × 6.25 μm
2
, respectively,
projected onto each detector element. Each element of the linear detector records
the spectrum of the corresponding projected pixel size onto the sample. Both the
single and linear detectors can operate in conjunction with thermal or synchrotron
sources [51, 52]. Chemical images are generated by mapping, i.e. the spectral signal
is delivered to the detector in a stepwise manner from different areas of a sample in
a grid manner with the use of a chosen aperture size and a computer-controlled motorized x, y, z stage. the spatial resolution in this case is also limited by the size of
the aperture. Lasch and Naumann have showed that chemical maps of human colon
mucosa recorded with the use of a linear detector in 1:4 imaging mode have a spatial
resolution of 8.8 μm at 4000 cm
−1
[42]. due to the small aperture transmissivity, the
point and linear mapping of a small region of interest produces an inefficient S/N
K. malek et al.
it does not exceed 2–3 μm in cells and tissues [43]. Compared to transmission or
transflection modes, AtR FtIR spectra do not show artefacts due to mie scattering
and the sample preparation is generally straight forward. Potential applications of
AtR FtIR imaging have been reported for both cells and tissues [45–48]. the other
advantage of this technique is that normalisation for the sample path length is usually not required because the depth of penetration is constant throughout the medium.
15.3.2 Linear and FPA Detectors
IR microspectrometers operate with the use of a mercury-Cadmium-telluride
(mCt) detector, which is more sensitive to small amounts of energy delivered
through the aperture than the deuterated L-alanine doped triglycine sulphate
(dLatgS) detector typically used for benchtop FtIR spectrometers. A single point
mCt detector is a hg 1–x Cd x te alloy, in which detection of IR radiation begins with
the excitation of an electron from the valence band into the conduction band. the
minimum photon energy required is equal to the bandgap, E g . the bandgap of a
mCt device is a function of its composition ratio “x” of Cdte to hgte, and the temperature of the material [49, 50]. It also affects the cut-off wavenumber. the single
and linear mCt detectors employed in IR microscopes operate in the photoconductive mode with the cut-off at ca. 650 and 720 cm
−1
, respectively, whereas a matrix
of mCt detectors (mCt FPA, mCt Focal Plane Array) acts as a photodiode and
enables recording an IR spectrum from 850 cm
−1
[49, 50]. the latter belongs to the
third-generation mCt devices—hyperspectral array, generating a hypercube image.
For such an application, standard second-generation hgCdte arrays are enhanced
with optical coatings and/or proximal filters [50].
Single point mCt detectors, which operate in IR microscopes, usually have a
size of 50 × 50, 100 × 100, or 250 × 250 μm
2
. A smaller area of the mCt element is
usually utilized, since only a part of the detector is illuminated by the infrared beam,
and the remaining part only contributes noise. A linear mCt detector consists of a
16 × 1 MCT array (400 μm × 25 μm). For instance, the PerkinElmer microspectrometer system coupled with a Spectrum Spotlight infrared microscope permits 1:1 or
4:1 imaging, resulting in sample areas of 25 × 25 or 6.25 × 6.25 μm
2
, respectively,
projected onto each detector element. Each element of the linear detector records
the spectrum of the corresponding projected pixel size onto the sample. Both the
single and linear detectors can operate in conjunction with thermal or synchrotron
sources [51, 52]. Chemical images are generated by mapping, i.e. the spectral signal
is delivered to the detector in a stepwise manner from different areas of a sample in
a grid manner with the use of a chosen aperture size and a computer-controlled motorized x, y, z stage. the spatial resolution in this case is also limited by the size of
the aperture. Lasch and Naumann have showed that chemical maps of human colon
mucosa recorded with the use of a linear detector in 1:4 imaging mode have a spatial
resolution of 8.8 μm at 4000 cm
−1
[42]. due to the small aperture transmissivity, the
point and linear mapping of a small region of interest produces an inefficient S/N