Quantum Cascade Laser Spectroscopy
377
causes the change in rotational and vibrational bending and stretching motions of the
bonded nuclei leading to exclusively assignment of specific isotopologues. Thus, the
isotopic ratio can be easily retrieved from the spectrum of sample gas and its known
isotopic composition.
From the past two decades, QCL has been exploited in different optical detection strategies as a laser source for isotopic trace gas detection with high selectivity
and high-sensitivity. In 1999, kosterev et al. reported the methane isotopes (
12 CH 4 ,
13 CH 4 and
12 CH 3 D) detection around 8.1 μm by probing n 4 fundamental vibrational
band of CH 4 by exploiting cryogenic cooled CW QCL in a transmission absorption
spectrometer [79]. Later, QCL-DFB laser was employed in a wavelength modulation spectrometer to investigate methane and nitrous oxide isotopomers around
8.06 mm [80]. The cryogenic cooled QCL emitted the optical power around 80 mW
with the operating temperature around 80–87 K. The tuning range of this QCL
enabled to probe fundamental transition mode of both CH 4 (
13 CH 4 and
12 CH 4 ) and
N 2 O (
14 N 2
16 O,
15 N
14 N
16 O,
14 N 2
18 O,
14 N 2
17 O) isotopes. Later they employed the
same QCL for frequency modulation spectroscopy for high-sensitive detection of
the ration of the isotopes such as
12 CH 4 /
13 CH 4 ,
14 N 2
16 O/
14 N 2
18 O,
14 N 2
16 O/
14 N 2
17 O
and
14 N 2
16 O/
15 N 14 N
17 O pairs by probing their simultaneous transition lines.
The first field-deployable mid-IR absorption spectrometer was developed by the
laser science group at Rice University, USA to investigate the CO 2 isotopes. They
exploited a Peltier-cooled pulsed QCL operating at 4.3 μm in a multi-pass cell to
quantitatively yield the
12 C and
13 C concentrations of the CO 2, which are temperatureinsensitive [81]. Moreover, they also demonstrated the simultaneous investigation of
16 O
12 C
16 O and
16 O
12 C
18 O concentrations in a 1% CO 2 mixture in N 2 using the same
prototype spectrometer. At the same time, another group of researchers from Aerodyne Research also developed a Peltier-cooled QCL coupled CO 2 isotopes prototype
analyser in the mid-IR region [82]. They utilised a pulsed-QCL at 2311 cm
−1 in dual
absorption cells with the longer path for
13 CO 2 and shorter path for major isotope
12 CO 2 and subsequently they were able to achieve measurement precision of 0.1‰
of δ
13 C by using novel optics and advanced signal processing techniques . Later they
improved same infrared QCL spectrometer to get sufficient precision (0.2‰ in 1 s,
0.02‰ in 60 s) for isotopic studies in different timescale for atmospheric air sample
monitoring by incorporating advanced regression analysis [83] (Fig. 8).
Furthermore, Tuzson et al. demonstrated a modification of a commercial QCL
coupled CO 2 isotope analyzer from Aerodyne Research Inc. in the mid-IR spectral
region [85]. This spectrometer was capable of simultaneous measuring of
12 CO 2 ,
13 CO 2 and
16 O
12 C
18 O isotopic species of atmospheric samples by carefully probing
their simultaneous three transition lines. The instrument achieved measurement
precision up to 0.16 and 0.25‰ for
13 C/
12 C and
18 O/
16 O respectively within acquisition time 9 min. Furthermore, QCL was also exploited in the determination of
N 2 O isotopomers using mid-IR absorption spectrometer at 4.6 μm [86]. Utilising
the potential of the QCL, they were able to record the high-resolution experimental
absorption spectra of two isotopomers of N 2 O with one heavy isotope of nitrogen
(
15 N), positioned at the centre (αsite) and at any of the end (β site) of the molecule.
This study paved new avenue regarding isotopomers analysis by laser spectroscopy,
377
causes the change in rotational and vibrational bending and stretching motions of the
bonded nuclei leading to exclusively assignment of specific isotopologues. Thus, the
isotopic ratio can be easily retrieved from the spectrum of sample gas and its known
isotopic composition.
From the past two decades, QCL has been exploited in different optical detection strategies as a laser source for isotopic trace gas detection with high selectivity
and high-sensitivity. In 1999, kosterev et al. reported the methane isotopes (
12 CH 4 ,
13 CH 4 and
12 CH 3 D) detection around 8.1 μm by probing n 4 fundamental vibrational
band of CH 4 by exploiting cryogenic cooled CW QCL in a transmission absorption
spectrometer [79]. Later, QCL-DFB laser was employed in a wavelength modulation spectrometer to investigate methane and nitrous oxide isotopomers around
8.06 mm [80]. The cryogenic cooled QCL emitted the optical power around 80 mW
with the operating temperature around 80–87 K. The tuning range of this QCL
enabled to probe fundamental transition mode of both CH 4 (
13 CH 4 and
12 CH 4 ) and
N 2 O (
14 N 2
16 O,
15 N
14 N
16 O,
14 N 2
18 O,
14 N 2
17 O) isotopes. Later they employed the
same QCL for frequency modulation spectroscopy for high-sensitive detection of
the ration of the isotopes such as
12 CH 4 /
13 CH 4 ,
14 N 2
16 O/
14 N 2
18 O,
14 N 2
16 O/
14 N 2
17 O
and
14 N 2
16 O/
15 N 14 N
17 O pairs by probing their simultaneous transition lines.
The first field-deployable mid-IR absorption spectrometer was developed by the
laser science group at Rice University, USA to investigate the CO 2 isotopes. They
exploited a Peltier-cooled pulsed QCL operating at 4.3 μm in a multi-pass cell to
quantitatively yield the
12 C and
13 C concentrations of the CO 2, which are temperatureinsensitive [81]. Moreover, they also demonstrated the simultaneous investigation of
16 O
12 C
16 O and
16 O
12 C
18 O concentrations in a 1% CO 2 mixture in N 2 using the same
prototype spectrometer. At the same time, another group of researchers from Aerodyne Research also developed a Peltier-cooled QCL coupled CO 2 isotopes prototype
analyser in the mid-IR region [82]. They utilised a pulsed-QCL at 2311 cm
−1 in dual
absorption cells with the longer path for
13 CO 2 and shorter path for major isotope
12 CO 2 and subsequently they were able to achieve measurement precision of 0.1‰
of δ
13 C by using novel optics and advanced signal processing techniques . Later they
improved same infrared QCL spectrometer to get sufficient precision (0.2‰ in 1 s,
0.02‰ in 60 s) for isotopic studies in different timescale for atmospheric air sample
monitoring by incorporating advanced regression analysis [83] (Fig. 8).
Furthermore, Tuzson et al. demonstrated a modification of a commercial QCL
coupled CO 2 isotope analyzer from Aerodyne Research Inc. in the mid-IR spectral
region [85]. This spectrometer was capable of simultaneous measuring of
12 CO 2 ,
13 CO 2 and
16 O
12 C
18 O isotopic species of atmospheric samples by carefully probing
their simultaneous three transition lines. The instrument achieved measurement
precision up to 0.16 and 0.25‰ for
13 C/
12 C and
18 O/
16 O respectively within acquisition time 9 min. Furthermore, QCL was also exploited in the determination of
N 2 O isotopomers using mid-IR absorption spectrometer at 4.6 μm [86]. Utilising
the potential of the QCL, they were able to record the high-resolution experimental
absorption spectra of two isotopomers of N 2 O with one heavy isotope of nitrogen
(
15 N), positioned at the centre (αsite) and at any of the end (β site) of the molecule.
This study paved new avenue regarding isotopomers analysis by laser spectroscopy,