Jasco Report Vol.63, No.2,45 (2021)

Development of a Vibrational Circular Dichroism Spectrometerusing a Quantum Cascade Laser
Keisuke Watanabe, Masaru Shimizu, Jun Koshoubu, JASCO Corporation
Jun Yoshida, Department of Chemistry, College of Humanities & Sciences, Nihon University
Izuru Kawamura, Graduate School of Engineering Science, Yokohama National University
Hisako Sato, Graduate School of Science and Engineering, Ehime University
Abstract
Vibrational circular dichroism spectrometer based on Fourier transform infrared optical system (FT-VCD) is used todetermine the absolute configuration of chiral compounds, because it provides three-dimensional structural information
about vibrational transitions in the infrared region. However, since the intensity (ΔAbs) in the vibrational circulardichroism (VCD) spectrum can be as low as 10 -4 to 10 -5, it is necessary to increase the sensitivity of the measurement
system. To achieve this, we have newly developed a vibrational circular dichroism spectrometer using a quantum
cascade laser (QCL-VCD) as a monochromatic light source instead of a Fourier transform infrared spectrometer (FT-IR).
The QCL-VCD (wavenumber range: 1740 to 1500 cm-1) was combined with the FT-VCD (wavenumber range: 4000 to
850 cm-1) to realize a system (MultiD-VCD-MIRAI -2020) that achieves both high sensitivity and a wide measurement
range.
Compared to the light sources used in conventional FT-VCD, quantum cascade lasers produce a higher-intensity
output, so in addition to increasing the sensitivity of the measurement system, it is also possible to measure samples
exhibiting strong absorption. The QCL-VCD can be applied to measurements of highly concentrated samples, inaddition to aqueous solutions that have been difficult to measure in the past. Furthermore, by taking advantage of thehigh sensitivity of this system, it is expected that it can be used to measure extremely small regions of a sample, andfor in-plane mapping measurements. In this report, we describe the effectiveness of the newly developed QCL-VCD byproviding application examples.
Keywords: Vibrational Circular Dichroism, Quantum Cascade Laser, Aqueous Solutions, Glycyl-leucine, Mapping,
Alanine1. Introduction
Vibrational circular dichroism spectroscopy (VCD)
is used for the analysis of chiral compounds. Since many
absorption bands arising from the vibrational transitionsof molecules are observed in the infrared region, theabsolute configuration of a sample can be determinedby analyzing its VCD spectra [1, 2]. However, the signal
intensity (ΔAbs) of VCD spectra is as weak as 10 -4 to
10 -5, and it often makes it difficult to measure samples.
To develop VCD as an analytical method for a wider
range of samples, the sensitivity of the system needs tobe improved further.
To solve this problem of low sensitivity, we havedeveloped a new v ibrat ional ci rcula r d ich roism
spectrometer (MultiD-VCD-MIR AI-2020) using aquantum cascade laser (QCL, wavenumber range: 1740 to1500 cm-1) as a light source. Compared to the vibrational
circular dichroism spectrometer (FV-VCD) based on theconventional Fourier transform infrared spectrometer(FT-IR), the system using QCL has a higher sensitivity
due to its high brightness light source and enables us
to measure samples with high absorption intensities.
Therefore, the system using QCL can be applied tothe measurement of highly concentrated samples andaqueous solutions, which have been considered difficult
to measure in the past. In addition, since QCL isa monochromatic light source, it is possible to obtain
results in a shorter time than FT-VCD in measuring
specific wavenumbers. These characteristics of highspeed and high sensitivity are expected to allow one toapply the system to the measurement of microregions
and to mapping measurement, which can analyze the inplane distribution of chiral compounds within the sample.
In this paper, we report the effectiveness of the VCD
measurement system using a quantum cascade laser.

2. MultiD-VCD-MIRAI-2020
2-1 System
Fig. 1 shows the external view of MultiD-VCDMIRAI-2020, and Fig. 2 shows the optical system
around its sample chamber. MultiD-VCD-MIRAI-2020
is a vibrational circular dichroism spectrometer whoselight source can be selected between the FT-VCD (FT-IR)
and the QCL-VCD (QCL) which share an optical system
around the sample chamber. The section of FT-VCD uses
FVS-6000 VCD spectrometer (JASCO Corporation,
Japan). The light source can be switched between the FTVCD and the QCL-VCD to measure the same sample bywitching mirror in front of the sample chamber.
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Jasco Report Vol.63, No.2,45 (2021)
FT-VCD light source(ceramic light source)

Polarizer and PEM

XY stage

FT-VCD op�cal system(interferometer)

QCL light source

QCL op�cal system

Sample compartment

Switching mirror
for op�cal system

Detector

Stage controller
PEM controller

Lock-in ampliﬁer

Figure 1 Photograph of MultiD-VCD-MIRAI-2020

Figure 3 Photograph of XY-Stage

2-2 QCL Light Source
The MIRcat-QT mid-infrared laser manufactured
by DAYLIGHT SOLUTIONS was adopted as the QCLlight source. The oscillation mode of the "MIRcatQT mid-infrared laser" is CW (continuous wave),and infrared light of 1740 ~ 1500 cm -1 is emitted.
The maximum output power is 300 mW, and thewavenumber accuracy is within 1 cm-1. The MIRcatQT can be equipped with up to four laser heads
with different oscillation bandwidths, so that thewavenumber range for measurement can be extended
in the future.
Before integrating the QCL into the system, we
compared the output power of the QCL with that of thehigh-intensity infrared light source initially installedin the FT-VCD to confirm its performance as a lightsource. Fig. 4 shows the output spectra of these lightsources measured using FT-IR. The left figure in Fig.
4 is the spectrum measured by scanning the oscillation
wavenumber of the QCL around 1600 cm-1, and theright figure the spectrum measured by the infrared
light source in the FT-VCD. The intensity at 1600 cm-1
was 250,626 for the QCL and 4.4 for the infrared lightsource in the FT-VCD, showing that the output of the
QCL is more than 55,000 times larger than that of theinfrared light source. In addition, light from the QCL
is not led to the FT-IR in this system, so that the power
energy loss by the Michelson interferometer can be
avoided.

FT-VCD light source&
op�cal system
Polarizer
Detector

Pre amp.

Switching mirror forop�cal system
SampleQCL light source&
op�cal system

PEM
50 kHzOp�cal chopper
PC

1 kHz
DAQ

AC Signal
Lock-in
ampliﬁer

35 kHz
HPF

Lock-in
ampliﬁer

1.6 kHzLPF
DC Signal

Figure 2 MultiD-VCD-MIRAI-2020 optical system

Infrared monochromatic light emitted from the
QCL-VCD is introduced into the QCL optical system
and modulated by a chopper at a frequency of 1 kHz.
The light is then linearly polarized by a polarizer andmodulated into left and right circularly polarized lightat 50 kHz by a photoelastic modulator (PEM). The
sample is irradiated with circularly polarized light, andthe transmitted infrared light is observed by a mercury
cadmium telluride (MCT) detector cooled with liquid
nitrogen. The detected signal is a superposition of a DC
signal, which is the intensity of infrared monochromatic
light corresponding to the modulation frequency of the
chopper, and an AC signal, which is the difference inintensity between the transmitted light with left circularpolarization and that with right circular polarization
corresponding to the modulation frequency of the PEM.
The DC signal is separated from the high-frequency
noise signal by a low-pass filter that eliminates signals
above 1.6 kHz, and the AC signal is separated from thelow frequency noise signal by a high-pass filter thateliminates signals below 35 kHz, and then the signal
components synchronized with the modulation frequency
are acquired by a lock-in amplifier. The ratio of the DC
signal to the one without the sample is output as the IR
spectrum, and the ratio of the AC signal to the DC signal(AC/DC) as the VCD spectrum.
The sample stage (Fig. 3) is installed in the sample
chamber, and mapping measurement is realized bymoving the sample position.

FT-VCD(Ceramic light source)

QCL light source

250,626.0
1600 cm-1

1600 cm-1

4.4

Figure 4 Comparison of light source brightness for
QCL-VCD and FT-VCD
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Jasco Report Vol.63, No.2,45 (2021)
2-3 Characteristics
Using a QCL as a high brightness light source in thesystem is expected to realize "a high system sensitivity,""the measurement of samples with strong absorption intensity," and "its use for mapping measurement."
In addition to the above, it is also a characteristicthat MultiD-VCD-MIRAI-2020 can be used with its
light source selected between the FT-VCD and the QCLVCD. While the measurement range of the QCL-VCD
is limited to 1740 ~ 1500 cm-1, the QCL was selected sothat it covered the absorption peaks of mainly amide I
and amide II in measuring them. In contrast to the QCLVCD, the FT-VCD can measure from 4000 to 850 cm-1
and obtain information of various functional groups. The
system allows us to switch between the FT-VCD and the
QCL-VCD depending on the purpose of measurement.

signal intensity of IR. When IR absorption is large,
however, the infrared light observed by the detector
becomes weak in a logar ith mically propor tionalmanner, and the influence of noise rapidly increases.
If the influence of noise becomes larger than the signal
intensity of VCD, the accurate VCD spectral shapecannot be maintained. It was expected that using a QCL
with high brightness as a light source would secure theintensity of infrared light transmitted through the sample
and reduce the influence of noise, which would allow us
to accurately measure the VCD spectra of samples withhigh absorbance. The samples used were NEAT liquidsof (1R)-(+)-α-Pinene and (1S)-(-)-α-Pinene, and the IR
absorbance at 1658 cm-1 was adjusted to about 2 using acell with an optical path length of 500 μm. The resultsof QCL-VCD are shown on the left in Fig. 7, and thoseof FT-VCD on the right. As for the VCD spectrum,
FT-VCD shows large VCD noises, failing to produce
symmetric VCD signals. On the other hand, QCL-VCD
produced symmetric VCD signals. Therefore, QCL-VCD
was shown to be effective in measuring samples withstrong absorption.

3. Measurement
3-1 Measurement of High Absorbance Samples
We studied the measurement of high absorbances a mple s , wh ich wa s ex p e c t e d t o b e one of t hecharacteristics of the QCL. Before conducting the highabsorbance measurement of VCD, we examined how
accurately the system could measure IR absorption
intensity. As samples for this, we used polystyrene filmsof different thicknesses for different absorptions. We
also used DLATGS (Deuterated L-Alanine Triglycine
Sulphate), which operates at room temperature, as thedetector. Fig. 5 shows the IR spectrum of polystyrene.
Focusing on the absorption intensity at 1601 cm -1,
we examined the relationship between the thicknessof the sample and the absorbance at 1601 cm -1 of thepolystyrene films. The left figure in Fig. 6 shows theresult of QCL-IR, and the right figure the result of FTIR. They show that while the linearity of absorption for
QCL-IR was maintained up to around an absorbance of 5,
the linearity for FT-IR was broken above an absorbanceof 3.
We then studied the VCD measurement of highabsorbance samples. In general, the signal intensity of
VCD increases proportionally with the increase in the【QCL-VCD】

IR

VCD

VCD

【QCL-IR】【FT-IR】
No linearity

Figure 6 Relationship between poystyrene thikness andbsorbance at 1601 cm-1
Left: QCL-IR, Right: FT-IR(1R)-(+)-α-Pinene(1S)-(-)-α-Pinene

ΔAbs

VCD Noise
ΔAbs

Figure 5 Polystyrene IR spectrum【FT-VCD】

IR

ΔAbs

1601 cm-1

VCD Noise
ΔAbs

ΔAbs

Figure 7 VCD results of high absorbance sample
Left: QCL-IR, Right: FT-IR
3【Measurement parameters】
Cell：
Sealed liquid cell
Window plate:
BaF2
Path length:
500 µmTime:a li�le over 10 min.
Baseline:
Racemic body
Sample:(1R)-(+)-α-Pinene(1S)-(-)-α-Pinene
Concentra�on:neatJasco Report Vol.63, No.2,45 (2021)
3-2 Measurement of Aqueous Solutions
One of the applications of VCD high absorbancemeasurement is the measurement of aqueous solutions.
Aqueous solutions are studied in various fields, andtheir measurement is useful especially for the biologicalsample. However, the infrared absorption of water isvery strong, and the path length of sample cells generally
used is several μm. Fig. 8 shows the IR spectrum ofwater measured using a spacer of thickness 4 μm. The
absorption due to the bending vibration of OH of water
is seen at around 1640 cm-1, and it is a strong signal ofabsorbance about 0.8. As described in "3-1 Measurementof High Absorbance Samples" above, the measurement ofhigh absorbance samples is difficult in ordinary FT-VCD
measurement, and thus it is difficult to measure aqueous
solutions, which have strong absorption. In addition, if
the optical path length is short, the absorption intensityof a sample dissolved in water becomes small, and theresulting VCD signal also becomes weak. A short cellpath length also makes sampling difficult. Furthermore,
the sampling of a sample with high concentration into acell of a short optical path tends to cause the clogging or
contamination of the cell. Using QCL-VCD for measuring
aqueous solutions possibly solve these problems.
IR

We thus measured the VCD spectra of aqueous solutions,using the QCL-VCD system.
We used the D-isomer and the L-isomer of Glycylleucine (Gly-Leu) (concentration: 0.3 M) as aqueous
samples. We also used a spacer of thickness 25 μm,
which made the optical path length longer than theordinary path length. Fig. 9 shows the results of themeasurement. The figure on the left in Fig. 9 is the
IR spectra of the Gly-Leu solutions and water, andthe figures on the right the IR, VCD, and VCD noise
spectra of Gly-Leu solutions after subtracting the water
spectrum. The figure on the left shows that absorption
by water is dominant among the IR spectra, while theamide II absorption of the samples can be seen at around1580 cm-1. The absorbance of this amide II absorption
as well as that of water is very large, being about 1 ~ 2.
Nevertheless, the VCD signal of the amide II absorption
band at 1580 cm-1 was clearly observed in the figure on
the right in Fig. 9. This clearly shows that QCL-VCD isvery effective in the measurement of aqueous solutions,which have been difficult in the past.
3-3 Mapping Measurement
The QCL-VCD system can reduce the scanning
time because of its high sensitivity and also allows one
to obtain spectra in a shorter time than FT-VCD bymeasuring only the wavenumber range in focus. As aresult, the system is expected to be applied to mapping
measurement, in which multiple VCD spectra aremeasured by moving the sample. We thus investigatedmapping measurement, which is one of the characteristicsof the system. Fig. 10 shows images of the sample used.
To confirm the validity of the system, we prepared atablet of KBr containing D-Alanine in the right half and

Measurement parameters
Cell:demountable cell
Window plate: CaF2
Spacer:
4 μm

Figure 8 IR spectrum of waterIR

IR

Gly-L-Leu
Gly-D-Leu
H2O

VCD
ΔAbs

Measurement parameters
Cell:demountable cell
Window plate:
CaF2
Spacer:
25 μm
Meas. �me
40 min.
Baseline:
Racemic body
Sample:Gly-L-Leu,
Gly-D-Leu
Concentra�on:
0.3 M

VCD Noise
ΔAbs

Figure 9 Measurement results of Gly-Leu aqueous solution
Left: Gly-Leu IR spectrum
Right: IR, VCD, VCD noise spectra of Gly-Leu (after subtracting water spectrum)
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Jasco Report Vol.63, No.2,45 (2021)
Measurement points
L-Alanine

Measurement parameters
Interval:
2.5 mmNo. of points: 5 x 5Meas. Time: 75 min.
(approx. 3 min./1 spectrum)
D-Alanine

KBr tablet (φ 10mm)

KBr tablet (φ 10mm)

Figure 10 Sample of mapping measurement

IR

L-Alanine in the left half as a sample with known
distributions of chiral compounds. In preparing thesample, each of D-Alanine and L-Alanine was mixed
with KBr at a ratio of 0.15 wt% and ground in an agate
mortar. These portions were spatially separated in a
hydraulic press for tableting, and a tablet of diameter
10 mm and thickness 0.58 mm was produced. Mapping
measurement was perfor med at 25 points (5 x 5)
disposed at intervals of 2.5 mm on the tablet to obtain
the distribution of chiral compounds. The diameter of thebeam irradiated on the sample was 10 mm (the spatial
resolution being 5.0 mm [3]). The time of VCD spectrum
measurement was about 3 minutes for each point, andthe total time for the mapping measurement was about 75
minutes.
Before discussing the results of the measurement,
we show the IR and VCD spectra of D-Alanine and
L-Alanine in Fig. 11 as a reference. It shows that while
the IR spectra have a similar shape, the VCD spectra
have symmetric shapes. The peak at 1603 cm-1 in the
VCD spectrum is positive for D-Alanine and negative for
L-Alanine. Therefore, D-Alanine and L-Alanine can be
distinguished by identifying the sign of this peak.
Fig. 12 shows the results of the mapping measurement.

D-Alanine
L-Alanine

VCD

1603 cm-1

ΔAbs

Figure 11 Spectrum of pure d-alanine and pure l-alanine

VCD
1603 cm-1
10 mm

IR

(plus)
+

10 mm

VCD

L-Ala

D-Ala

Chemical image(minus)ΔAbs

X : -5.0 mm ,X : -2.5 mm ,X : 0.0 mm ,X : 2.5 mm ,X : 5.0 mm ,Figure 12 Mapping results
Left: Chemical image, Right: IR and VCD spectra5

Y : 0 mm
Y : 0 mm
Y : 0 mm
Y : 0 mm
Y : 0 mm

Jasco Report Vol.63, No.2,45 (2021)
The figure on the left shows an image of the measured
points distinguished by color based on the VCD peakintensity at 1603 cm-1, where yellow-green is used if the
VCD peak at 1603 cm-1 was positive (D-Alanine), andorange if it was negative (L-Alanine). As we expected, itshows the existence of D-Alanine in the right half of thetablet and that of L-Alanine in the left half. In addition,
the spectra obtained by mapping measurement (on theright in Fig. 12) show that the right side of the sample(X = 2.5, 5.0 mm) produced the spectra of D-Alanine,
and the left side of the sample (X = -2.5, -5.0 mm) the
spectra of L-Alanine. These results show that QCL-VCD
can be applied to mapping measurement to examine the
distribution of chiral compounds.
4. Summary
We developed MultiD-VCD-MIRAI-2020, which
combi nes QCL -VCD a nd FT-VCD. QCL -VCD iseffectivein measuring aqueous solutions, as an applicationof high absorbance measurement. It can be also applied
to mapping measurement, by which the distribution ofchiral compounds in a sample can be visualized. We hopethat the developed system will provide more occasions
to gain new insights into measurements and samples thatare difficult to apply conventional FT-VCD.
It is noted here for your reference that while we did
not describe the microscopic measurement system formeasuring microregions in this report, it was reported inreference [4, 5].
5. Acknowledgements
This work was supported by JST-Mirai Program
JPMJMI18GC of Japan Science and Technology Agency(JST). We would like to express our deepest gratitude
for it.

REFERENCES
1. Keisuke Watanabe, “JASCO REPORT”, Vol. 58, Issue 1,
pp. 19-25 (2016)
2. Taniguchi, Tohru and Monde, Kenji, “Trends in Glycoscience and Glycotechnology” Vol.19 No.107, pp.147–
164 (2007)
3. Hisako Sato et al., “Analytical Chemistry”, Vol. 93,
Issue 5, pp. 2742-2748 (2021)
4. Hisako Sato et al., “Chem. Lett.”, May 15, 2021 accepted Advance Publication on the web May 28, 2021
doi:10.1246/cl.210258
5. Hisako Sato et al., “J.Phys. Chem. Lett.”, Vol. 12, Issue
32, pp.7733-7737 (2021)

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