Photoionization mass spectrometry: A useful method to evaluate the pyrolysis process of glycoside flavor precursor

Article abstract of DOI:10.1002/jccs.201190027

The thermal decomposition behavior and the pyrolysis products of benzyl-2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside (BGLU) were studied with synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry at temperatures of 300, 500 and 700 °C at 0.062 Pa. Several pyrolysis products and intermediates were identified by the measurement of photoionization mass spectra at different photon energies. The results indicated that the primary decomposition reaction was the cleavage of O-glycosidic bond of the glycoside at low temperature, proven by the discoveries of benzyloxy radical (m/z = 107) and glycon radical (m/z = 331) in mass spectra. As pyrolysis temperature increased from 300 to 700 °C, two possible pyrolytic modes were observed. This work reported an application of synchrotron VUV photoionization mass spectrometry in the study of the thermal decomposition of glycoside flavor precursor, which was expected to help understand the thermal decomposition mechanism of this type of compound. The possibility of this glycoside to be used as a flavor precursor in high temperature process was evaluated.

Full text of DOI:10.1002/jccs.201190027

2
90
Journal of the Chinese Chemical Society, 2011, 58, 290-295
Communication
Photoionization Mass Spectrometry: A Useful Method to Evaluate the
Pyrolysis Process of Glycoside Flavor Precursor
a
b
c
a,
Yu Ding, Ji-Bao Cai, * Fei Qi and Qing-De Su *
Department of Chemistry, University of Science and Technology of China, Hefei, 230026, P. R. China
a
b
Center of R&D, China Tobacco Jiangxi Industrial Co. Ltd., Nanchang, 330096, P. R. China
c
National Synchrotron Radiation Laboratory, University of Science and Technology of China,
Hefei, 230026, P. R. China
Received November 7, 2010; Accepted March 7, 2011; Published Online March 24, 2011
The thermal decomposition behavior and the pyrolysis products of benzyl-2,3,4,6-tetra-O-acetyl-b-
D-glucopyranoside (BGLU) were studied with synchrotron vacuum ultraviolet (VUV) photoionization
mass spectrometry at temperatures of 300, 500 and 700 °C at 0.062 Pa. Several pyrolysis products and in-
termediates were identified by the measurement of photoionization mass spectra at different photon ener-
gies. The results indicated that the primary decomposition reaction was the cleavage of O-glycosidic bond
of the glycoside at low temperature, proven by the discoveries of benzyloxy radical (m/z = 107) and
glycon radical (m/z = 331) in mass spectra. As pyrolysis temperature increased from 300 to 700 °C, two
possible pyrolytic modes were observed. This work reported an application of synchrotron VUV photo-
ionization mass spectrometry in the study of the thermal decomposition of glycoside flavor precursor,
which was expected to help understand the thermal decomposition mechanism of this type of compound.
The possibility of this glycoside to be used as a flavor precursor in high temperature process was evalu-
ated.
Keywords: Glycoside; Flavor precursor; Thermal decomposition; Synchrotron VUV
photoionization; Mass spectrometry.
INTRODUCTION
plant materials and played an important role in the flavor of
the plants.
Glycosides widely existing in nature have been ex-
tensively studied in recent decades, and interests usually
In tobacco leaves, there also a large number of flavor
1
-3
10
focus on their role as flavor precursors. Nowadays, it is
well established that flavorless glycosides represent one
accumulation form of aroma substances in fruits and other
compounds are in glycosylated form. And it has been re-
vealed that the glycoside in tobacco leaves could contribute
to generating aroma and taste in the curing and aging pro-
4
11
plant tissues. In general, biochemical aroma formation is
cesses of flue-cured tobacco. However, there are also
thought to proceed by two different processes. One type
follows normal biosynthetic routes and the other type in-
volves the hydrolysis of glycosides having aroma com-
some gaps in our understanding of the process of aroma
formation from this kind of flavor precursor during high
temperature process, such as cigarette burning. In general,
the cleavage of O-glycosidic bond could be achieved either
5
,6
pounds as their aglycons. Many studies on tea leaves and
grape wine showed that a significant portion of volatile fla-
vor compounds, such as linalool, geraniol, etc., were dis-
covered to be present as glycosides and liberated from their
glycosidic aroma precursors by the action of endogenous
5
by enzymatic/chemical hydrolysis or by pyrolysis. Previ-
ous studies on the glycoside flavor precursor were mainly
1
2,13
focused on the enzymatic/chemical hydrolysis.
The
thermolysis study of this compound had been rarely re-
7
-9
14
enzymes during the manufacturing process. As a matter
of fact, glycosidic flavor precursors presented in many
ported. Until recently, Wancui Xie et al. studied the ther-
mal decomposition of menthyl-glycoside by pyrolysis-gas
*
Corresponding author. Tel & Fax: 0086-551-3606642; E-mail: qdsu@ustc.edu.cn (Qing-De Su); jbcai@ustc.edu.cn (Ji-Bao Cai)

Photoionization Mass Spectrometry Pyrolysis Glycoside
J. Chin. Chem. Soc., Vol. 58, No. 3, 2011 291
1
chromatography-mass spectroscopy (Py-GC-MS) analy-
sis.
Table 1. HNMR, IR, EI-MS identification of the synthetic
BGLY
-
1
However, it is known that above technique have some
drawbacks in analyzing the thermal decomposition process
of analyte, such as the problems in real-time analysis and
secondary reaction. During Py-GC-MS analysis, the pri-
mary rupture of bonds is usually followed by a complex se-
ries of competitive reactions between the radicals and the
fragment molecules produced in thermal decomposition
process. These secondary reactions complicated the pyrol-
ysis products and made the mechanism of decomposition
process unclear.
IR (cm , KBr
micropollets)
EI-MS
(M/Z)
1
HNMR d (in CDCl3)
2960
1.99-2.10 (12H, C-2,3,4,6-Acetyl)
3.68 (1H, C-5)
4.16-4.28 (2H, C-6)
4.53-4.64 (2H, C-1¢)
4.90 (1H, C-1)
331
245
139
97
91
43
1
1
1
1
753
499
453
428
5.03-5.20 (3H, C-2,3,4)
.27-7.37 (5H, Benzene ring)
7
rolysis product we concerned was aromatic benzyl alcohol.
In our study, since the photon energy was tunable, the iden-
tification of pyrolysis products of glycoside, such as aro-
matic alcohol, could be deduced from the measurements of
photoionization mass spectra. With the variation of photon
energy, a series of mass spectra could be taken at the fixed
temperature for identification of pyrolysis products, based
mainly on the comparison of appearance energy region and
known ionization energies (IEs) of pyrolysis products. For
Here, we reported an experimental study of thermal
decomposition of synthetic benzyl-2,3,4,6-tetra-O-acetyl-
b-D-glucopyranoside (BGLU) at low pressure by means of
synchrotron vacuum ultraviolet (VUV) photoionization
combined with molecular-beam mass spectrometry (MBMS),
also called as synchrotron VUV photoionization mass spec-
1
5
trometry (SVUV-PIMS). The experimental apparatus ex-
hibited some advantages in analyzing the pyrolytic process
of glycoside flavor precursor, such as real-time analysis
and less secondary reactions, compared with conventional
method. Benefitted from the wide tunability and high en-
ergy resolution in VUV region, synchrotron VUV photo-
ionization can minimize fragmentation and detect radi-
example, benzyl alcohol (C
tant decomposition product in our study. However, there
are several possible isomers of C O (m/z = 108), such as
benzyl alcohol (IE = 9.53 eV) and methyl-phenol isomers
7 8
H O, m/z = 108) is very impor-
7
H
8
1
9
2
0-21
1
6-18
(IEs below 8.5 eV).
In our study, no signal could be ob-
cals
. Therefore, SVUV-PIMS can selectively detect
served at the photon energy below 9.00 eV, which excluded
the existence of phenolic compounds; and thus the signal of
m/z = 108 could be attributed to the substance of benzyl
alcohol. Some other products and intermediates could be
identified by the same way as above.
most pyrolysis products in real time. The results observed
in this work were significant to this type of flavor precursor
to be used as flavor additive in high temperature process.
RESULTS AND DISCUSSION
Fig. 2 shows the mass spectra at the temperature of
7
00 °C and with four photon energies labeled on the figure.
Sample characterization
1
The selection of photon energy in our experiment was im-
portant. The final photonionization energy was selected as
Some techniques such as IR, HNMR and EI-MS
were adopted to identify the structure of the glycoside.
1
From the results of the HNMR and IR, we can see that the
data were perfectly corresponding to the molecular struc-
ture of BGLU (see Table 1). The purity of the product can
1
be evaluated from the HNMR spectra data. There was no
1
impurity peak in HNMR spectra (see supporting informa-
tion S-Figure 1). The data of MS spectra showed that the
molecular ion peak was absent, possibly because the en-
ergy of EI source was too high (see Fig. 1). The yield of the
product was about 35%.
Identification of the thermal decomposition products
As for BGLU to be used as flavor precursor, the py-
Fig. 1. The EI spectrum of BGLU.

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92 J. Chin. Chem. Soc., Vol. 58, No. 3, 2011
Ding et al.
1
1.5 eV, at which most of the pyrolysis intermediates and
number of pyrolysis products at m/z from 59 to 331 were
detected. As the temperature increased, it can be seen that
more pyrolysis products and intermediates ion signals
ranging from m/z = 43 to 347 were observed in our study.
From the products and intermediates study, we con-
clude that the cleavage of glycosidic bond can be carried
out by two possible modes in the pyrolysis process of
BGLU. As shown in Scheme I, the intermediates of ben-
zyloxy radical (m/z = 107) and glycon moiety radical (m/z =
331) would be observed in mass spectra by Route 1. As
compared with Route 1, Route 2 directly resulted into the
formation of glycon moiety radical (m/z = 347) and benzyl
radical (m/z = 91). From Fig. 3, we can see that the cleavage
of glycoside was carried out by Route 1 mode at low tem-
perature (300 °C). Because there was no presence of the
signal at m/z = 347. When the temperature increased to 500
and 700 °C, Route 2 was becoming an alternative mode in
the pyrolysis process of BGLU. The formation of benzyl
alcohol from benzyloxy radical should be easier than
benzyl radical in gas phase. For the H radical was easier to
be captured than OH radical in gas phase. The former was
just via H addition reaction and the latter needed OH addi-
tion reaction. It was possibly the reason why the aromatic
flavor component would dramatically decrease at high
temperature according to previous Py-GC-MS analy-
products could be ionized and detected.
Possible pyrolysis mechanism of BGLU
The studies on components of pyrolysis products
were important for the potential application of the glyco-
side as flavor additives in high temperature process. Differ-
ent temperature can usually result to the difference in py-
rolysis products. The thermal decomposition of glycoside
starts at about 250-300 °C. Fig. 3 shows the mass spectra
recorded for BGLU with various temperatures of 300, 500,
7
00 °C. As shown in Fig. 3, the number of pyrolysis prod-
ucts and intermediates increased with increasing tempera-
ture, which was in good agreement with previous Py-GC-
1
4
MS studies. At low temperature (300 °C), only a small
2
2,23
sis.
Scheme I The schematic diagram of possible pyroly-
Fig. 2. Photoionization mass spectra of the thermal de-
composition products of BGLU at the tempera-
ture of 700 °C and various photon energies: (a)
sis pathways of BGLU
1
4.5 eV, (b) 11.5 eV, (c) 10.0 eV and (d) 9.0 eV.
Scheme II shows the principal pyrolysis products of
BGLU observed in PIMS study. The thermal decomposi-
tion of glycon moiety was almost the same both by Route 1
and Route 2 mode. Some intermediates and radicals can be
observed by this technique. In comparison, from previous
Py-GC-MS study on menthyl-glycoside we couldn’t ob-
serve some useful information about the pyrolysis process
of glycon moiety. The major pyrolysis products for menth-
Fig. 3. Photoionization mass spectra of the thermal de-
composition products of BGLU at the photon
energy of 11.5 eV and various temperatures: (a)
3
00 °C, (b) 500 °C, and (c) 700 °C.

Photoionization Mass Spectrometry Pyrolysis Glycoside
J. Chin. Chem. Soc., Vol. 58, No. 3, 2011 293
yl-glycoside identified by Py-GC-MS were methanol, etha-
nol, menthol, acetic acid, dodecanal, geraniol, 1,6-anhy-
dro-triacetate-b-D-glucose and 1,6-anhydro-b-D-glucose.
There were seldom pyrolysis intermediates and radicals
Table 2. Py-GC-MS result of BGLU at the temperature of 700
°
C
Retention
Time (min)
Compounds of pyrolysis
Area (%)
1
4
identified.
3.52
Toluene
14.385
8.161
8.098
8.648
1.100
6.437
11.935
1.073
1.279
0.527
1.148
1.760
1.117
0.870
2.359
0.945
0.575
6.198
0.571
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
.68
.12
.56
.11
.31
.40
0.16
0.68
1.02
1.42
1.80
1.98
2.54
5.37
7.04
7.10
0.69
1.32
Ethylbenzene
Styrene
Benzaldehyde
2-Furanmethanol, acetate
Benzyl Alcohol
Formic acid, phenylmethyl ester
Phenylethyl Alcohol
4-Methyl-Benzenemethanol
2-Methylindene
Acetic acid, phenylmethyl ester
Formic acid, 2-phenylethyl ester
Naphthalene
1-(Bromomethyl)-4-methyl-Benzene
5-Acetoxymethyl-2-furaldehyde
Biphenyl
1,2-Benzenediol, diacetate
Bibenzyl
Scheme II Proposed pyrolysis scheme of BGLU
1,1¢-(1-methyl-1,2-ethanediyl)bis-
benzene
2
2
3
3
3
4.56
4.83
1.70
3.22
6.46
Heptadecane
(E)-Stilbene
a-D-Glucopyranose, pentaacetate
Hexanoic acid, 2-phenylethyl ester
3-Methyl-butanoic acid, 2-phenylethyl
ester
0.278
0.506
0.384
1.337
0.736
was separated by using a column chromatography and then
the pyrolysis was carried out. All reagents used were of an-
alytical grade and purchased from Sinopharm Chemical
Reagent Co. (Shanghai, China).
As a comparison with PIMS method, the pyrolysis re-
sults of BGLU produced by Py-GC-MS under high temper-
ature was also obtained in our study. Fig. 4 shows the total
ion chromatogram of BGLU obtained by Py-GC-MS at the
temperature of 700 °C. Major pyrolysis products were
listed in Table 2. From the results we can see that there were
many derivatives (mainly include some benzene deriva-
tives) detected by this technique.
Synthetic process
Acetobromo-a-D-glucose (2,3,4,6-tetra-O-acetyl-
EXPERIMENTAL
General
The Koenigs-Knorr method for the synthesis of glyc-
osides probably represents the most widely used procedure
in the chemistry of carbohydrate derivatives. In our work,
BGLU was synthesized by the modified Koenigs-Knorr re-
Fig. 4. Total ion chromatogram of BGLU produced by
2
4
Py-GC-MS at the temperature of 700 °C.
action under strictly anhydrous condition. The product

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94 J. Chin. Chem. Soc., Vol. 58, No. 3, 2011
Ding et al.
a-D-glucopyranosyl bromide, III in Scheme III) was pre-
data were obtained using a GCT mass spectrometer (Micor-
mass, UK).
pared in 40 mL acetic anhydride via successive addition of
1
0 g dry glucose, 3.1 g red phosphorous, 6 mL liquid bro-
Pyrolysis
mine and 3.6 mL water. At the end of the reaction, 30 mL
dichloromethane and 2 × 80 mL of ice water were added
into the mixture, respectively. The mixture was vigorously
shaken and the phases were allowed to separate. The upper
aqueous phase was discarded. And then, 80 mL saturated
sodium bicarbonate solution was added and the same ex-
traction process was performed as above. The acetobromo-
a-D-glucose was eventually retained in dichloromethane.
Then, 1.62 g benzyl alcohol and 5 g 4 Å molecular sieve
were mixed with acetobromo-a-D-glucose dispersed in di-
chloromethane. In the end, 4.14 g freshly prepared, dried
silver carbonate was added into the mixture. The reaction
mixture was stirred free of light for about 48 H. On the
completion of the reaction, the mixture was filtered and
condensed and separated on the silica gel column chroma-
tography.
The experimental work was carried out at the Na-
tional Synchrotron Radiation Laboratory (NSRL) in Hefei,
China. A detailed description of the instrument setup has
1
5
been reported elsewhere, and only a brief description is
presented here. An undulator radiation from the 800 MeV
electron storage ring dispersed by a 1m Seya-Namioka
monochromator with a 1,500 grooves/mm grating was used
in our study. The available photon energy range covers
from 7.8 to 24 eV with an average photon flux exceeding
1
3
10 photons/sec. A gas filter with Ne or Ar filled in the gas
cell was used to avoid higher-order harmonic radiation.
The thermal decomposition apparatus is composed of
a pyrolysis chamber with a Shimadzu pyrolyzer (PYR-2A,
Nakagyo-ku, Kyoto, Japan) and a photoionization chamber
with a reflectron time-of-flight mass spectrometer (RTOF-
MS) (see Fig. 1). Argon was used as the carrier gas with a
flow rate of 20.0 standard cubic centimeters per minute,
which was controlled by a mass flow controller. In our
work, a stainless steel pole with a diameter of 6 mm was
used to put the sample into the middle of the furnace (seen
Scheme III General synthetic process of BGLU
Product separation
Silica gel column chromatography was performed us-
ing a glass column (40 cm length, 3 cm i.d.) filled with sil-
ica gel (0.03-0.06 mm, Qingdao ocean chemistry factory,
China), eluted with petroleum ether-ethyl acetate (7:1,
v/v). The eluent was monitored by thin layer chromatogra-
phy. The outflow containing BGLU was collected and
evaporated to dryness and white solid BGLU was obtained,
immediately.
Structural characterization
Fig. 5. The schematic diagram of the thermal decom-
position apparatus with molecular-beam mass
IR spectra was recorded with an EQUIVOX55 infra-
red spectrometer (Bruker, German), the sample was ana-
15
spectrometry. Parts I and II are the pyrolysis
1
lyzed as KBr micropellets. HNMR spectra data was ob-
chamber and the photoionization chamber, re-
spectively. The synchrotron VUV light crosses
vertically with the molecular beam.
tained on a Bruker 300 MHz Spectrometer with CDCl
3
as
solvent and TMS as internal reference standard. EI-MS

Photoionization Mass Spectrometry Pyrolysis Glycoside
J. Chin. Chem. Soc., Vol. 58, No. 3, 2011 295
from the amplified part in Fig. 1). Argon was used as the
carrier gas. The pyrolysis gas passes through a nickel skim-
mer to enter the ionization chamber, where it intersects
with the synchrotron VUV light perpendicularly. The di-
ameter of the skimmer is about 1.25 mm. Species produced
at thermal decomposition process absorbed a VUV photon,
and were then ionized. A homemade RTOF mass spectrom-
eter was used to detect the positive ions with an approxi-
mate mass resolving power of 1400. The ion signal was re-
corded by a multiscaler P7888 (FAST Comtec, Germany)
after it was amplified with a preamplifier VT120C (EG &
G, ORTEC, USA). The temperature of the furnace and the
pyrolyzer can be controlled by a temperature controller
Naoharu, W. Phytochemistry 1998, 47, 1527.
. Wang, D. M.; Yoshimura, T.; Kubota, K.; Kobayashi, A. J.
Agric. Food Chem. 2000, 48, 5411.
2
3
4
5
6
. Natalia, L.; Purificacion, H. O.; Juan, C.; Vicente, F. J.
Agric. Food Chem. 2007, 55, 6674.
. Wilson, B.; Strauss, C. R.; Williams, P. J. J. Agric. Food
Chem. 1984, 32, 919.
. Matsumura, S.; Takahashi, S.; Nishikitani, M.; Kubota, K.;
Kobayashi, A. J. Agric. Food Chem. 1997, 45, 2674.
. Nishikitani, M.; Kubota, K.; Kobayashi, A.; Sugawara, F.
Biosci. Biotechnol. Biochem. 1996, 60, 929.
7. Bureau, S.; Razungles, A.; Baumes, R.; Bayonove, C. J.
Food Sci. 1996, 61, 557.
8
. Ma, S. J.; Watanabe, N.; Yagi, A.; Sakata, K. Phytochemistry
001, 56, 819.
2
(
SKY Technology Development Co., Ltd., Shenyang,
9
. Yang, Z.; Kinoshita, T.; Tanida, A.; Sayama, H.; Morita, A.;
China). During the experiment, the furnace was heated to a
specific temperature, and the sample of about 50 mg inside
the scoop was then pushed into the furnace. A series of
mass spectra was measured with variations of photon en-
ergy or temperature. In our work, the mass spectra were
collected at the photon energies of 14.50, 11.50, 10.00,
Watanabe, N. Food Chem. 2009, 114, 289.
1
0. Heckman, R. A.; Dule, M. F.; River, L. D. Rec. Adv. Tob. Sci.
981, 7, 107.
1. Ito, K.; Tanabe, Y.; Kato, S.; Yamamoto, T.; Saito, A. Biosci.
1
1
Biotechnol. Biochem. 2000, 64, 584.
1
2. Boulanger, R.; Crouzet, J. Food Chem. 2001, 74, 209.
9
.00 eV at selected temperatures ranged from 300 to 700
13. Vasserot, Y.; Arnaud, A.; Galzy, P. Acta Biotechnol. 1995,
15, 77.
°
C. With this ‘soft’ tunable photoionization method all the
+
plyrolysis products were detected as molecular ions, M . In
order to avoid any secondary reactions, the experiment was
performed at low pressure (0.062 Pa).
14. Xie, W.-C.; Tang, J.; Gu, X.-H.; Luo, C.-R.; Wang, G.-Y. J.
Anal. Appl. Pyrolysis 2007, 78, 180.
1
5. Li, J.; Cai, J.; Yuan, T.; Guo, H.; Qi, F. Rapid Commun. Mass
Spectrom. 2009, 23, 1269.
1
6. Cool, T. A.; Nakajima, K.; Mostefaoui, T. A.; Qi, F.;
McIlroy, A.; Westmoreland, P. R.; Law, M. E.; Poisson, L.;
Peterka, D. S.; Ahemd, M. J. Chem. Phys. 2003, 119, 8356.
7. Cool, T. A.; Nakajima, K.; Taatjes, C. A.; McIlroy, A.;
Westmoreland, P. R.; Law, M. E. Proc. Combust. Inst. 2005,
CONCLUSION
The pyrolysis behavior and the pyrolysis products of
synthetic BGLU were studied with synchrotron VUV pho-
toionization mass spectrometry at the temperatures of 300,
1
5
00 and 700 °C. According to the results, it can be con-
3
0, 1681.
cluded that the primary decomposition reaction of the
glycoside flavor precursor was the cleavage of O-glyco-
sidic bond at low decomposition temperature. As tempera-
ture increased, two different pyrolysis modes were ob-
served in our study for the first time. This study would be
significant to glycoside flavor precursor to be used as
flavor additive in high temperature process.
18. Hansen, N.; Klippenstein, S. J.; Taatjes, C. A.; Miller, J. A.;
Wang, J.; Cool, T. A.; Yang, B.; Wei, L. X.; Huang, C. Q.;
Wang, J.; Qi, F.; Law, M. E.; Westmoreland, P. R. J. Phys.
Chem. A 2006, 110, 3670.
1
2
2
2
9. Deshmukh, P.; Dutta, T. K.; Hwang, J. L.; Housecroft, C. E.;
Fehlner, T. P. J. Am. Chem. Soc. 1982, 104, 1740.
0. Johnstone, R. A.; Mellon, F. A. J. Chem. Soc. Fraraday
Trans. 1973, 69, 36.
1. Oikawa, A.; Abe, H.; Mikami, N.; Ito, M. Chem. Phys. Lett.
ACKNOWLEDGEMENTS
1
985, 116, 50.
This work was completed in National Synchrotron
Radiation Laboratory. The authors thank to National Natu-
ral Science Foundation of China (No. 20405013) for the fi-
nancial support.
2. Paine, III, J. B.; Pithawalla, Y. B.; Naworal, J. D.; Thomas,
Jr., C. E. J. Anal. Appl. Pyrolysis 2007, 80, 297.
23. Paine, III, J. B.; Pithawalla, Y. B.; Naworal, J. D. J. Anal.
Appl. Pyrolysis 2008, 83, 37.
24. Koenigs, W.; Knorr, E. Ber. Dtsch. Chem. Ges. 1901, 34,
9
57.
REFERENCES
1
. Noriaki, O.; Atsuko, I.; Mami, O.; Kanzo, S.; Akihito, Y.;