Applied_Spectroscopy_Practica_01_01 - 34

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Applied Spectroscopy Practica
The authors developed a handheld spectrometer in the
400-760 nm range by using a visible sensor (256 channels).13
In this study, the authors combined two sensors to develop a
handheld visible-NIR (Vis-NIR) spectrometer in the 400-
1050 nm range covering wider spectral regions including the
short wavelength NIR region. This new instrument was used
to monitor the daily changes of a mini tomato for about 100
days. Daily changes of color L*a*b* values and Vis-NIR spectra
were followed during the ripening of the same position of the
mini tomato. Daily changes of chlorophyll, lycopene, and carotenoids
and their relations to color values were examined.
Materials and Methods
A commercially available seedling of a mini tomato (Solanum
lycopersicum; Suntory, Sweets Red) was purchased and
planted in a planter set on a veranda of the first author's
home in Osaka, Japan. The seedling was grown on the
veranda by supplying just water every day from 22 April
2022 to 13 August 2022. No fertilizer was added.
Handheld Vis-NIR Spectrometer
A handheld Vis-NIR spectrometer has been originally developed
(Fig. 1). This instrument uses two monochromators
with 256 channels in the wavelength range of 340-780 nm
(Hamamatsu, C12666MA) and 640-1050 nm (Hamamatsu,
C11708MA-01). Since signals at shorter and longer wavelength
regions are weak, effective wavelength regions are
from 400 to 1000 nm by connecting at 700 nm. In the present
study, spectra are presented in the 400-1050 nm
range with about 330 channels, since signal-to-noise ratios
are good at the longer wavelength region (Fig. 1).
A xenon (Xe) lamp was used as a light source. This light
was applied to the sample surface using a fiber guide and
reflected lights from the sample were collected through
fiber guides at angles of 45°. These reflected lights were
then dispersed by the two monochromators (Figs. 1a and 1b).
The power of direct current 5 V can be supplied to the spectrometer
from the universal serial bus (USB) cable connected to
a personal computer (PC). The signals recorded by the two
monochromators can be transferred to a Windows PC through
the USB cable and aWindows program was used to measure and
store spectra. By using four AAA batteries, the signals were sent
via WiFi to a PC or iPhone. An iPhone application (PRISMO
Mirage) can also be used to control the spectrometer (Fig. 1c).
A dark signal was first measured without the light source by
using an original cover plate covering the measuring aperture of
about 6 mm in diameter. Then, a reference signal was measured
with the light source by using another face of the original
plate with a white reference made of MgO with known reflectance
(R) values (Fig. 1c). Reflected light intensities from a sample
were obtained by the following equation:
Reflectance R = (Sample-Dark) / (Reference-Dark) (1)
The obtained reflection spectra were connected at 700 nm at
the R-value obtained from the visible monochromator by shifting
the R values of longer wavelength regions than 700 nm.
The connected reflection spectra are from 400 to 1050 nm
with about 330 channels with about 2.4 nm intervals.
The R spectrum of the white reference of MgO measured by
this spectrometer is shown in Fig. 1d. By taking Abs = -log R,
its absorbance (Abs) spectrum can be obtained as shown in
Fig. 1e.
Since the signals of the two monochromators change
with temperature, a temperature-humidity sensor was
set inside the spectrometer. The temperature and relative
humidity (RH) inside the spectrometer at the daily measurements
were 30.1 ± 4.3 °C and 60.3 ± 13.2%, respectively,
during the measurement period. Therefore,
measured signals by the monochromators were used
without correction.
Daily Measurements
The same position of a mini tomato grown on the veranda was
measured daily around 8:00 a.m. by the handheld Vis-NIR spectrometer
from 24 April 2022 to 13 August 2022 for 111 days
(Fig. 2a).
A handheld Brix meter (Atago, PAL Light Sensor 3 Mini
Cat. No. 5403) was also used to measure Brix values at
the same time as the above spectroscopic measurements.
Temperature, humidity, and luminance values on the veranda
were monitored every hour during this period by a temperature,
humidity, and luminance sensor.
Results
Temperature, Humidity, and Luminance Values During
Mini Tomato Ripening
Temperatures on the veranda during the measurement
period of the mini tomato are plotted in Fig. 2b. Minimum
and maximum temperatures were 10.2 °C and 41.8 °C,
respectively, with an average value of 25.9 °C. The veranda
is facing southeast and the direct sunlight is shining on the
mini tomato around 7:00 to 9:00 a.m. Therefore, the temperature
at the sample can increase under direct sunlight leading
to a high temperature as high as 41.8 °C.
Relative humidity values at the veranda changed from
17.0% to 89.0% with an average value of 56.3% (Fig. 2c).
Luminance values changed from 0 to 8882 Lux during this
period (figure not shown).
Brix and Color Values During Mini Tomato Ripening
Brix values are around 10 at the start of the ripening and
reached a maximum of about 13 at around 45 days, then
decreased down to three (Fig. 2d).
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