Optics and Photonics Journal, 2011, 1, 24-35
doi:10.4236/opj.2011.12005 Published Online June 2011 (http://www.SciRP.org/journal/opj/)
Copyright © 2011 SciRes. OPJ
Trace Gas Detection from Plant Leaves, Flowers and Seeds
Using Conventional and Photothermal Light Deflection
Mohammad Ibrahim Abu-Taha, Yasmeen Abduljaleel Abu-Rayan
Physics Department, College of Science and Technology, Al-Quds University, Jerusalem, Palestine
E-mail: mabutaha@science.alquds.edu
Received March 14 , 20 11; revised April 14, 2011; accepted April 25, 2011
Photothermal deflection spectroscopy is a method used indirectly to measure optical absorption of a sample.
Different techniques can be employed to measure the amount of deflection, hence evaluate optical absorption
of sample. This work investigates an alternative method both in principle and technique to measure sample’s
optical absorption. The new method employed for the first time, relies on the simple idea of light beam de-
flection from the medium under investigation as a result of change in the index of refraction in its vicinity.
The amount of deviation executed by the deviated beam is estimated using new technique that is used for the
first time in deflection spectroscopy. As the deviated beam is allowed to pass through a single slit, the value
of beam deflection is estimated from the resulting diffraction pattern, i.e. indicating the value of changes
taking place in the sample and or measure sample’s optical absorption. The new detection technique used in
the estimation of probe beam deflection was also applied in photothermal spectroscopy. Results from both
methods were compared and revealed the ease of use of the new method, in addition it cuts cost and experi-
mental efforts although its sensitivity is less than the conventional photothermal method.
Keywords: Photothermal Deflection Spectroscopy, Deflection Spectroscopy, Trace Detection
1. Introduction
In photothermal deflection spectroscopy, sample under
investigation undergoes a photo-excitation, i.e. optical
absorption, which subsequently causes a change in state
of the sample and a thermal response. The method hence,
constitutes an indirect method to measure optical ab-
sorption. In regular optical absorption spectroscopy, the
intensity of the exciting light beam and passing through
the sample is monitored and compared with the intensity
of light beam before it enters the sample. However, in
photothermal spectroscopy, the transmission of light is
not used in measuring, and instead, sample heating, which
is a direct consequence of optical absorption, is what is
studied by measuring the value of deflection suffered by
a probe beam.
Many researchers carried out different investigations
for the theoretical aspects of photothermal method [1-3].
Photothermal deflection spectroscopy (PTDS) is widely
used in many applications for example in the investiga-
tions of thermal diffusivity of bulk solids and thin films
[4], measurements of low absorption coefficients [5],
temperature measurem ent s i n flam e by photothermal spec-
troscopy was also used [6-7], study of solids [8] and
electrochromism of synthetic metals [9].
In most of the experimental set ups employing PTDS a
photosensitive position sensor [10] is used and a com-
plicated equipment and electronic circuitry involved [11 ].
Some studies even used two sensors, and concluded that
PTDS is very sensitive for measurements of low absorp-
tion coefficients [12].
In this work the conventional optical effect, i.e. when
light passes from one medium to another, it changes speed,
and bends depending on the refractive index change of the
mediums is used as a spectroscopic indic ator. This means,
that idea of light bending toward or away from the normal
drawn perpendicular to the line dividing the two mediums
can act as a spectroscopic indicator, for example if light is
passing in the same medium and in center part of it an
index change does occur, then the beam change position
accordingly confirming the index change, hence leading to
a spectroscopic result related to the concerned medium.
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The amount of deflection is proportional to the mount of
change in the medium properties.
2. Experimental
2.1.Experimental Set Up
A schematic diagram showing the complete experimental
setup for conventional deflection and phtothermal spec-
troscopy used in the present work are shown in the Fig-
ure 1 and Figure 2 respectively. Experiments are carried
out using the two methods, i.e. the conventional light
deflection (CLD) and the photothermal light deflection
(PTLD) spectroscopic techniques. In both methods we
applied one technique for measuring the deflection of the
probe beam resulting from the effect taking place in the
cell. Firstly, results are taken for known samples used to
authenticate the systems, followed by the use of un-
known samples being taken using bo th method s and their
results compared.
It is worth noting that both arrangements comprise:
Power function generator, simple rectangular glass box,
He-Ne laser to act as a probe beam, single slit, light
photometer and a plane mirror fixed at one side of the
chamber. The only difference between the two set ups is
that the PTLD method employs an Infra red light source
to act as the power beam.
The infrared light source used in this study [14], is a
wideband pulsed mid-IR thermal source based on elec-
trical heating of a thin metal alloy foil up to red heat
(~900˚C) and cooled by its own radiation. The device,
emitting a wideband in the rang e 2 - 15 µm with emitting
area diameters of 25 mm, is based on a suspended bispi-
ral metal foil geometry which offers substantial thermal
isolation from the supporting structure. Although its
power is in the range of microwatts it proved enough in
the above arrangement, making the use of the source
advantageous in replacing hefty laser sources.
2.2. Light Deflection Detection Scheme
It is customary in PTLDS to use sensitive position sensors.
Various types of posit i on sens ors can be used to detect the
change in the position of the light beam that is reflected
from a specific t arget absor bi n g zo ne, employing different
measuring principles and involving difficulties and limit-
ing efficiencies accordingly. In the present work a new
detection scheme is suggested that can avoid experimen-
tal complications and cost. The new detection scheme
serves two goals: Firstly, and most importantly, it allows
the simple conventional light deflection to become a
practical spectroscopic method that uses only probe beam
laser; And secondly, it replaces position sensors in the
known PTLDS method, making it more manageable and
less complicated. The suggested method can be summa-
rized as follows: When the light beam from He-Ne laser
is allowed to pass through the medium under investiga-
tion and reflected from a mirror to a single slits, then
bright and a dark spots are formed on a screen, i.e. dif-
fraction pattern. A central peak containing most of the
light intensity accompanied by secondary higher-order
maxima and intensity minima governed according to:
dsin(θ) = mλ, and the first dark fringe occurs at the angle
given by: sin(θ) = λ/d, where θ is the angle between the
central incident propagation direction and the first mini-
mum of the diffraction pattern, and m indicates the se-
quential number of the higher-order maxima, λ is the
wavelength of the probe beam and d is the width of the
slit. Usually, interest is in the location of the first mini-
mum, when m = 1, because most of the light energy is
located in the central diffraction maximum. Light inten-
sity is maximum at θ = zero degrees, and decreases as we
move to away from the center point at angles dictated by
the equation above, see Figure 3 belo w.
In the present work the diffraction pattern is initiated
with a heliumneon laser beam allowed to fall on a single
aperture after being reflected from the region of interest
where a sample under investigation is placed. Abu-Taha,
2008, suggested the technique using a light photometer
head to be placed right at the center of the principal
maxima, then any slight changes in the index of refrac-
tion of the sample due to an activity or physical effect,
will result in a shift of the reflected beam, hence, a cor-
responding shift in the center of the principal maxima.
This resulted in certain angle change corresponding to a
change in the relative intensity amplitude falling on the
photometer sensor. In this case a change due to any ac-
tivity/effect taking place in the sample contained in the
cell will be translated into change in intensity amplitude
being recorded b y the photometer. This technique, allows
determining any minute sh ifts of the light beam position,
hence, the complicated position sensor in photothermal
deflection spectroscopy can be replaced by a simple slit.
2.3. Detection Scheme and Sensitivity
The reflected beam passes through the single slit forming
diffraction pattern. Photometer’s head is adjusted such
that it is sensing the principle maximum central spot,
then the head was manually moved across the maxima
spot in both sides. This is a trial carried out to confirm
the sensitivity of the detection scheme and to make sure
that data will result in a diagram similar to that of the
diffraction pattern as shown in Figure 4 below. The sen-
sitivity of the system is limited by the sensitivity of the
photometer being used. In the system used a displacement
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Glass box
Reflected beam after the introduction of
Photometer’s head
Reflected beam prior to introduction
of sample
He Ne Laser beam
Figure 1. chematic sh owing the conventional Light Deflection cell u s ed to detect gas trace emissions from d ifferent samples.
IR Source
Glass box
Aft er a bs orp tion
Photometer’s head
Before absorption
He Ne Laser b eam
Figure 2. Schematic showing the PTLD experimen tal s etup used to detect trace gas IR abs orptions.
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Figure 3. Single aperture diffraction pattern (Reproduced from: http://www.saburchill.com/physics/cha pter s2/00081.html-2010.
Figure 4. The relation between light intensity in (a.u.)and the traversed distance in (mm) to the left and right sides of t he c enter
of the principle maxima.)
of ~ 80 µm show itself as a change of light intensity as
small as 0.1 lux, the more sensitive the photometer could
be the more sensitive detection level becomes.
2.4. Samples
Using both experimental set ups different samples were
used. To start with experiment was first carried out using
known sample such as Methanol (CH3OH) to confirm
system reliability. Different volumes of Methanol (20, 40
and 60 µL) were allowed to evaporate in the cell, and trace
gas detection is carried out in the cell using both methods.
Other samples; such as plant seeds, flowers and leaves, of
unknown gas species were investigated using both CLD
and PTLD methods. For example, a definite mass of the
plant part were used (0.25 g and/or 0.50 g) of fresh leaves
and/or flowers (intact or crushed) were taken from the
garden in the same day of experiment and their emission
monitored against time, i.e. their rate of trace gas emission
monitored. Such applications are so important to monitor
seeds during storage period in a warehouse prior to mar-
keting, or investigate ripening conditions of plant parts.
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Healthy and caries infected samples were used so as to
monitor the live activity of insects on seeds.
3. Results
It is worth noting that since, the center of the principle
maxima spot was made to coincide with the fixed pho-
tometer’s fiber versus optic head at the start of measure-
ments, then the photometer measurement of intensity
would be maximum at first. As time goes intensity read-
ing would be decreased as the beam deflected away from
the principle max-spot, hence, for measurements to re-
flect the physics behind the effect taking place in the cell
the intensity difference is plotted versu s time as ind icated
in all figures of the experimental results.
Figure 5(a) and Figure 5(b) show the rate of evapora-
tion from different volumes of methanol placed in the cell
as they monitored using CLD and PTLD respectively.
This experiment with known chemical sample is used to
authenticate the viability of t he CLD method in particul ar.
Plant leaves is an important source of essential oil of
some plants. The level of trace emission also reflects the
ripening situation of the plant. Mint is chosen as an im-
portant example as Figur es 7(a)-(d) show.
Two different kinds of flowers namely; Jasmine and
Rose were investigated using CLD and PTLD techniques.
For each flower type two flower colors were used.
Two different plant seeds namely wheat and chickpeas
were investigated using CLD and PTLD techniques. It’s
known that seeds can be attacked by caries insects or
start to germinate during the storage period, so the avai-
lability of a monitoring technique in the warehouse is
advantageous. The present study was employed to con-
firm the ability of the system to check for insect actions
in seeds, this is done by detecting their gas emissions
when insect contaminated seeds were introduced into the
testing box.
In the above trace gas results using CLD and PTLD are
presented. A comparison between both techniques could
help identify the technique that is more sensitive. Com-
parison results between the two techniques are shown in
Figures 9(a)-(e). To further prove the technique which is
most sensitive the PTLD technique was used to detect
trace gas from only one Jasmine flower, see Figure 9(d).
It is believed that each plant leaves emit a trace gas that
contain part of the gases emitted by its flower. To inves-
tigate this fact the relation between gas emissions from
jasmine leaves and flowers are shown in Figure 9(e).
4. Discussion
The data obtained from the experiments using both me-
thods were discussed along three paths; one involves re-
sults of gas emissions using CLD technique and the sec-
ond involves results of gas emission from samples using
the well known PTLD technique. And, th e third involves
comparison between results of CLD and PTLD. Both
CLD and PTLD signals were monitored as a function of
time. In PTLD experimental part samples were allowed
to absorb wideband IR radiation from a pulsed IR source,
i.e. for the experiment a probe and a pump beams are
used; contrary to CLD technique which involves only
one beam namely the probe beam. As mentioned earlier,
no position sensor is used to determine the deflection of
beams in both techniques, and instead a diffraction slit
proved very useful and satisfactory.
The new less sophisticated technique CLD method was
used to detect trace gas emissions from different chemical
and plant part samples. The first impression confirms our
hypothesis that CLD method are giving comparable re-
sults to that of PTLD. Different trials to detect trace gas
emission from plant parts were carried out. Group of
Figures 6(a)-(d), show detection possibility for trace
detection using plant leaves at different conditions using
both methods. Plant flowers are important in the perfume
industry, both experimental techniques were checked for
their ability to detect trace emissions from different types
of flowers and even different flower colors in the same
flower type (see Figures 7(a)-(d). Trace emissions from
seeds provides a way to follow up the storage conditions
in the warehouse. Trials to mimic storage conditions of
humidity and insect attack on the stored seeds were stud-
ied (see Figures 8(a)-(c)). The activity of only 5 caries
attacking chickpeas could be monitored using both meth-
ods. Finally, Comparisons Figures 9(a)-(e) between se-
lected situations using both methods speak for them-
selves. It obvious that the PTLD is superior to the CLD
method, since it employs the advantage of trace gas ab-
sorption of IR radiation of the wide band light source
emitting in the range 2 - 15 µm.
As far as the sensitivity of the methods are concerned,
it can be estimated from a known volume of methanol,
detection levels were estimated to be as low as 18 and 24
ppm for PTLD and CLD respectively. It is obvious that
the PTLD methods is more sensitive than CLD, but the
reward from the later is simplicity and easy to implement.
Open area detection (no cell used to hold sample) was
carried out using the simple conventional light deflection
method, which was used to detect smoke emission from
one cigarette placed such that its smoke is flown across
the path of the laser beam. More, the effect of air speed
going through the path of the laser beam were invest-
tigated too. In this experiment the laser beam allowed to
fall on a mirror fixed on the wall inside the lab room and
be reflected through a slit into the photometer’s head.
The CLD method is found applicable for both smoke
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Figure 5. (a) Monitoring trace gas emissions from three different Methanol’s volumes using CLD technique; (b) Monitoring
gas trace emissions of three different Methanol’s volumes using PTLD technique.
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Figure 6. (a) Monitoring trace gas emissions from two different crushed mint’s masses using laser CLD technique; (b) Com-
parison between trace gas emissions from 0.5 g crushed and uncrushed mint’s leaves using laser CLD technique; (c) Moni-
toring trace gas emissions from two different crushed mint's masses using laser PTLD technique; (d) Comparison between
trace gas emissions from 0.5 g crushed and unc r ushe d mint’s le ave s using laser PTLD technique.
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Figure 7. (a) Comparison between trace gas emissions from white and yellow Jasmine flowers using CLD technique; (b)
Comparison between trace gas emissions from crushed and uncrushed yellow Jasmine flowers using CLD technique; (c)
Comparison between trace gas emissions from white and yellow Jasmine flowers using PTLD technique; (d) Comparison
between tr ac e gas emissions from white and red ro se flowers using CLD technique.
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Figure 8. (a) Comparison between trace gas emissions from non contaminated and insect contaminated chick peas using CLD
technique; (b) Comparison between trace gas emissions from non contaminated and insect contaminated chick peas using
PTLD technique; (c) Comparison be tween trace gas emissions from germinated and un germinated wheat grains using CLD
Copyright © 2011 SciRes. OPJ
Figure 9. (a) Comparison between the use of CLD and PTLD techniques to detect trace gas emissions from 40 µL drop of
Methanol; (b) Comparison between the use of CLD and PTLD techniques to detect trace gas e missions from 0.25 g of mint; (c)
Comparison betwee n the use of CLD and PTLD technique to detect trace gas emissions from insect contaminated chickpeas;
(d) Trace gas emissions from one Jasmine white flower using PTLD technique; (e) Comparison between trace gas emissions
from Jasmine's one flower and (0.5 g) of Jasmine’s plants’ leaves using PTLD technique.
Copyright © 2011 SciRes. OPJ
detection and changes of air speed in the form of draft. It
was possible to determine the side into which air is
blown. This experiment indicates the ability of CLD
method for open area applications, for example detecting
insect sprays vapors carried away from fields toward
inhabited areas.
5. Conclusions
A set of experiments were performed successfully using
two simple, sensitive, easy to handle, safely, inexpensive
and accurate systems. The basic conclusions of this work,
is drawn from the use of CLD and PTLD methods to de-
tect gaseous species at the trace level for different sam-
ples. This experiment prov ed the ability of the two me th-
ods to distinguish different kinds of samples emitting at
low ppm level. Both methods were used without the need
for position sensor, instead a slit initiated diffraction pat-
tern was used, i.e. an expensive difficult to handle posi-
tion sensor could be replaced successfully with a single
diffraction slit. This is considered an achievement for
such experiments, resulting in the enhancement and ease
of their use. Although the CLD gave encouraging results
comparable to the PTLD method, its sensitivity co uld be
improved using more sensitive light photometer. CLD
can have a wide variety of agricultural applications, for
example, to monitor storage conditions in a warehouse,
conditions of animal barns, industrial plants, hospitals
and many different environmental studies.
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