designing a low cost portable vis spectrometer
TRANSCRIPT
DESIGNING A LOW COST PORTABLE VIS SPECTROMETER
Rajat Nag (15202684), Michelle Savian (15203989) and Mayukh Bhattacharjee (15202910) UCD School of Biosystems and Food Engineering, University College Dublin, Belfield, Dublin 4, Ireland.
Aim
The main objective of this experiment is to design a basic spectrometer. In this experiment we
are asked to design a basic spectrometer using low cost materials. Spectrometry is the
quantitative measurement of the reflection or transmission properties of a material as a function
of wavelength. The Beer-Lambert law is adopted for the experiment.
Introduction
Atoms and molecules prefer to be in their ground state. When they get energy (e.g. from light)
they jump into an excited state. Excited species will spontaneously emit radiation as they relax
back into their ground states. Atoms and molecules exist in a number of defined energy levels.
Because light is a form of energy, absorption of light by your sample causes the energy content
of the molecules to increase. The energy of a photon absorbed during a transition from one
molecular energy level to another is given by the equation:
E=hc/λ=hν
Where h is Planck’s constant = 6.62x10-34Js; c is the speed of light and λ is the wave length.
When radiation passes through a layer of solid liquid or gas certain wavelengths may be
selectively removed by absorption. Wavelengths absorbed by a substance are related to its
molecular structure. If the light has energy (E = hc/λ) enough to promote excitation, it is
absorbed. E.g. In the visible wavelength range, chlorophyll absorbs light in the blue and red
regions of the visible spectrum. The remaining reflected light thus appears green.
Diffraction grating
It is an optical device used to separate different wavelengths or colors contained in a beam of
light. It consists of thousands of narrow, closely spaced parallel lines.
Figure 1. The diagram of the reflective and transmissive Diffraction Gratings
Diffraction Gratings can be either transmissive or reflective (Figure 1). In our experiment
transmissive Diffraction Grating is used. As light transmits through a Grating, it diffract the light
into its component wavelengths.
Beer’s Law
For monochromatic radiation, absorbance A is directly proportional to the path length b through
the medium and the concentration c of the absorbing species (Figure 2).
A = abc
Where a = absorptivity coefficient (a constant, depends on sample). The units on a must be such
that A is unit less.
Figure 2. The explanation of Beer’s law.
Materials and methods
Provided materials
The following materials are used to build a low cost portable spectrometer based on Beer’s law.
Diffraction grating
Plastic cuvette
LED light source
Black paper
Blu tack
Cello tape
Scissor
Dimension measuring Scale
Design development of experiment setup
Using the materials provided which is listed above a model spectrometer was prepared as
discussed with the group members. The schematic diagram of the prepared model spectrometer
is presented below (Figure 3). A black box was prepared with help of given black paper, cutting
and sizing with the scissor wherever required. The box was held together with the application of
cello tape. A pin hole was created on one side of the box and a larger hole which was meant for
viewing was created on the exact opposite side of the pin hole. The box was held at stationary
position by the application of blu tack on the table. The cuvette which was filled with the sample
liquid was placed inside the box at a distance of 40 mm from the pin hole, the cuvette was held
stationary by the application of blu tack underneath it. The diffraction grating was placed inside
the box, just in front of the viewing hole as referred in the Figure 3. The distance between the
cuvette and the diffraction grating was measured to be 97mm. Figure 4 presents the actual
picture of the setup of the experiment.
Figure 3. The schematic diagram of the experiment set up
Figure 4. The photograph of the experiment set up
First part of the experiment
Using the above mentioned setup the light passing through the sample (taken in cuvette) was
diffracted with the help of diffraction grating and the spectrum was captured with a mobile
phone camera. Three different solutions; green, red and blue (Figure 5) of 100ppm have been
analyzed to obtain the spectra.
Figure 5. The three samples provided green, red and blue; 100 ppm each
The obtained images are listed in Figure 6a, 6b and 6c, analysis of those will be discussed in
discussion and result section of the report.
Figure 6a. Image obtained with 100 ppm green solution
Figure 6b. Image obtained with 100 ppm red solution
Figure 6c. Image obtained with 100 ppm blue solution
Second part of the experiment
Red colored solution was selected for second part of the experiment. The concentration of the
solution was broken down to 20, 40, 60 and 80 ppm with the help of C1*V1 = C2*V2 method.
Where C1 is the known concentration which is 10ppm and C2 is 20, 40, 60 and 80 ppm. V2 was
considered as 5 ml whereas V2 was determined as 4 ml, 3 ml, 2 ml and 1 ml for the preparation
of 80, 60, 40 and 20 ppm solution respectively. With the help of prepared spectrometer the same
process mentioned in the first part of the experiment was repeated for the standard solution of 20,
40, 60, 80 ppm and of course a blank solution of zero ppm. The obtained images are listed in
Figure 7a, 7b, 7c, 7d and 7e.
Figure 7a. Image obtained with 0 ppm red solution
Figure 7b. Image obtained with 20 ppm red solution
Figure 7c. Image obtained with 40 ppm red solution
Figure 7d. Image obtained with 60 ppm red solution
Figure 7e. Image obtained with 80 ppm red solution
Results and Discussions
Development of method to estimate absorbance and λ max for each of the colored solution
The spectrums produce during the experiment, and capture by the camera, were saved as a jpeg
image. After that, the pictures were cropping using Paint Program, where all the unwanted area
from the spectrum was removed (Figure 8), to obtain the central part that was used in Rsudio.
Figure 8. A single strip of spectra (Red 100 ppm)
In Rstudio, the spectrum images were processing in EBImage, that is a R package (available
onhttps://www.bioconductor.org/packages/3.3/bioc/html/EBImage.html), which provides general
purpose functionality for reading, writing, processing and analyzing of images. After open the
image in Rstudio and display it, the spectrum was extract to find out its wavelength size.
This procedure was repeated for the 7 spectrum image obtained during the experiment, see table
1. The wavelength ranged from 350 to 700 nm. The final step was plot the graph absorbance
versus wavelength, which allowed us to obtain the absorption peak and “lambda-max” λ max,
which is the wavelength that correspond to the highest absorption. Detail about the Rstudio script
is presented below.
library(EBImage)
Im=readImage('B100.jpg')
display(Im)
Im_sub=Im
display(Im_sub)
#extract spectrum from sub-image
N=dim(Im_sub)
Im_unfold=matrix(Im_sub,N[1]*N[2],N[3])
Im_mean1<-matrix(rowMeans(Im_unfold),N[1],N[2])
Im_mean2<-rowMeans(Im_mean1)
#find wavelength step size (depends on distance from camera to light source & diffraction
grating)
WL_step=350/N[1]
WL=seq(350,700,by=WL_step)
plot(WL[1:444],Im_mean2,type="l",ann=F)
title("Blue 100", xlab="Wavelength (nm)",ylab="Absorbance")
# The wavelength e.g. WL[1:444] is variable, depend on the spectrum image.
The summary of the results obtained by the analysis of absorbance versus “lambda-max” λ max,
for all spectrum images obtained during the experiment, is presented in table 2.
Table 1: The graphs obtained from R studio output for different concentration of the solutions
Table 2: The λ max and Absorbance obtained from the graphs for different concentrations
Solution
Colour
Concent-
ration
ppm
Predicted
Wavelength
value (nm)
Actual
Waveleng
th λ max
(nm)
Actual
Colour
Absobed
Absorbance
Observations
according to
Table 3
Green 100 620 - 780 _
RED 428 & 606
Indigo and
Orange 0.58
Taking the max as
606
Blue 100 585 - 620 _
Orange 433 & 586
Indigo and
Orange 0.425
Taking the max as
586, perfectly
matched
Red 100 490 -
570_Green 610 Orange 0.8 Slightly higher
Red 80 490 -
570_Green 555 Green 0.62 Perfectly matched
Red 60 490 -
570_Green 581 Yellow 0.58
The impurity
dominates this and
the downward
results as the
sensitivity of our
instrumental set
up is not error free
as UV
spectrometer
Red 40 490 -
570_Green 584 Yellow 0.48
Red 20 490 -
570_Green 381 & 582
Ultra violet
- Violet
transition
range and
Yellow
0.37
Blank 0
Ultra violet -
Violet
transition range
383 & 620
Ultra violet
- Violet
transition
range and
Red
0.3
UV, due to
organic impurities,
620 nm adopted
for absorbance
value
Note: Overall observation: Absorbance values to be considered only as the light is not monichromatic
(single colored) before entering the sample, it has other eliment's influence
Table 3: Characteristics of wavelength and color
Color Wave length
(nm)
Complimentary color
Violet: 400 - 420
Indigo: 420 - 440
Blue: 440 - 490
Green: 490 - 570
Yellow: 570 - 585
Orange: 585 - 620
Red: 620 - 780
Conclusion
Now if we compare the concentration of the red solution of different concentrations we can find
that there is a strong co relation between concentration and absorbance.
Table 4: Data for Concentration vs Absorbance plot
Solution
Colour
Concentration
(ppm) Absorbance
Red 100 0.8
Red 80 0.62
Red 60 0.58
Red 40 0.48
Red 20 0.37
Blank 0 0.3
Graph 1: Concentration vs Absorbance plot
The relationship between Absorbance and concentration is liner (y = 0.0048x + 0.2857) and we
came to a conclusion that there is strong correlation between concentration and absorbance as R2
value is 0.9755. From the graph we can conclude that Absorbance is directly proportional to the
concentration.
Absorbance ∞ concentration
The more the concentration of the solution the more the absorbance will be. And more
absorbance means lesser transmittance. Hence Beer’s low was followed by the experiment.
y = 0.0048x + 0.2857
R² = 0.9755
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 20 40 60 80 100 120
Abso
rban
ce
Concentration (ppm)
Concentration vs Absorbance
Ideally at zero concentration of red solution the absorbance should be zero. As the instrument is
not comparable with costly conventional spectrometer regarding the elimination of error (refer
observation section of table 2) it is beyond scope of our experiment. However there is minimal
error in the experiment resulting from the strong correlation of the graph 1. With the help of this
portable VIS spectrometer we can easily construct a calibration curve and the unknown
concentration could be measured with accuracy.
References
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