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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