SPECTROPHOTOMETER USING CELL PHONEPROJECT PROPOSAL

PREPARED BY, GUIDED BY,

DEVANSH DESAI (233) Dr. ARUN PATEL

CHINTAN PATEL (247) St. Xavier’s College,

NTRODUCTION:

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A spectrophotometer measures either the amount of light reflected from a sample object or the

amount of light that is absorbed by the sample object. It is based on electromagnetic spectra. In short, the

sequence of events in a modern spectrophotometer is as follows:

1. The light source shines on the sample.

2. A fraction of the light is transmitted or reflected from the sample

3. The light from the sample is directed to the entrance slit of the monochromator

4. The mono-chromator separates the wavelengths of light and focuses each of them onto the photo-

detector sequentially.

There are two kinds of spectrophotometers: single beam and double beam. A double beam

spectrophotometer compares the light intensity between two light paths. One path containing a reference

sample. And the other has test sample. A single beam spectrophotometer measures the relative light

intensity of the beam before and after a test sample is inserted. A double beam machine makes

comparison readings easier and more stable. But a single beam machine can have measure a wider range

of light frequencies. Single beam machines have simple optical systems and are more compact. When the

spectrophotometer is built into another device (like microscopes or telescopes) only single beam

machines will work.

Many older spectrophotometers must be calibrated by a procedure known as "zeroing." The

absorbancy of a reference substance is set as a baseline value, so the absorbancies of all other substances

are recorded relative to the initial "zeroed" substance. The spectrophotometer then displays % absorbancy

(the amount of light absorbed relative to the initial substance).

Spectrophotometers can also measure lumniscence. For example, the machine can shine

ultraviolet light of one frequency on the sample. This will excite the sample and make it glow. The

detectors can the measuring the light glowing from the sample at a different frequency.

A study of the interaction of light (or other electromagnetic radiation) with matter is an important

and versatile tool for the chemist. Indeed, much of our knowledge of chemical substances comes from

their specific absorption or emission of light. In this experiment, we are interested in analytical

procedures based on the amount of light absorbed (or transmitted) as it passes through a sample.

Suppose you look at two solutions of the same substance, one a deeper color than the other. Your

common sense tells you that the darker colored one is the more concentrated. In other words, as the color

of the solution deepens,you infer that its concentration also increases. This is an underlying principle of

spectrophotometry: the intensity of color is a measure of the amount of a material in solution.

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A second principle of spectrophotometry is that every substance absorbs or transmits certain

wavelengths of radiant energy but not other wavelengths. The light energy absorbed or transmitted must

match exactly the energy required to cause an electronic transition (a movement of an electron from one

quantum level to another) in the substance under consideration. Only certain wavelength photons satisfy

this energy condition. Thus absorption and transmission is a characteristic of a substance and it serves as

finger print for that substance.

In recent years lot of development has taken place in spectrophotometer techniques. They are

applicable to many industrial and clinical problems involving the quantitative determination of

compounds that are colored or that react to form a colored product.

LIGHT AND THE PERCEPTION OF COLOR

Light is a form of electromagnetic radiation. When it falls on a substance, three things can happen:

• the light can be reflected by the substance

• it can be absorbed by the substance

• certain wavelengths can be absorbed and the remainder transmitted or reflected

Since reflection of light is of minimal interest in spectrophotometry, we will ignore it and turn to the

absorbance and transmittance of light.

The color we see in a sample of solution is due to the selective absorption of certain wavelengths

of visible light and transmittance of the remaining wavelengths. If a sample absorbs all wavelengths in the

visible region of the spectrum, it will appear black; if it absorbs none of them, it will appear white or

colorless. We see the various colors when particular wavelengths of radiant energy strike our eyes. For

example, the wavelength we perceive as green is 0.0000195 inches or, expressed more scientifically, 495

nanometers. We can see only that color that is transmitted by the substance.

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Wavelength in nm Color absorbed Color observed

400 Violet Yellow green

435 Blue Yellow

495 Green Purple

560 Yellow Blue

650 Orange Greenish blue

800 Red Redish blue

We define transmittance as the ratio of the amount of light transmitted to the amount of light that initially

fell on the surface.

Absorbance is defined as the negative logarithm of the transmittance, and you will note that

absorbance and transmittance bear an inverse relationship.

Absorbance = - log T = - log P/Po

Going back to our example of chlorophyll, if you have two colored solutions, you may deduce

that the darker colored green solution appears darker because it absorbs more of the light falling on it.

Because the darker solution is also the more concentrated one, you can also say that the more

concentrateone absorbs more of the light.

Next, suppose that there are two test tubes, both containing the same solution at the same

concentration. The only difference is that one of the test tubes is thicker than the other. We shine light of

the same intensity (Po) on both containers. In the first case the light has to travel through only a short

distance, whereas in the second case it has to pass through a much longer length of the sample. We might

deduce that in the second case more of the light will be absorbed or cut off, since the path length is

longer.

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In other words, absorbance increases as pathlength increases. The two observations described

above (those dealing with the relationship between absorbance and concentration and absorbance and

path length) constitute the BEER-LAMBERT LAW.

Beer-Lambert Law

Absorbance path length (l) • concentration∝A = ε l c

where

• A is a dimensionless number.

• ε the proportionality constant, is called the molar extinction coefficient or molar absorptivity. It is a

constant for a given substance. l and c have the usual units of length (cm) and concentration (mol/liter).

The quantitative measurement of light absorption as a function of wavelength can establish both the

identity and the concentration of a substance in solution. The spectrophotometer is an instrument that

separates electromagnetic radiation into its component wavelengths and selectively measures the

intensity of radiation after passing through a sample.

Introduction to LEARN LIGHT SOFTWARE:

As with most modern instruments, software is used to convert raw signals to useful results. As this lab is

being written, the software is incomplete. Rather than write pages and pages of documentation (which, if

everything's written correctly, should be needed anyway!), let me outline what's there. There are 3

graphical insets and 2 control sections. In the upper LEFT of the screen, put the JPG for the SAMPLE

(the picture from which you want to find I(λ)). In the upper RIGHT of the screen, put the JPG for the

REFERENCE (the picture from which you want to find I0(λ)). I have not designed this for "drag and

drop." You must use the directory functions in the middle of the screen to select the files by name. If you

regard this as a serious limitation, let me know, and after we make sure the science parts of the software

are error-free. After the JPGs are in place, you have to tell the software where the spectrum is. Point to

either the red or blue end of the spectrum with the mouse. When you click, you'll get to choose whether

it's the red or blue end. Once you have both ends marked, you can use the data fields in the lower right of

the screen to tell the software what wavelengths correspond to your clicks. You won't know exactly, but

give the machine your best estimate. As long as you are consistent between the two frames, your results

will be precise,though perhaps not accurate.

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BRIEF IDEA ABOUT THE WORK:

ABSTRACT:

The wide availability of cell phones equipped with CMOS cameras (and of digital cameras

directly exporting JPG files) opens many opportunities for inexpensive, portable photometric

Measurements. Spectrophotometry makes more sense when we can see light change intensity when

passed through a sample and when we can see equations, sketches, or the output of a computer screen or

meter. Using the basic concepts of absorption, transmittance and reflection of light we have designed the

spectrophotometer. The light is allowed to pass through a sample and then reflected using a mirror.

Grating is placed just before cell-phone and then the spectrum is analyzed. The software called Learn

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Light is used to compare the spectrum, with and without sample. The plot of absorption is drawn and then

λmax (absorption) is calculated. Spectrophotometry is covered in every introductory text on quantitative analysis

or instrumental analysis. In this poster we present the construction, hardware, software, and laboratory

instructions for a diffraction spectrograph/cell phone (or digital camera) array detector suitable for

common people.

EXPERIMENTAL SETUP:

APPARATUS:

Acrylic box

Test tubes

Reflecting glass

Diffused glass

Grating

Cell phone

Learn light software.

LED’s

PROCEDURE:

Arrange the apparatus as shown in fig.

Switch on the light source and observe the spectrum on the screen of cellphone.

In first case allow the light to pass without placing any sample

Capture the spectrum

Now place a solution in test tube and repeat the exp.

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Compare both the spectrum using a learn light software.

Plot the curves for absorbance and transmittance.

RESULTS AND ANALYSIS:

The above graph is plotted for the analysis of potassium permanganate solution. The graph shows that the

absorbance (λmax) is maximum for 590 nm while the theoretical result shows it is for 550nm.

The graph fot transmittance can be plotted as follows:

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The max transmittance is at 580nm.

Further work to be done:

We would try to analyse the potassium permanganate solutions for various concentration and

many other solution for getting the idea about the accuracy of the instrument.

We would try to replace the LED source with sodium source and would try to see the results.

We expect the work to be finished by 27-2-2014.

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