103exp3 Spectrophotometer

EXPERIMENT 3 SPECTROPHOTOMETRIC MEASUREMENT OF ABSORPTION

1. INTRODUCTION AND AIM What is the aim of using a spectrophotometer in biological experiments? The aim of using a spectrophotometer is measuring the relative amounts of radiant energy as a function of wavelenght, usually it is used to analyze unknown substance in a solution or concentration of a solution according to the substance's capacity to absorb radiant energy. What are the basic components of a spectrophotometer? Describe their functions. • • • • •

Light source: Releasing light Diffraction grating: splitting the light into its components colors. Aperture: it is used to control the passing of the light with the selected wavelength. Photoelectric tube: Creating an electric current proportional to the number of photon striking it. Meter: Displaying the out put of the phototube.

Please define transmittance (T) and absorption (A) in your own words. Transmittance: The ratio of the radiant power transmitted by an object to the incident radiant power. Absorption: The process in which radiant energy is retained by a substance.

Which property of a solution affects its absorbance value? Explain. Concentration of a solution affects the absorbance value. Because absorbance value is the radiant energy retained by a substance and concentration is related to the amount of the substance, concentration affects the absorbance value and they have directly and linearly a relationship.

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2. MATERIALS AND METHODS Part 1: How did you operate the spectrophotometer? First of all I switched it on and waited for a while to be ready to use. After a few minutes it is ready. To operate the spectrophotometer I put a cuvette of dH2O in it and pushed the button “REF” to reference the absorbance value to zero. I used dH2O because it is the most transparent element we know, choosing it enables us to get the most accurate calculations. Finally, it is ready to use the wavelength that I decide. Describe the protocol you have used to determine the wavelength of maximum absorbance by bromophenol blue. Firstly I chose minimum and maximum wavelengths (525nm-650nm). From the 525nm to 650nm I increased the wavelength 25nm. At each wavelength I referenced the spectrophotometer by using dH2O then I measure the absorbance value of 0,02mg/ml bromophenol blue. After I finished the measuring, I looked at the values; there was a decrease after 600nm. In conclusion, this decrease meant that 600 nm is the wavelength of maximum absorbance by bromophenol blue. Part 2: Describe the protocol you have used to demonstrate the linear relationship between absorbance and concentration. In the beginning, I set the wavelength 600 nm because t is the wavelength of maximum absorbance by bromophenol blue. I measured the dilutions that I have five different concentrations of bromophenol blue from 0,0025mb/ml to 0,0200mg/ml. Then I calculate the absorbance value of all five concentrations. I realized that, when the concentration is increase absorbance value is also increase. How did you determine the concentration of bromophenol blue in the unknown solution? I measured the absorbance value of different concentration which were 0,0025, 0,0050, 0,0100, 0,0150, 0,0200mg/ml. Then I drew a graph. After I drew the graph of absorbance value and concentration I wrote equation. After I wrote the equation I decided the slope of the graph. Finally I found the unknown with mathematic.

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3. RESULTS Part1: Record the absorbance values (OD) of the 0.02 mg/ml dye solution for 475-675 nm interval in the following table. Wavelength 525 (nm) Absorbance 0.547 (OD)

550

575

600

625

650

0.856

1.452

1.583

0.248

0.015

Absorbance (OD)

Plot these values on a graph (independent variable should be on the X-axis). What is the wavelength of maximum absorbance?

1,7 1,6 1,5 1,4 1,3 1,2 1,1 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0

1,583 1,452

0,856

0,547

0,248 0,015 525

550

575

600

625

650

Wavelength (nm)

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Part 2: Record your absorbance values to the table below. Concentratio n (mg/ml) Absorbance (OD)

0.0025

0.0050

0.0100

0.0150

0.0200

Unknown

0.213

0.401

0.818

1.237

1.583

0,622

Absorbance (OD)

Plot these values on a graph.

1,7 1,6 1,5 1,4 1,3 1,2 1,1 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0

1,583 y =0,273x 1,237

0,818 0,622 0,401 0,213 0,0025

0,0050

Unknown

0,0100

0,0150

0,0200

Concentration (mg/ml)

What is the concentration of the dye in the unknown solution? UNKNOWN1 Because the graphic is not linear, I choose 2 slope points to obtain the most correct point. SP1(0,0050, 0,401) SP2(0,0100, 0,818) y2 – y1 Slope= -------------- =83,4 x2-x1 83,4= (0,622-0,401) / (Unknown-0,0050) Unknown= 0,0076 mg/ml

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4. DISCUSSION: Evaluate your results for part I. In the first part of the experiment, I calculated the absorbance value of 0,02mg/ml bromophenol blue for each wavelength of 525nm,550nm,575nm,600nm,625nm, and 650nm. Then, I determined the highest value for bromophenol blue by looking the absorbance values. It was 600nm but because we made it for 5 value, we cannot say it is 600nm exactly. To make the most accurate value we must make the experiment again between 575nm and 625nm. After finding a new value again we must decrease the interval of wavelengths that we use. We can reach the real absorbance value of bromophenol blue. Evaluate your results for part II. In the second part of the experiment, I measured absorbance value of five different concentration of bromophenol blue in 600nm. After the measurement when I draw the graph, it must be linear because of the Lambert-Beer Law. Because of some problems probably concentrations we could not get a linear function. To minimize the mistakes we must be more careful and first keep the interval wide then slowly make the interval small. However I saw the relationship between concentration and absorbance. Lambert-Law is correct. Write a short paragraph describing what you have learned in this lab session. First of all I learnt what spectrophotometer is and using spectrophotometer, and what we mean by transmittance and absorbance. After this experiment I saw the relationship between concentration and absorbance value which is linear.

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