BIOL 244 Lecture Notes - Lecture 9: Diffraction Grating, Cuvette, Spectrophotometry

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25 Sep 2016

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In many fields of biology, the spectrophotometer is an important analytical instrument. The concentration of specific compounds in a solution can be determined by measuring the amount of light (at a particular wavelength) that has been absorbed by a solution. The amount of absorbance depends on the concentration of the compound and the ability of that compound to absorb that particular wavelength of light. A beam of light of a single wavelength (we call this beam monochromatic light") is passed through) or is absorbed by the solution. The spectrophotometer tells us the amount of the monochromatic light that is absorbed or transmitted. In a spectrophotometer, a white light source (for visible wavelengths) is focused through a prism or diffraction grating (see figure 9. 1). The white light is then separated into its spectrum consisting of various wavelengths. A particular wavelength is selected by focusing the light through a narrow slit.

Related Questions

1.What was the purpose of measuring the stock solutionâs absorbance at different wavelengths? Or in other words, why did you need to make an absorbance spectrum for this dye?

2.Explain why you used a blank when using the spectrophotometer. Make certain to explain why you needed to use the blank between each measurement in the first part of the experiment, but only once in the second part of the experiment.

3.You mix up a stock solution of green dye by adding 5 mg of green dye in 100 ml of water and find that there is maximum absorbance of 0.75A at 425nm for this dye.

What wavelengths of light do you expect to be transmitted through this solution (refer to Table 3.1 in your lab manual)?

If you were to add another 2 mg of the green dye to your stock solution would you expect the absorbance at 425nm to be higher or lower than 0.75 A₄₂₅?

This laboratory relates to the following Terminal Course Objective(TCO):

8 Given the two major problems of global warming and ozone layerdepletion, compare and contrast the causes, environmental andhealth impacts, as well as the success or failure of internationalpolicies to solve these problems.

Introduction

We have heard for a long time that air pollution and theaccumulation of greenhouse gases lead to global warming. In theanimation below the rise in global temperature and the rise in sealevel is predicted by a model built by the Intergovernmental Panelon Climate Change (IPCC). This model is based on a number ofassumptions on the amount of ozone, particulate matter and otherair pollutants.

Depletion of the Ozone Layer

Solar radiation emits electromagnetic waves with a wide range ofwavelengths. Visible light is the part of the electromagneticspectrum that can be detected by the eye's photoreceptors.Ultraviolet radiation has slightly shorter wavelengths than visiblelight and can cause damage to the biological tissues that absorb itby destroying protein and DNA molecules. The damaging effects ofultraviolet radiation are mitigated by the ozone (O3) in theatmosphere. The ozone in the stratosphere is called the OzoneLayer. Without the Ozone Layer, it is doubtful that any life onearth could survive. Since World War ll, many atmosphericscientists began to measure the concentration of ozone in thestratosphere around the South Pole. They have noted someinteresting trends. In the exercise below we will explore the issueof Ozone Depletion.

Wind Circulation in the Atmosphere

Wind circulation in the atmosphere is largely responsible for ourweather, our climate zones, and the movement and concentration ofgreenhouse gases and other air pollutants. The learning objectbelow demonstrates the circulation of winds vertically andhorizontally around the earth.

Man and Global Climate Change

The role of man in the creation of greenhouse gases and globalwarming has long been debated. The Intergovernmental Panel onClimate Change (IPCC) commissioned a model to judge the effects ofman on global temperature, the role of natural environmentalprocesses, and the combination of both. This model is exploredbelow.

One of the greenhouse gases that can be directly related to theactivities of man is carbon dioxide (CO2), which is a product ofindustrial and transportation activities. While it is true thatother processes, like respiration, can produce CO2, it is simple tolook at the concentration of CO2 before and after the industrialrevolution and clearly see the effects of man.

Concern about greenhouse gases led to representatives of 166nations to sign the Kyoto Protocol of 1997, which attempted tolimit emissions of carbon dioxide and other greenhouse gases. Therise of the global economy has complicated the issue, as isdemonstrated in the learning object below.

Greenhouse Gases and Health

In recent years we have been measuring the Air Quality Index (AQI),which is a measure of such pollutants as ozone, particulate matter,and CO2. In the exercise below you may explore the AQI in citiesacross the U.S.

Let's look at two locations less than 500 miles apart thatrepresent both ends of the spectrum of the issue of greenhousegases and global warming. Explore both virtual field trips andevaluate them in terms of whether they contribute to the greenhousegas problem or help reduce it.

Kuala Lumpur

Rainforest / Terraced Rice Fields

Exercises

Exercise 1. Have a class debate between people representing thedeveloping countries and people representing the developedcountries OR write an essay comparing and contrasting the two.Characterize each side in terms of the issues surroundingpopulation growth, energy use, resource use, and sustainabledevelopment. Find common interests between the two.

Exercise 2. Look at the two virtual field trips and define each ofthe environments depicted in terms of the global carbon cyclediagram below. Assuming that the cycle is in balance, how wouldeach site change the balance.

Exercise 3. Develop a series of international agreements that wouldlead to the kind of reductions in greenhouse gas emissions calledfor in the Kyoto Protocol and that would satisfy the concerns forglobal equity between developed and developing countries.

Exercise 4. Using the table below keep track of your dailyactivities and how they affect the greenhouse gas crisis in theworld. What does each activity produce? Are the products positiveor negative. How do the sizes (volumes of gas) of the variousproducts vary? After you have created your table define yourpersonal global carbon footprint. Do you have a positive effect ora negative one?

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BIOL 244 Lecture Notes - Lecture 9: Diffraction Grating, Cuvette, Spectrophotometry

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